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Introduction to electrospinning - CNR-ISMAC Biella 1

CNR-ISMAC Biella

National Research Council (CNR) is the largestpublic research institution in Italy, with about 7800researchers and technicians.

CNR is distributed all over Italy through a networkof 107 Institutes.

Institute for Macromolecular Studies (ISMAC) has3 offices: Milan (headquarters), Biella and Genoa.

ISMAC-CNR Biella is an internationally recognizedcentre of fibres and textile science.

Website: www.bi.ismac.cnr.it

Main activities

• Antibacterial finishes for textile products;• Electrospinning for nanofibres production;• Extraction and processing of wool keratin for filtration and

biomedical applications;• Synthesis of electro-conducting polymers on textiles;

• Phys iological comfort evaluation and objective measurements of texti le handle properties;

• Labelling of textile materials and development of analytical

methods for the quality assessment of fibres and textiles.

Piedmont region

Introduction to electrospinning

Alessio Varesano

CNR-ISMAC, Istituto per lo Studio delle Macromolecole

C.so G. Pella 16 - 13900 Biella (Italy)

Tel.: (+39) 015 8493043Fax : (+39) 015 8408387

Website: http://www.bi.ismac.cnr.it


First article on electrospinning

“electrospinning” means “electrostatic spinning”

electrospinning is a concept used to describe a class of fibre forming processes in

which electrostatic forces are used to produce fibres.


Electrospinning in short

A basic electrospinning apparatus for electrospinning polymer solutions consists of:

• syringe (with capillary/needle)• pump• voltage generator

• collector

The polymer solution in the syringe is pushed by the pump through the

capillary. The solution is electrically charged by the generator and the drop is

stretched into a cone (so-called Taylor cone).

Reneker and Yarin, Polymer 49 (2008) 2387

Initial straight segment

Bending instability

(trajectory becomes chaotic):the oscillation evolves into a spiral,

similar to the movement

of a whip (whipping motion).

NanofibresSolvent


Atomic force microscopy (AFM)

Scanning electron microscopy (SEM) Light microscopy

Electrospun nanofibres


Non-woven structure

Annu. Rev. Mater. Res. 2006.36:333-368


Main industrial technologies for fine-fibres production


Fibrillation

Applicable to man-made cellulose fibres (lyocell, tencel) and few otherpolymer fibres, such as polyacrylonitrile (PAN), depending on the internalstructure of the filament.

Filaments composed of a single component are longitudinally separated in finerfibres (fibrils) usually from 50 to 500 nm by means of high pressure waterstreams, ultrasound or strong mechanical means.

Biomacromolecules, 2008. DOI: 10.1021/bm701157n

J Appl Polym Sci, 2010. DOI: 10.1002/app.30160


Splitting (Island-in-the-sea)

The matrix (sea) is dissolved in solvents or high temperature water leaving the internal fibres (islands).

Controlled diameter and narrow diameter distribution.

bicomponente fibres cross-section (Kuraray)

island

sea

(matrix)


Spinneret design for bi-component filaments by melt-spinning

Fine control of the molten polymers feedingProduction plants are expensive

US Patent 005162074


Modified spunbond for microfibre production by splitting

Spinneret for spinning multi-component filaments

The “sea” melts by hot air leaving “island” fibres

hot air

Non-woven

High productivity


Melt-blown

Conventional melt-blown process produces filaments with diameter from 2 to 10 µm with the shape of non-woven

Molten polymer

Filaments

Non-woven

FinishingRolling press


Melt-blown spinneret design for fine-fibre production

Filaments of ~1 µm in polypropilene using smaller nozzles, higher flux of air,higher temperature and pressure

Drawbacks: fibre with defects (breaks, droplets), wide size distribution,polymer degradation

Advantages: productivity, melt-spinning (no solvents)


PTFE membrane

Gore-tex (polytetrafluoroethylene, PTFE)

Application of a nanofibre-based technology:Membranes have nodes interconnected each otherby fibrils produced by sintering and drawing at hightemperature

Process and material are expensive


Why nanofibres are so interesting?

Nanofibres are ultra-fine fibres

(fibre diameter below 1 micron).

Materials composed of nanofibres have:

� High specific surface;

� Nanofibres are highly interconnected;

� Low pore size;

� High porosity.

WoolCoarse: 30÷100 µmMerino: 12÷25 µm

Silk: ~20 µmCashmere: 10÷18 µmCotton: 11÷15 µmMan-made fibres:

10÷100 µmMicrofibres: 2÷5 µmNanofibres: ~0.03÷1 µm


Specific surfaces (ssp)

36

30


3 grams to reach the Moon

The mass of such a long but very thin nanofibre is very low.

For example, if we take the distance between the Earth and the Moon to be

L = 380 000 km

it takes only x grams of a polymer fibre filament of the polymer density ρ = 1 g/cm3 and fibre diameter d = 2r = 100 nm, where

x = V · ρ = πr2L · ρ = π × (50 nm)2 × (380 000 km) × (1 g/cm3) == π × (5.0 × 10-8 m)2 × (3.8 × 10+8 m) × (1 × 10+6 g/m3) ≈ 3 g!

Some numbers about scientific production


Electrospinning is fast-moving

Currently, electrospinning field has been evaluated as the

fast-moving front in Materials science by Thomson, ISI(Web of Knowledge).Till now, the electrospinning technique has become moreinterdisciplinary, numerous cooperations have beenestablished among the experts in diverse research fields.

Z. Li and C. Wang, One-Dimensional Nanostructures,SpringerBriefs in Materials, DOI: 10.1007/978-3-642-36427-3_1, 2013


Scientific interest in electrospinningP

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Top 10 countries in electrospinning

Source: Web of Knowledge


Criticism

In the 2009 Sander De Vrieze and Karen De Clerck wrote:“The number of publications about electrospinning is exponentially growing.Predictions for this year is that the curve will start to flatten. However, thenumber of publications is still enormously high and comparable to other hottopics in the field of science like stem cells or alternative energy sources.It is only when the content is graded that another remark can be made with allthese electrospinning articles. Most of the publications don’t contribute to thefield of electrospinning. They don’t innovate the field, but repeat knowledgebuild up by other studies. This repetition is caused by the relative simplicity tobuild an electrospinning setup and to electrospin a polymer. The danger in thisphenomenon is that there is no real progress in electrospinning with thesepublications. They reinvent the wheel.”

That’s right: a such huge number of publications makes difficult to collect quickly right information.Moreover, since the quality of some papers is low, data are often conflicting or unclear.

S. De Vrieze, K. De Clerck, 2009. “80 Years of Electrospinning.” In International Conference on Latest Advances

in High-Tech Textiles and Textile-Based Materials, Proceedings. Ed. Paul Kiekens, 60–63. Ghent, Belgium: Ghent University. Department of Textiles.


Industrial interest in electrospinning

Patents: 52.400 results on Google Patents searching “electrospinning” (Sept. 2014)

Companies are involved in electrospinning for:- Production of electrospinning plants;- Production of electrospun materials.

Commercial products in filtration by Finetex, Donaldson Company, Freudenberg, United Air Specialists.

The nanofiber products market was $80.7 million in 2009, according to BCCResearch. Nanofibre-based products market is forecast to reach $2.2 billion intotal revenues by 2020.Mechanical and chemical applications account for more than 70 % of thatmarket right now, but medical applications will have a place in that growingmarket. MedCity News - June 21th, 2011

History of electrospinning

Introduction to electrospinning - CNR-ISMAC Biella 25Available at The New Zealand Institute for Plant & Food Research website


Old patents (1)In the past, “electrospinning” was named “electrostatic spinning”.

The term “electrospinning” was introduced in the early 1990s by Reneker.

Then, the aim was to

produce yarns, threads,

artificial fibres, etc., not

small-sized fibres.

Remember: SEM did not exist!

At the beginning, electro-

spinning is a method for

dispersing fluids.


Old patents (2)

High-tech applications

Non-woven structures


Selected old publications (1)1600 William Gilbert “De magnete magnetcisque corporibus, et de magno magnete tellure” (On the magnet and magnetic

bodies, and on that great magnet the earth)1610 Jean Beguin (1550–1620) “Tyrocinium chymicum” (“Beginners chemistry")1629 Nicholaus Cabæus (1586 –1650) “Phi losophia Magnetica”, Ferrara 1629

1644 Evangelista Torricelli (1608– 1647) “De motu gravium” (On the motion of heavy bodies)1665 Robert Hooke (1635–1703) “Micrographia”

1744 Georg Mathias Bose “Die Electri tat nach ihrer entdeckung und fortgang, mit poetischer feder entworfen” (Electrici ty – itsdiscovery and development, with poetic sketches)

1745 Georg Mathias Bose “Recherches sur la cause et sur la veri table théorie de l ’electricité” (Research on the cause and the true

theory of electricity)1753 Giovanni Battista Beccaria (1716–1781) “D’ell electrismo naturalet artificial”(On naturaland artificialelectricity)

1786 Charles–Augustin Coulomb (1736–1806) Quatrième Mémoire sur l ’Electricité et le Magnétisme, où l’on démontre deuxprincipaux propriétés du fluide électrique” (Fourth Paper on electrici ty and magnetism – where two principles of theelectric fluid are demonstrated) Mem. Acad. Sci., Paris (1786)[1788],pp67–77

1812 Jöns Jakob Berzelius publishes (1779–1848) “Über die Theorie der Chemischen Proprtionen und über die chemischeWirkung der Elektrizität” (On the theory of chemical proportions and on the chemical effects of electricity)

1827 Georg Simon Ohm (1789–1854) “Die Galvanische Kette, mathematisch bearbeitet” (The galvanic ci rcui t investigatedmathematically)

1855 George Audemars. GB Patent 283 “Obtainingand treatingvegetable fibres”

1873 Joseph Antoine Ferdinand Plateau “Statique expérimentale et théorique des liquides soumis aux seules forces moléculai res”(Static experiments and theoretical considerations concerning fluid subject to molecular forces)

1878 Lord Rayleigh “On the instability of jets” Proc. Lond. Math. Soc. 1878 s1–10(1):4–11879 Lord Rayleigh “The influence ofelectricity on colliding water drops” Proc. RoyalSoc. Lond., Vol. 28 (1878–1879) pp405–4091879 Lord Rayleigh “On the capillary phenomena of jets” Proc. Royal Soc. Lond., Vol. 29 (1879) pp71–97

1882 Lord Rayleigh “On the equilibrium of liquid conducting masses charged with electrici ty” The London, Edinburgh and DublinPhi losophicalMagazine and Journal of Science, Taylor and Francis, London 1882, pp184–186


Selected old publications (2)1887 Boys , C.V, “On the Production, Properties , and some suggested Uses of the Finest Threads”. Proceedings of the Physical

Society of London. 1887, 9, 8.1890 D. Tommasi “Traité theorique et pratique d’electrochemie” (A theoreticaland practical treatise on electrochemistry”)1907 J. Zeleny “The discharge of electrici ty from pointed conductors differing in size” The Physical Review, Vol . XXV, No. 5, pp305–

3331912 E.F Burton and W.B Wiegand “Effect of electricity on streams of water drops” Philosophical magazine, XII, series 6, Vol . 23,

i ssue 133, 1912, pp148–1651914 J. Zeleny “The electrical discharge from liquid points, and a hydrostatic method of measuring the electric intensity at their

surfaces” The PhysicalReview, Second Series, Vol. I II, No. 2, February 1914, pp69–90

1914 J. Zeleny “On the conditions of instability of electri fied drops , with applications to the electrical discharge from liquidpoints” Proc. Cambridge Philosophical Soc.: mathematical and physical sciences, 1914, pp71–83

1917 J. Zeleny “Instability of electrified liquid surfaces” The Physical Review, Second Series, Vol. X, No. 1, July 1917, pp1–61920 J. Zeleny, “ElectricalDischargesfrom Pointed Conductors” Phys. Rev 16 (2) p 1021931 W.A Macky “Some investigations on the deformation and breaking of water drops in strong electric fields” Proc. R. Soc.

Lond. A. 1931, 133, pp565–5871936 L. Onsager “Electric moments of molecules in liquids” J. Am. Chem. Soc. 58, 1486 (1936)

1948 W.N Engl ish “Corona from a water drop” The Physical Review, Vol. 74, No. 2, 1948, pp179–1891952 Vonnegut, B. and Neubauer, R. L. “Production of monodisperse liquid particles by electrical atomization”. Journal of Colloid

Science. 1952, 7, 616–22

1955 V.G. Drozin “The electrical dispersion of liquids as aerosols” J. Col loid Sci., 10 (1955) pp158–1641959 J.D. Teja Patent US 2908545 “Spinning nonfused glass fibres from an aqueous dispersion”

1959 Richard P. Feynman “There's Plenty of Room at the Bottom – An Invi tation to Enter a New Field of Physics” talk at the annualmeetingof the American Physical Society at the California Institute of Technology (Caltech)

1964 A. Doyle, D. Read Moffett, B. Vonnegut “Behavior of evaporating electrically charged objects” ” J. Colloid Sci ., 19 (1964)

pp136–143


Selected old publications (3)1964 G.I Taylor “Disintegration of waterdrops in an electric field” Proc. Roy. Soc. Lond. Series A: Mathematical and Physical

Sciences, 1964, 280, 1382, 3831966 G. I Taylor (with an appendix by M.D. Van Dyke) “The force exerted by an electric field on a long cylindrical conductor”

Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences , Volume 291, Issue 1425, pp.

145–158, 1966.1969 G.I Taylor “Electrically driven jets” Proc. Roy. Soc. Lond. Series A: Mathematical and Physical Sciences, 1969, 313, 1515, 453

1969 J.R Melcher and G.I Taylor “Electrohydrodynamics : a review of the role of interfacial shear stresses” Annual Review of FluidMechanics, 1969, 1, 1, 111

1981 L. Larrondo, R. St. John Manley “Electrostatic fiber spinning from polymer melts. I . Experimental observations on fiber

formation and properties” J. polymer science: polymer physics edition, vol. 19, pp909–920 (1981)1981 L. Larrondo, R. St. John Manley “Electrostatic fiber spinning from polymer melts . II . Examination of the flow field in an

electrically driven jet” J. polymer science: polymer physics edition, vol. 19, pp921–932 (1981)1981 L. Larrondo, R. St. John Manley “Electrostatic fiber spinning from polymer melts . III . Electrostatic deformation of a pendant

drop of polymer melt” J. polymer science: polymer physics edition, vol. 19, pp933–940 (1981)

1995 J. Doshi, D.H Reneker “Electrospinning processand application of electrospun fibers” J. Electrostatics 35(1995) pp151–160

Taylor cone

Rayleigh break-up instability


Electrospinners of note

First industrial application of electrospinning (secret outside the Soviet Union).

This information became public after his death [Lushnikov A. Obituaries: Igor Vasilievich

Petryanov-Sokolov (1907-1996), J. Aerosol Sci 28 (1997) 545-546].

Such research continues at the Aerosol Department of Karpov Institute of Physical

Chemistry where it is now published openly.


Recent milestones

2001 Fabrication of inorganic fibers by electrospinning [Wang Y, et al. “ElectrostaticSynthesis and Characterization of Pb(ZrxTi1-x)O3 Micro/nano-fibers”. 2001 E-MRS

Fall Meeting.]

2001 Fabrication of nanofibres with porous surface [Bognitzki M, et al. Adv. Mater. 13

(2001) 70.]

2003 Multi-nozzle electrospinning (modern conception) [Gupta P and Wilkes GL.Polymer 44 (2003), 6353.]

2003 Core-shell electrospinning process [Sun Z, et al. Adv. Mater. 15 (2003) 1929.]

2005 Electrospun continuous yarn [Smit E, et al. Polymer 46 (2005) 2419.]

2005 Needle-less electrospinning (modern conception) [Jirsak O, et al. PatentWO2005024101, 2005.]


Conclusion for the history of electrospinning

The history of electrospinning is mainly characterised by bad timing. The technique becameonly interesting in the same time period as the invention of the polyamides. After theSecond World War, electrospinning was further developed in the Cold War secretlaboratories. This resulted in an industrialization in the USSR.There was no clear development in the Western world during the Cold War in the field ofelectrospinning. There is of course the theoretical work of Taylor in the years ’60s that ledto a mathematical model of electrospinning. The characteristic droplet shape of a fluiddroplet in an electric field is still known as the “Taylor cone”.But there was no clear research going on in the Western world until the ‘90s of last century.Clearly, the non-governmental driven capitalistic industry didn’t put their effort in the fieldof electrospinning. There is one exception though, that is the Donaldson company in theUS. They claim that they developed their own electrospinning equipment in the ‘70s of thelast century [probably in connection with the US Army Research Office]. With their process,they make enhanced air filters for the automotive industry.It is however difficult to really know what was already done in the past by secretlaboratories in the USSR (or even in the US). So, perhaps there will be some rediscoveriesdone by modern science. These rediscoveries will however result in common and not insecret knowledge.

S. De Vrieze, K. De Clerck, 2009. “80 Years of Electrospinning”

Applications


Nanofibres applications

Electrospinning

Nanofibres

Acoustic insulationAntimicrobial

Carbon fibres

CatalysisComposites

ElectronicsSensorsOptoelectronics

Energy generation and

storageEnzymes

Filtration and

protective clothingGarments

MedicineBone reconstructionCel l growth/Scaffolding

Drug releaseVascular/Tubular devicesWound dressing

Metal oxides fibres

Photo-catalysis

Main patent applications are related to Medicine (>60% patents) and in Filtration (~30% patents).


Number of publications per applications

J Mater Sci (2014) 49:6143–6159


Biomedical applications


Other applications (polymers)


Expertises involved:• Physics:

� Electrohydrodynamics: motion of fluids in a electric field;

� Rheology: study of fluids under mechanical stresses.• Chemistry:

� Polymer/solvent systems;� Materials (synthesis, modification, nanoparticles, sol-gel, …);

� Characterization.

• Mathematics:

� Modelling and prediction (e.g. DOE is widely used).

• Engineering:� Design of plants (spinning heads, collectors, electric fields, …);

� Feeding solution/Pumping systems;

� High voltage generators;� Processing control.

• Other, depending on the application: Medicine, Biology, …

Polymer solution + High voltage = Nanofibres (simple!)

Complicated? Complex!Electrospinning is not complicated, but it is complex.Electrospinning is multi-disciplinary.


Productivity

If we take a 10 wt% polymer solution, deliver it through a spinneret at afeeding rate of q = 1 ml h−1, collect the nanofibers as a nonwoven mat with afiber diameter of d = 500 nm (which is fairly large by electrospun fiberdiameter standards), and assume a polymer density and solution density of ρ =1 g cm−3, the jet stream velocity v, in the absence of bifurcation, is:

v ≈ q / π (d/2)2 = (1 × 10 wt%) / (19.6 × 10−10) cm h−1 ≈ 500 km h−1

(around the speed of sound)

Under normal operating conditions, it is unlikely that one can exceed this fibre-spinning velocity by a large margin. Yet, with this production rate, the net fiberformation rate is 0.1 g h−1. So, even if we were to operate the process 24 hoursper day, each spinneret would produce fewer than 2.4 grams of fibres per day.


Productivity is the bottleneckDespite the several advantages and wide potential applicationsoffered by electrospinning, the throughput of nanofibers has beena serious bottleneck problem that limits their real application.

Increase the production rate of electrospun fibres is challenging.

Some issues:• Process stability/Reproducibility• Jet-jet interactions/Mutual repulsion• High voltages (safety)• Solvents (environment)

However, industrial applications exist and large-scale plants arecommercially available, but not all problems are solved.


Commercially available nanofibre mass production plants

Sci . Technol. Adv. Mater. 12 (2011) 013002


Some commercially available electrospinning setups

Nanon by Mecc, Co. (Japan)

Rotating drum collector

Yflow Nanotechnologies (Spain)

Multi-nozzle

Nano-I by Toptec (South Korea)

Deposition on moving substrate

Nanospider by Elmarco, sro (Czech Rep.)

Rotating drum jet source (needle-less)

Spinbow (Italy)

Multi-nozzle + Rotating drum collector

4spin (Czech Rep.)

Multi-jet from orifices


Cost-effectiveness

It is difficult to compare productivities of nanofibre mass-production plantsbased on official information. The size of equipment, particularly the spinneretand the spinning volume per time, and the unit cost of samples are necessaryto evaluate the productivity; unfortunately, this information is unavailable.For example, the productivity of needle-nozzle spinning increases with thenumber of needle-nozzles, but so does the space occupied by the nozzles.When determining the cost-effectiveness of electrospinning, it is insufficient toexamine only at the fibre production rate. The setup requirements forelectrospinning and conventional spinning, such as dry spinning and meltspinning are very different. In dry spinning or melt spinning, each spinningchamber can be 6–10 m long, as this length is required to stretch the fiber;however, an electrospinning chamber can be as short as 10 cm.



Basic economic problem

productivity

pro

du

ct c

ost

e.g. high-tech medical applications

e.g. conventional filtration

pro

du

ctiv

ity

lim

it

product cost limit

market cost

gap


Safety

In addition to productivity, the cost of supplying more charges and the risk of

electric discharge are higher for spinning at a high voltage.In terms of the hazards of mass-production electrospinning, the rapid build-upof inflammable solvent vapour in the environment poses a fire hazard andmay affect fibre drying.The chamber must be properly ventilated without disturbing theelectrospinning process. Scrubbing and recycling of the (inflammable or toxic)solvent in solution-based electrospinning should also be considered forenvironmental safety. Such additional equipments can have a cost

comparable to the cost of the electrospinning equipment.


Industrialisation of electrospinning

Despite the huge literature there are some technical issues inelectrospinning that were almost neglected till now:• Efficient spinning head design, including the feeding solution system;• Processing control strategies;• Operative parameters of large-scale electrospinning related to theresulting materials;• Risk management.

Requirements:• Flowing to the electrospinning head;• Complete modeling of the fibre-forming process at large-scale;• Relations between parameters and products.

Fibre-forming processes


Fibres by Nature

Zhang and He, Textile Research Journal 2009 79: 243


Conventional spinning

By temperature:

• melt-spinning, solid filaments by cooling

From solution (in a solvent or a solvent system):

• wet-spinning, solid filaments by removal of the solvent by means of another liquid (e.g. water)• dry-spinning, solid filaments by evaporation of the solvent


Extrusion

Polymer fluid (melt or dissolved) athigh pressure passes through a metalholed plate (spinneret).


Mechanical strain

Drawing between two godets will control the filament diameter (filament must be deformable)

Godet 1(low speed)

Godet 2(high speed) Size reduction


Drawing in electrospinning

Electrospinning (electrostatic spinning) is a physical phenomenon that can be includedwithin the electro-hydrodynamics (dynamic of electrically charged liquids).

The process is based on theelectrostatic repulsion of charges

ruled by the Coulomb law.

An electrospinning apparatus can generate (an) electrically-driven polymer fluid streams

(molten or dissolved in a solvent) by means of an electrostatic field.

The electrically-driven polymer fluid stream is usually simply called jet.

Each jet, ejected from the source, solidifies (by cooling or solvent evaporation) producing

an infinite solid filament with a diameter generally lower than 1 micron (nanofibre).

Electrospinning is a spontaneous process.

Nanofibre production from an electrically-driven polymer fluid stream is a self-induced

process.

ε is electric permettivity


Electrostatic forces

Electrospinning involves the ejection of a viscous solution or a melt jet from anelectrically-charged drop. Subsequent drawing and solidification of the jet leadto the formation of a uniform, thin fibre. Unlike the conventional spinningprocesses, the driving forces for electrospinning are based on electrostatic

interactions, instead of mechanical stretching or pulling.

The polymeric fluid jet contains electrical charges inside. The charges repeleach other by Coulomb forces. The forces are homogeneous and strong. Theseelectrostatic forces are generated internally to the jet. Therefore, the jet

elongates it-self.

The most remarkable feature of electrospinning is that this technique iscapable of spinning fibres with diameters down to tens of nanometers.


Basic elements and its functions

1. Fluid supply

2. Electric field

3. Collector(s)

• Feeds the polymer fluid within the electric field

• Induces charges to the fluid• Produces a jet• Elongates the jet• Rules the trajectory of the jet

(secondary electrode)

• Collects the nanofibres• Can induce nanofibre orientation

(shape, movement, conductivity)


Process variants overview• Liquid state using:

– Temperature = molten polymer (thermoplastic)

– Solvent = dissolution (polymer solubility)

• Jetting (production of jet):– Nozzle/Needle/Capillary/Orifice = jet from a liquid droplet

(usually a single jet, i.e. one nozzle = one jet)

– Needle-less = jet(s) from a free liquid surface

(usually production of several jets simultaneously)

• Collector (nanofibre collection) = nanofibre textures:– Randomly oriented

– Alligned

– Other: looping, single wire, 3D-shaped (e.g. tube), etc.


Solvent vs. Temperature

advantages disadvantages

� by solution diameter <1 µm solventsimple equipment

flexible (broad range of materials)

� melt no solvent complex (additional equipments)polymers with low melting pointdiameter >1 µm


Nozzle/Needle vs. Needle-less

NanospiderTM (Elmarco s.r.o.) Donaldson Company Inc.

Rotor with several nozzles

advantages disadvantages

� nozzle reduced solvent evaporation complex design for scale upnot uniform deposition

� needle-less simple design for scale up huge solvent evaporationmaterials wasteonly stable solutionnarrow range of materials


Collector

The collector is the element on which the solidified jet (nanofibre) lands up.It is usually ground (electrical circuite close).

Shape, motion, conductivity, charge, etc. of the collector can influence the spatial deposition of the electrospun nanofibres

Basic apparatusfor solution electrospinning


First electrospinning plant at CNR-ISMAC

High-voltage generator: +/-30 kV

Pump: 0.1-0.001 ml/h

Capillary: 0.20-0.65 mm Ø

Collettor: 5.5 cm Ø

polymer capillary

generator

collector

syringe

Scheme


Pump and syringe

Rotating metallic collector

Metal needle


Charged needle

Collector

Electric field

Electrospinning process (1/2)

Jet:- Rectilinear

pathway- Solvent

evaporation

starts

Capillarytip

Onset of bending instability- Whipping motion (looping chaotic

trajectory)- Complete solvent evaporation

Surface tension

Elettrostatic force Jet: evident

reduction of the

diameter

Taylorcone

When the electrostatic force

overcomes the surface tensionthe drop at the tip deforms into acone and an electrically charged

jet of fluid is ejected from theapex of the cone.


Collector

Electrospinning process (2/2)non-woven structure whipping motion

Nanofibre handling


Typical nanofibre morphologies

Regular Beads

BranchingRibbons

Fibrils

T-shaped

Rayleigh and

axisymmetric

instabilities

Electric effectsEvaporation of

the core solution

through a

solidified shell

Flat fibres

Small droplets


Electrospinning parameters

• Solution (polymer/solvent system) properties

• Processing parameters

• Ambient conditions

Electrospinning parameters can be devided in three

classes:


Solution properties

Standard unit Non-standard unit

• (Zero-shear) viscosity [Pa s] [cPoise]• Rheological properties

– Shear behaviour

– Storage (G‘) and loss (G") moduli [mPa]

– Stress relaxation time [ms]

• Polymer concentration [% w/w] or [% w/v]• Molecular weight of polymer [kDa] or [g/mol]• Molecular weight distribution• Topology of polymer chain• Surface tension [mN/m] [dyn/cm]

• Electrical conductivity [mS/cm]• Dielectric constant [-]• Heat of vaporisation of solvent [mJ/g] [cal/g]• Vapour pressure of solvent [mPa]• Boiling point of solvent [°C]

• Mechanical properties of polymer– Young modulus [MPa] or [N/mm2]

– Bending moment/length [Nm/m]


Processing parameters


• Applied voltage [kV]• Polarity• Distance needle-to-collector [cm]• Volume feed rate [ml/h]• Internal nozzle diameter [mm]• External nozzle diameter [mm] [Gauge number]• Nozzle length [mm]• Nozzle shape• Collector size [cm × cm] or [cm Ø] …• Collector shape• Collector speed [m/s] or [rad/s] [rpm]


Ambient conditions


• Temperature [°C]

• Relative humidity [%]

• Atmospheric pressure [kPa] [atm] or [mmHg]


Parameters affect morphology (same examples)

Solution parameters

↑ Viscosity: Low-beads generation, high-increase in fibre diameter, disappearance of beads.

↑ Polymer concentration: Increase in fibre diameter.

↑ Molecular weight of polymer: Reduction in the number of beads and droplets.↑ Conductivity: Decrease in fibre diameter.

↑ Surface tension: No conclusive relation with fibre morphology, high surface tension results in instability of jet.

Processing parameters

↑ Applied voltage: Decrease in fibre diameter with increase in voltage.― Distance between tip and collector: Generation of beads with too small and too large distance, minimum

distance required for uniform fibres.

― Flow rate: Decrease in fibre diameter with decrease in flow rate, generation of beads with too high flow rate.

Ambient conditions

↑ Temperature: Decrease in fibre diameter.

↑ Relative humidity: Circular pores on the fibre surface, instability of the jet.


Processing mapA “processing map” can be obtained basedon a systematic parameter study.

Understanding of the concept how each ofprocessing parameter affect the morphology

(fibre diameter, for instance) of theelectrospun nanofibers is essential. All theparameters have been divided into twogroups:

(a) parameters which affect an electricalforce during electrospinning, i.e. jet

elongation/an electrical force (affected byelectrical conductivity of solution, appliedvoltage, for example)

(b) parameters which affect the mass ofpolymer fed out from a tip of needle, i.e.mass of the jet (affected by polymerconcentration, applied voltage, volume feed

rate, for example).

Tan, et a l. Polymer 46 (2005) 6128


Applied voltage

The role of the applied voltage is not completlyclear. It probably depends on the polymersolution, and it could be related to other

processing parameters and ambient conditions.

Generally, as the voltage increases the fibre

diameter decreases. However, rarely an increasein the voltage results in an increase of the fibre

diameter.

Applying a high voltage to the tip generates anelectrostatic field. The field is extremely strongclose to the tip, and weakens far from the tip.

On the left, a simulation results with 30 kVapplied voltage. At 6 cm from the tip theelectrostatic force reduces by 2 order ofmagnitude compared to the force acting at 0.5

cm.

The voltage is often confused with the electric field,

considered as the ratio voltage/tip-to-collector distance

(kV/cm). This is not right because the electric field

could be not uniform.


Critical voltage

In order to produce an accelerated jet inelectrospinning, electrostatic forcesmust overcome other forces, mainlysurface tension, but also viscoelasticstresses and inertia.

At some critical value of the appliedvoltage, proposed by Taylor, theelectrostatic force surpasses the surfacetension and results in jetting from thepolymer drop (i.e. Taylor cone).

Journal of Nanomaterials, 2011. DOI: 10.1155/2011/317673


Flow rate and voltage (1)

Fixed flow-rate

Voltage

Capillary

Jet

Optimal condition

Flow rate and voltage are connected and process stability

greatly depends on their relation.

directly from the nozzle

the Taylor conea dropThe jet emanates from:


Flow rate and voltage (2)

Fixed voltage

Flow-rate

Capillary

Jet

Optimal condition

directly from the nozzle

the Taylor conea dropThe jet emanates from:


Process stability

stable

Mass balance at the cone:

Mass per second fed to the tip (flow rate × solution density) must be egual to the mass

ejected from the cone, that depends on the voltage. Solvent evaporation from the cone

should be negligible.

instable

Capillary will emply out (there is not sufficient fluid for the formation

of the Taylor cone)

Clogging is probable

instable

Dripping/Spraying Beaded fibres


Viscosity

Beads

Bead-free fibres

Beaded fibres

Viscosity mainly depends on concentration and molecular weight of the polymer


Polymer concentration

A

B

CDEFGH

Polymer concentration affects the nanofibresize, other than viscosity and related effects.

Nanofibre diameter increases as polymerconcentration increase


Molecular weight of the polymer

Beaded nanofibres

Beads

Molecular weight increase

Bead-free nanofibres

Ribbons


Electrospinning direction

Gravity seems to be negligible compared to electrostatic force.The weight force of 1 cm of a 220-nm sized nanofibre is ~1.6 × 10-10 N

(considering a polymer density of 1.1 g/cm3).Electristatic force acting on it is ~8 × 10-7 N at the collector.Electrostatic force is 3 order of magnitude higher than gravity.

Calculation based on experimental data from Kalayci et al., Polymer 46, 2005.

From top to bottom From left to right From bottom to top

Setups used in order to avoid dripping of solution on the sample

Examples of electrospun materials and applications


Different application,different morphology

Different fibre morphologies: (a) beaded; (b)

smooth; (c) core-shell; and (d) porous fibres.

Beaded and core-shell nanofibres can alsobe used to encapsulate therapeutics for

drug delivery applications.

The ability to form porous fibres through

electrospinning means that the surfacearea of the fibre mesh can be increased

tremendously.


Different application,different material

Almost any soluble polymer with sufficiently high molecular weight can be

electrospun.

Nanofibres made of natural polymers, polymer blends, nanoparticle- or drug-

impregnated polymers, and ceramic precursors have been successfully produced.


Medical applicationsSince stretching of the solution arises from repulsivecharges, the electrospinning jet path is very chaotic andonly non-woven (random) structures are produced using a

typical setup.Nevertheless, more ordered assemblies that allow theporosity of the mesh to be controlled have been producedthrough clever manipulation of the setup.Several methods have been developed that yield aligned

fibres with various degrees of order and fiber directionsfor two- and three-dimensional assemblies (e.g. tubes).Such assemblies are usually achieved through control ofthe electric field between the tip of the spinneret and thecollector, or use a rotating mandrel at a speed of >1000

rpm to collect aligned fibres.To fabricate a tubular scaffold, electrospun fibres can bedeposited on a rotating mandrel and the deposited fibrelayer subsequently extracted from the tube.One of the aims of the research applied to medicine is to

fabricate electrospun polymer nanofibre scaffolds forblood vessels, nerves, ligaments, skin, cartilage, and bone

reconstruction.


Future perspectives in nanomedicine by electrospinning

The ECM-like (Extra-Cellular Matrix) properties of the nanofibre scaffolds mimic thenatural tissue environment.

Further research is required to elucidate the influence of nanofibres on the

biochemical pathways and cellular signaling mechanisms that regulate:• Cell morphology;

• Growth and proliferation;• Differentiation;

• Motility;

• Genotyping.

Insight into how natural ECM components secreted by cells replace the biodegradablepolymeric scaffolds is also needed. This complete understanding of cell-nanofibre

interactions will pave the way for successful engineering of various tissues and organs.

A higher polymer degradation rate in the presence of degradative enzymes secreted by

the cells, which could be desirable for applications like wound dressing, where aninitially higher but sustained release of antibiotics is preferred.


Electrospinning forpharma and cosmetics

In the pharmaceutical and cosmetic industry, nanofibres are promising toolsfor controlled delivery of drugs, therapeutics, molecular medicines, and body-care supplements.In cosmetics, nanofibre masks and patches can be impregnated with skin-revitalizing factors for skin health and renewal, other than used in wads for

skin cleansing.


Portable electrospinning

Researchers have developed aportable electrospinning device

able to electrospun nanofibres

for the directly treating wound

by the deposition of functional

nanofibres containg drugs.Moreover, the nanofibre layer

cound act as barrier against

bacteria.This device could have

application also in cosmetics, forlocal treatment of skin by

enzymes.

Chem. Unserer Zeit, 2005, 39, 26–35. DOI: 10.1002/ciuz.200400321


Environmental engineering applications: mechanical separation

High porosity, interconnectivity, microscale interstitial space, and a large surface-to-volume ratio

mean that non-woven electrospun nanofibre meshes are an excellent material for filtration andmembrane preparation, especially in environmental engineering applications.

(a) surface; (b) cross-section; and (c) magnified cross-section pictures

Electrospun nanofibers can form an effective filter medium for particulate removal fromwastewater, gas stream and air. The nanofibre filter media show an extremely effective removal

of airborne particles by both physical trapping and adsorption. No particles were trapped in the

membrane, so the membrane could be effectively recovered after cleaning.This opens up new avenues of application of electrospun membranes such as for the pretreatment

of water prior to reverse osmosis, for the air cleaning in open-cycle gas turbine and in air

conditioning systems, including High Volume Air Conditioning (HVAC).


Environmental engineering applications: chemical separation

Water pollution is now becoming a critical global issue. One important class of inorganic pollutantof great physiological significance is heavy metals, e.g. copper, mercury, lead, cromium andcadmium. The distribution of these metals in the environment is mainly attributed to the release of

metal-containing wastewaters from industries, but also from high-tech devices improperlydismissed.It is impossible to eliminate some classes of environmental contaminants completely, such asmetals, by conventional water purification methods. Affinity membranes will play a critical role inwastewater treatment to remove (or recycle) heavy metals ions in the future.

We are working on the production of protein-based nanofibres that are a reactive nanomaterialthat can attract toxic heavy metal ions, such as cromium and copper, by adsorption and

chemisorption and electrostatic attraction mechanisms.

Problems of air pollution are not only due to the particulate, but also by the presence of VolatileOrganic Compounds (VOCs), i.e. chemicals or solvents. Adsorption of VOCs on porous substrates isthe most cost-effective way to remove it.

Nanofibres can be used for this purpose. In particular, our protein-based nanofibres are able to

chemically bind formaldehyde, a toxic VOC used for instance as binder for furniture, in synthesis ofresins, and for anti-crease treatments on textiles. Formaldehyde can be release over time.


Sensor applications

Electrospun nanofibres have also received great attention for sensor applications because of theirunique high surface area. This is one of the most desirable properties for improving the sensitivity

of conductometric sensors because a larger surface area will absorb more of a gas analyte and

change the sensor conductivity more significantly.Nanofibres functionalized with or composed of a semi-conductor metal oxide show an electricalresistance that is sensitive to harmful chemical gases.

Chem. Unserer Zeit, 2005, 39, 26–35. DOI: 10.1002/ciuz.200400321

Titanium dioxide (a, b) and chrome (c) hollow nanofibres for gas sensoringThin core-shell

nanofibres


Ceramic nanofibres

Nano. Lett., 3, 555–60. 2003

An example describing the typical procedure for preparing ceramic nanofibresby electrospinning.

The SEM image in the middle shows TiO2 nanofibers electrospun from anethanol solution that contained titanium tetraisopropoxide (precursor, 0.1g/mL) and poly(vinyl pyrrolidone) (PVP, 0.03 g/mL). The as-spun fibers weremade of a composite of amorphous titania and PVP.After calcination at 500°C in air, PVP was removed and amorphous titania wasconverted into anatase (ceramic nanofibres).


Yarns productionElectrospinning has been considered as a fibre and yarn fabrication

method since as early as the 1930s. However, probably owing to thehigher production rate, other fabrication methods such as dry, wet and

melt spinning have dominated the industry.Nevertheless, the interest in nanotechnology has led researchers todevelop continuous electrospun nanofibrous yarns that aresufficiently strong to be handled. Many different setups weredeveloped to produce yarns by electrospinning. At the moment it is

not clear the advantages in producing dernier filaments composedfrom nanofibres compared to conventional (bulk) filaments.



Breathable garments and protective clothing

Fabrics for protecting clothing applications are usually laminated together fromdifferent fibrous layers sharing many specific tasks such as mechanical support,

resistance against convective air flow and moisture transport, and adsorption layers to

trap airborne particles, including micro-organisms.Electrospun nanofibrous layers of elastomeric nanofibres can combine some required

flexibility in shape with sufficient tensile strength. The electrospun layers are, ofcourse, also responsible for the filtering properties of the protective laminated fabrics.

They also require low impedance to water-vapour diffusion (breathability); this is a

significant improvement compared with protective clothing containing a layer ofactivated carbon, which has poor water-vapour permeability.

Nanofibres have been already successfully applied as PPE (Personal Protection

Equipment) as gas masks in Soviet Union thanks to Petryanov-Sokolov. In this field, the

most challenging applications for nanofibres are (1) in enhancing NBC (Nuclear

Biological Chemical) suits to be able to trap and even destroy almost all kinds ofairborne hazardous materials, particles or substances, and (2) in replacing PTFE

membranes in water-proof brethable garments in sportswear.

Phases of electrospinning


Citation

“It can scarcely be denied that thesupreme goal of all theory is to make theirreducible basic elements as simple and asfew as possible without having to surrenderthe adequate representation of a singledatum of experience.”

Albert Einstein

“On the Method of Theoretical Physics”, The Herbert Spencer Lecture, Oxford (June 10, 1933).


Studies on the electrified jet (1)

Journal of Materials Processing Technology 209 (2009) 3156–3165

Taylor initiated the first detailed mathematical study on the subject of electrified fluid jetin 1960s when he introduced the “Leaky Dielectric Model”. This model suggests that

most of the charges for this class of dielectric accumulate only on the surface and not in

the bulk fluid. Consequently, these fluids contain a nonzero electrical field tangent to theinterface of the fluids. This nonzero electrical field causes a nonzero tangential stress on

the interface that is balanced by the tangential viscous force of the fluid. Under theseconditions the fluid will be in dynamic equilibrium. This model has been successfully

used to compare the experimental results of neutrally pendant drops elongated by an

electric field. The resulting fluid shape is the well known “Taylor cone.”Based on Taylor’s work, Saville made a detailed discussion and derivation of the

assumptions for the Taylor’s leaky dielectric model. In the seventies, he developed alinear stability model of an uncharged jet under the electrical field. His qualitative

analysis on the characteristics of electrospinning is consistent with the experiments. He

identified the presence of experimentally observed axisymmetric and oscillatory

“whipping” (bending) instabilities of the centerline of the electrospinning jet.


Studies on the electrified jet (2)

In subsequent research in electrospinning during the nineties, Reneker et al. (2000) andFong et al. (1999) studied bending instability of the electrospinning process. They

further identified the influence of solution properties on the formation of electrically

charged jets. These include viscosity, surface tension and conductivity of the fluid.Rutledge et al. developed a “whipping model” for the electrospinning process that

mathematically describes the interaction between the electric field and fluid properties.They used this model to predict “terminal” jet diameter. The mathematically derived

limiting diameter has been equated with experimentally measured fiber diameter. The

key assumptions in the derivation of this “terminal” diameter include uniform electricfield, no phase change and no inelastic stretching of the jet. Their model is qualitatively

valid for selected polymeric solutions including polyethylene oxide (PEO) andpolycaprolactone (PCL) in low concentrations. Feng eliminated the “ballooning”

instability of Rutledge model by including the effect of non-Newtonian extensional

viscosity in electrospinning.


Phases of electrospinning✔ Taylor cone generation and emission of the jet

✔ Rectilinear trajectory

✔ Whipping motion

✔ Nanofibre collection

The travel of the jetStretching and solidification

About 1700 results on “electrospinning” at YouTube.com

Some are really amazing to watch the process.

https://www.youtube.com/watch?v=MwniZlsLJlY https://www.youtube.com/watch?v=AS6SHzeSgB4

https://www.youtube.com/watch?v=Hr9qWgFmii0

https://www.youtube.com/watch?v=YZx1mu7kpSk

https://www.youtube.com/watch?v=OitaLc3E534

Theories of defects generation(beads, branching, buckling)

(a) Taylor cone

(b) Whipping (or bending instability)

Taylor cone generation and emission of the jet


Charging a fluid

Generation of charges on the fluid usually occurs by virtue of high electric

field polarization between the positive and negative potentials, referred to asinduction charging. At this time, free electrons, ions or ion pairs will be

generated as charge carriers in the fluid and form double layer in the fluidowing to the ion mobility. In the absence of flow, the double layer thickness isdetermined by the ion mobility within the fluid.In the presence of flow, ions may be convected away from the electrode andthe double layer continually replenished. Inductive charging is generallysuitable for fluids with the conductivities of the order of 0.1 μS/cm (or above).

Li and Wang, SpringerBriefs in Materials, DOI: 10.1007/978-3-642-36427-3_1, 2013


Taylor cone theory

In 1964, Taylor established the Taylor cone theory todescribe the deformation of small-volume liquid underthe high electric field:1. As a small volume of electrically conductive liquid is

exposed to an electric field, stable shape can be

acquired owing to the equilibrium of the electric forces

and the surface tension in the cases of inviscid,Newtonian, and viscoelastic liquids (if the potential isnot too large).2. As the voltage is increased to the critical potential

and any further increase will destroy the equilibrium,

thus the liquid body acquires a conical shape, with a

half angle of 49.3° (a whole angle of 98.6°), referred toas the Taylor Cone.

Li and Wang, SpringerBriefs in Materials, DOI: 10.1007/978-3-642-36427-3_1, 2013


Critical voltage

Additionally, Taylor also demonstrated that the shape of such a coneapproached the theoretical shape just before jet formation within theelectrospinning (electrospraying) process. Taylor’s derivation is based on twoassumptions:1. The surface of the cone is an equipotential surface;2. The cone exists in steady state equilibrium.

Namely, exists a critical voltage Uc given by:

where H is the distance between the tip and the collector, L is the length ofthe tip, R is the diameter of the tip, and γ is the surface tension of solution.


Improved the theoryIn 2001, Yarin and Reneker modified the Taylor cone theory based on theexperimental data. They claimed that the Taylor cone corresponded essentiallyto a specific self-similar solution, whereas there existed nonself-similarsolutions which did not tend toward a Taylor cone. Namely, there existedanother shape, which was not self-similar.They found that the assumption (1) within the previous Taylor cone theory wasuncorrected. The theory predicted that, as the potential increases andapproaches the critical value, the stable shape becomes less and less prolate.However, in the experiment an increase in potential resulted in more prolatedroplets.As a result, when a liquid surface develops a critical shape, its configurationapproaches the shape of a cone with a half angle of 33.5°, rather than a Taylorcone of 49.3°. The critical half angle does not depend on fluid properties forNewtonian fluids, since an increase in surface tension is always accompanied byan increase in the critical electric field. However, the sharpness of the criticalhyperboloid depends on elastic forces and surface tension in elastic fluids or inunrelaxed viscoelastic fluids.

J Appl Phys 90(9):4836–4846


Explanation on Taylor cone onset

capillay

fluid+

+ ++

+

++ +

+

electric field

electrostatic force

+

+

+

+

+

++

Sum of the components

Maximum force is at the apex of the cone

Spherical drop at the tip of the capillary

Production of Taylor cone as the voltage raises and the fluid charges.


Experimental observation

Fast process

Still round



Taylor cone formation is complete



Jet is produced in about 0.1 ms



Taylor cone relaxation


Cone relaxation

The jet is charged too.It produces a repulsive

electrostatic force on

the cone.

The shape of the cone depends also by the force of gravity.

The jet is accelerating.The jet has a mass (even

small).

So, by action and reactionprinciple it produces a force

toward the cone:

F = mjet· a

+

+

+

+

+

+

+

++acceleration

Drop weight


Flowing inside the coneAn axisymmetric circulation pattern, with the maximum flow rate directedtoward the apex on the surface of the Taylor cone and backflow away on theapex at the centerline of the cone, has been revealed using steakphotography of tracer particles. The origine of the axisymmetric circulationwas attributed also to the presence of tangential electrical shear stresses atthe surface of the cone.

Melcher and Taylor. Annu Rev Fluid Mech 1, 1969, 111-146


Taylor cone profiles (experimental)PEO-water, Flow rate: 0.1 ml/min

Polymer 42 (2001) 9955

E∞= V/d = 0.47 0.53 0.6 1 kV/cmPoor conditions:

The Taylor cone is not longer observed. The jet is ejected

directly from the nozzle. Due to the high voltage, the jet

could convey an amount of solution higher that the flowrate. So at the tip, there is not enough solution for the

formation of the Taylor cone.

Fairly good conditionsGood conditions


Taylor cone profiles (experimental)

Figure delineates the five distinct Taylor cone regimes identified

for a flow rate, shown with the corresponding Taylor cone

images as a function of the applied voltage.

When the applied voltage is low, the electric force cannotovercome the surface tension of the fluid. No jet is ejected and

the fluid builds up at the needle and falls in drops (regime 1).

When the voltage is increased above a critical voltage, a liquid

jet emerges. However, due to the low electric field strength and

charge density, the rate at which the electric force removes fluidfrom the Taylor cone, cannot match the flow supply rate. Thus,

fiber deposition is accompanied by droplets falling

intermittently from the needle tip (regime 2). With increased

voltage, a continuous jet occurs (regime 3). However, there is a

large variation of Taylor cone volumes in this region. With afurther increase of the voltage, the variation decreases

significantly and its shape becomes more steady. Then, the

electrospinning can be maintained stably for long periods with

minimal process variations (regime 4). It is noted that the

voltage range for this minimal jet fluctuation regime is afunction of flow rate Q: it requires higher voltage for higher flow

rate, as shown by the dashed upper and lower lines. Further

increse in electric field produce a narrow, variable jet because of

insufficient feeding of solution (regime 5). J Mater Sci (2013) 48:7812–7826 7825


Modeling for Taylor cone (1)In 2012, a mesoscale particle-based model, dissipative particle dynamics (DPD), has been applied tosimulate the Taylor cone.

Dissipative particle dynamics:

Computer Physics Communications

183 (2012) 2405–2412


Modeling for Taylor cone (2)

Equations for electrostatics:


183 (2012) 2405–2412



Z

The maximum absolute value

of electric field corresponds

to the cone apex where the

charge density is maximal, in agreement with the “Leaky

Dielectric Model” of Taylor.


As the applied voltage V0 increases, the shape of the drop

deviates from its initial spherical shape and turns into a cone,

such that the curvature at the interface increases. For high

enough voltage, a Taylor cone with an angle of 98.6° is formed.If V0 is increased above a critical value, the cone becomes

unstable, and particles are detached from the cone tip.

The electrical Bond number (Ne) is a measure for the importance

of electrical forces compared to the surface tension force. A high

bond number indicates that the system is relatively unaffected

by surface tension effects; a low number (typically less than 1 is

the requirement) indicates that surface tension dominates.Intermediate numbers indicate a non-trivial balance between

the two effects.

The jet does not form in this simulation. Improved models, e.g.

using a polymeric model, should be able to also capture thissecond stage of electrospinning.



183 (2012) 2405–2412

Multiple jet ejections


Multiple jet ejections

https://www.youtube.com/watch?v=eWGPW1tS38U

Appling a high voltage to a large capillary ordrop, the emission of more than one jet wasobserved (multiple jet ejections).


Study on multiple jet ejection

A high electrical field allows multiple jet ejections on a drop. The apex of the drop has the shortestdistance to the opposite collector, and thus the strongest electric force is exerted at this point.However, the jets are ejected not only at the apex of the drop but also at multiple positions when

enough electric force is exerted to overcome the surface tension of the drop.

J Macromol Sci, Part B: Physics, 48:1, 77-91


Ejection angle

The ejection angle (θ) is the half-angle between a pair of jets.Changes in the applied voltages resulted in changes in the ejection angle

between dual jets from a drop. To explore the existence of a relation betweenthe ejection angle and the voltage, the voltage was increased to the maximumvalue within the maintainable range of dual jets. It is clear that the

electrospinning ejection angle between dual jets is directly proportional to the

applied voltage. Therefore, the force of mutual Coulomb repulsion between thecharged jets is a function of the applied voltage.


Deposition zones

The electrostatic repulsion exerted between the two charged jets from adrop results in the separation into two deposition zones of thenanofibres on the collector.



Increasing nozzle diameterUsing a nozzle that has a larger diameter

increases polymer solution flow rate, andit induces a large number of jet ejections.A large number of jets consume the droprapidly. Therefore, the orbit of jet

formation on a drop changes with the

number of jets.If the quantity of the electrospun polymersolution by multiple jet ejectionincreases, the orbit of the multi-jet rises

closer to the needle compared to theposition of the single jet ejection on adrop. If the quantity of the jet ejection isgreater than the solution flow rate, thedrop will rapidly contract and the flow isintermittent.



Stability: Drop area


The drop area cyclically changes

during the electrospinning as afunction of spinning time.By varying the applied voltage, thefrequency of the cyclical growing andfalling of the drop and the amplitudeof the drop area were changed. Withincreasing the applied voltage, the

maximum amplitude of the drop

area decreased and the frequency of

the drop formation decreased, too.

Picture Area


Stability: Number of jets


The number of jets ejected from adrop changes with the time and thevoltage.A single stable jet was produced atthe lowest voltages (10-15 kV). As thevoltage was increases to 20 kV twojets were ejected, in a quite stablefashion.Intermittent electrospinning wasobserved with a further voltageincrease to 25-30 kV.

PVA-water 8 wt. %


Induced by collector

It has been reported that the production of multiple jet is promoted by the useof a wavy collector. Contouring the shape of the ground colletor from flat to oneor two with significant curvature, the formation of two to three jets wasobserved.

Appl. Phys. Lett. 90, 093115, 2007

flat wavy


Conclusion on multiple jet ejection

Multiple jet ejection can clearly increase the productivity producing twofold (or more) nanofibres than a single jet.

Unfortunately, the process seems not to be in a stable state (drop sizeat the tip changes, as well as the number of jets) and not to be

completely reproducible. Moreover, liquid was observed on the

collector probably due to dripping of solution or by incompleteevaporation of the solvent from the jet.

The travel of the jet

Stretching and solidification


Fibre formation process

As the jet leave the Taylor cone, itstarts the travel to the collector.

The transformation of a liquid jet to asolid nanofibre has in two regions:1. stability region (straight jet)2. instability region (whippingmode)

The whole mechanism depends alsoon the kind of fluids:Case a. Polymer solution

Case b. Molten polymer (few data)

1

2

https://www.youtube.com/watch?v=87uRQ7KwbB0


Coulomb law

Charges with the same sign repel each other and the force depends on the squared distance.

F: force on the chargesq: charged: distance between the chargesk: Coulomb constant

+

d

q1 q2

F F+


Stretching

+ +RdF F

+ +rDf f

As the jet radius reduced 10 times the jet length increased

100 times, and the resulting Coulomb forces reduced 100

times, too.

So, Coulomb repulsion is maximum at the beginning of the

process when the charges are close.

N.B.: Solvent evaporationhas not yet considered here.


Ion mobility is the driverWhen the free charges in the liquid polymer, which are generally ions, move inresponse to the electric field, they quickly transfer a force to the polymer

liquid. The drift velocity of charge carriers through a material is given by theproduct of the electric field and a parameter called the mobility of the carriers.For the electric fields of around 100 kV/m encountered within the polymer jetin electrospinning, the drift velocity of the ions is estimated to be around 0.15m/s which is much slower than most regions of the jet move. This estimate isbased on the observation that charge carriers such as sodium ions in water havea mobility of around 1.5 × 10−7 m2/(V s). Most charge carriers in organic solventsor polymer melts have even lower mobilities. This low mobility creates theinteresting situation that a charge can move through the liquid for significant

distances if enough time is allowed, for example near the base of the Taylor

cone where distances are short and the liquid velocity is low. In places where

the polymer is moving at velocities much higher than the drift velocity of thecharge, as occurs in the stable jet, the charge moves with the surrounding

molecules.Reneker and Chun, Nanotechnology 7 (1996) 216–223

Stability region


Just beyond the Taylor cone

Taylor showed that a viscous fluid is in equilibrium under an electric field when ajet leaving an orifice has the form of a cone (so-called Taylor cone) with a semi-vertical angle equal to 49.3°.

The generation of the Taylor cone is a balance of

surface tension and electrostatic forces.

As the voltage reaches a critical voltage (Uc), the

electrostatic forces overcome the surface tensionand a jet (fluid stream) is ejected from the apex

of the cone.

The stability region as a finite

length. It begins at the apex of thecone and ends as the bendinginstability starts. However, the

transition from the Taylor cone to

the jet is not sudden, but a zone

exists, i.e. there is a continuum.Malkin et al. Progress in Polymer Science 39 (2014) 959–978


Transition zoneThe fluid is going out of a nozzle, forming theTaylor cone which, in turn, is being transformed via thetransition zone into a jet.

The jet diameter decreases significantly from some hundreds μm to some tens μm in the Taylorcone. When the jet diameter becomes smaller, the slope of variation of jet diameter slow down via

the transition zone from cone to jet. Finally, as the solvent starts to evaporate the concentration

increases, and the variation of jet diameter before the transition zone (stability region) is moregentle [1].In the transition zone, the shape of the jet is strongly dependent on the evolution of surface charge

density and local electric field. They rise quickly to maximum values as charges relax to the jetsurface and surface advection current becomes more important relative to bulk conduction current.

The characteristic length (L) over which the initial dramatic thinning of the jet takes place can be

identified with the axial distance where advection and conduction currents are equal. L has beenempirically evaluated depending on conductivity, flow rate, density, diameter of the nozzle, initialdiameter of the jet, applied field, electric current, and dielectric constant of the outer fluid (typically,

air in conventional electrospinning). Beyond L, the jet thins more slowly. Sufficiently far from the

nozzle, the jet approaches the asymptotic regime where all terms except electrostatic and inertialmust eventually die out [2].

[1] Polym Eng Sci , 2010. DOI: 10.1002/pen

[2] SpringerBriefs in Materials. DOI: 10.1007/978-3-642-36427-3_1,


Characteristics of the straight jet

The jets emerging from the tip of the cone-shaped droplets are observed tomove towards the collector in a linear straight fashion for some distance.The jet diameter decreases along its trajectory typically by a factor of about 4over a distance of 10 cm which corresponds to the reduction of the crosssection by a factor of about 20.A part of this decrease probably comes from the evaporation of the solvents

yet a further contribution originates from longitudinally deformations of the

jet induced by electric forces. Tracer particle tracking techniques involvinghigh speed photography have revealed that elements of the fluid jet aresubjected to accelerations of up to 600 m/s2. These are thus two orders ofmagnitude larger then the acceleration coming from gravitational forces, i.e.the conclusion is that gravitational forces play no significant role in

electrospinning.


Characteristics of the straight jet

The velocity of fluid elements within the jets amounts to only a few cm/s veryclose to the beginning of the emerging jet. Yet it increases to up to several m/s

at a jet length in the range of 1 cm and above.

Finally, particle tracking techniques showed that the strain rates approachingvalues of the order of 1000 s−1 and elongational deformations of up to 1000. The

strain rates characteristic for this part of the jet are consequently large enough

to induce a certain degree of polymer chain extension.

Bel lan et al. J Appl Phys 2007;102, 094308/1–5.


Polymer chain extensionViscoelasticity (see next slide) is a pre-requisite for polymer solution spinnability,

meaning that spinnable solutions are

semidilute, highly entangled. Therefore, thepolymer system is assumed to be a network,

whose connectivity is provided by topological

“knots”. The sections of macromolecules

between two adjacent topological knots are

called subchains, and simulated by “springs”similar to the Rouse model.

The theoretical modeling demonstrated thatthe polymer network can transform from a

free state to an almost fully stretched state

under extreme longitudinal acceleration,within less than 1 mm from the jet start. The

stretching of the network is accompanied bysubstantial lateral contraction that leads to a

rise in polymer concentration at the jet

center.

(a) Illustration of polymer network stretching in an

electrospinning jet. (b) Definition of a 1D system

describing polymer network stretching in the axial

direction. (c) Definition of a 1D system describingpolymer network contraction in the radial direction.

Physical Review E 84, 041806 (2011)


Viscoelasticity

Viscoelasticity is a combination (or superposition) of properties characteristic

for liquids (viscous dissipative losses) and solids (storage of elastic energy).

Viscoelastic behaviour can be considered as a slow (or delayed) development of

stresses and deformations in time, and this delay must not be confused withinertial effects also characterized by a specific lag time.A very important, although not explicit, word in the last sentence is “slow”. Inorder to discover viscoelastic effects in regular liquids, we need to use ultra-highfrequencies (characteristic time of an experiment in this case is about 10-7 s),whereas time delay effects, in deformations of concrete rods, glass windows and

plastic tubes under pressure require years of observation (characteristic time isabout 108 s). Moreover, one can treat deformations of stones as a very slowprocess, realizing that it requires geological periods of time (characteristic time isof the order of 1017 s; as we know “only gods have enough time to observe it”).


Polymer chain extensionX-ray absorption measurements demonstrated that stretching is low and insignificant

close to the jet start at the apex of the Taylor cone. However, at a distance of only 0.5

mm from the jet start, a significant rise in polymer concentration is observed at the jet

center, indicating that stretching is already very high at this position.In addition, the experimental observations demonstrate that further jet acceleration is

possible, but no further polymer chain elongation.The reasonable question is: What happens with the polymer subsystem of the jet during

the further stage (i.e. after the transition zone) of the spinning?

To date, no experimental data are available with regard to the internal evolution of theelectrospinning jet at later stages of spinning, and this question remains an open

problem. Authors surmise that disentanglement and topological reordering of the

polymer network will be observed; these processes call for thorough examination.

Nevertheless, the obtained results allow predicting the stretched nonequilibrium

conformational state of a polymer matrix inside electrospun nanofibers, although partialrelaxation of this ordered microstructure can still occur after formation. Thus, the final

state of the internal microstructure of electrospun nanofibers remains nonequilibrium,affecting their mechanical and thermodynamic properties.

Physical Review E 84, 041806 (2011)


Initial stretching (straight jet)Beyond the Taylor cone the jet thin slowly.

Radius r of the jet versus axial distance z from the nozzle of glycerol at applied electric field of 2 kV/cm and flow rate 6 ml/min.

Sufficiently far from the Taylor cone, the experimental profile of the jet scales as r ∝ z-0.25 [1].For polymer solution added with salts, the jet scales as r ∝ z-0.5 [2], sharper reduction of radius.

Solid line corresponds to the slope of a line that scales as r ∝ z-0.25.

[1] Polymer 42 (2001) 9955

[2] Textile Research Journal81 (2010) 388


Internal capillary diameter = 1 mm

Flow rate = 3.2 ml/h

Jet size at Taylor cone apex = ~0.1 mm (100 µm)

Jet size (at steady state): ~0.02 mm (20 µm)Draw ratio = 50 = 1 / 0.02

Draw ratio with conventional spinning: <20(mechanical means)

Initial stretching (straight jet)


Initial velocity of the jet

The velocity of an electrospinning jet has been estimated using a moving collector, high-speed camera and Doppler effect to be 1–15 m/s (3.6–54 km/h).

Regarding the fibre spinning speed, electrospinning is comparable to traditional dry spinning (5 m/s).

Reneker et a l., Polymer 43 (2002) 6785

Kameoka and Craighead Appl. Phys. Lett. 83 (2003) 371Kameoka et al. Nanotechnology 14 (2003) 1124Wu et a l., Polymer 48 (2007) 5653


Raw estimation of the speed

Material balance:

Flow rate to the tip (Q) = Flow rate of the jet (q)

Q = 3.2 ml/h = 3.2 × 10-3 mm3/h

q = vz ∙ π/4 ∙ d2 � vz = 4/π ∙ Q/d2

d ≈ 0.02 mm = 2 × 10-2 mm (jet diameter in the stable region)vz ≈ 1.3 × 3.2 × 10-3 mm3/h / (2 × 10-2 mm)2 = 1.04 × 10+7 mm/h =

= 10.4 km/h = 2.9 m/s

Since length of the stability zone is usually in the order of few cm (e.g., 5 cm),the travel of the jet from the Taylor cone to the instability region lasts about 18 ms.


Experimental

Bel lan et al. J Appl Phys 2007;102, 094308/1–5.

Direct measurement of fluid velocity in an electrospinning jet using particle image velocimetry.


Modeling of the steady jet

Several models have been developed to describe the steady zone of anelectrospinning jet. Two of the models are:


Dimensionless numbers


Modeling the jet radiusThe macroscopic point of view allows one to describe the hydrodynamicevolution of the jet. Under the electric field the solution jet is moving withacceleration, so that the local velocity contains both longitudinal and radial

components:

therefore, the jet radius takes on an hyperbolic form:

where v0 is the jet initial velocity, and rJ0 is the jet initial radius. Thecharacteristic length z0 determines the scale of velocity increase, anddepends on the flow rate, viscosity, electric field and electric conductivity ofthe solution. This theoretical result is in good agreement with experimentalobservations.

Reznik and Zussman. Phys Rev E 2009;81, 026313/1–7.

Corona discharge


Corona discharge before jetting

Corona discharges (short flashes of around 100 ms) from electrospinning jetswere observed and photographed at the tip of the Taylor cone, and in acylindrical region around the jet, a few millimeters below the tip.

An aqueous solution of 6% PEO (MW 400 kDa)was held in a glass pipette. The electricalpotential of the solution was adjusted byelectrons flowing through a metal wireinserted into the solution.The picture shows a corona discharge near thetip of the Taylor cone before the jet formed. Anegative potential difference of 25 kV wasapplied.

Tripatanasuwan and Reneker, Polymer 50 (2009) 1835–1837


Corona discharge after jetting

After the jet formed, a corona discharge was observed when both a negativeand positive electrical potential was applied. The corona typically began about

1 or 2 mm below the tip and extended for about 1 mm along the jet. Thecorona was faintly visible in a dark room. The light from the corona became

more intense when the electrical potential, of either polarity, increased.

Fig. (a) shows a negative jet at a potential of 12kV. The position along the jet at which thecorona became visible is marked with a whitearrow. The measured radius of the jet at thisposition was 30±5 μm.

In Fig. (b) the negative potential was 20 kV.



Radial electric field calculationThe radial component of electric field at the surface of the jet increased, as the radius of thetapering jet decreased with distance from the tip.To calculate the radial electric field, the charged jet and its surroundings were approximated as a

cylindrical capacitor. The zero potential was assigned to a hypothetical outer metal cylinder with aradius A, of 10 cm. The radius of the outer cylinder was chosen to be equal to the radial distancefrom the jet to the nearest grounded metal support structure. The radial component of electric fieldbetween the concentric cylinders is given by:

E = φ0 / [r ln(A/r)]

where φ0 is the potential applied between the central cylinder and the grounded outer cylinder.The onset of the corona discharge was observed to occur when the jet radius (r) was ~30 μm whenthe applied potential was 12 kV. So, the radial electric field calculated from the above formula was

around 400 kV/cm.The electric field required to initiate a corona discharge around a metal wire can be estimated fromPeek’s empirical equation, Ec = 30 d[1 + 0.3/(d r)0.5], where Ec is the electric field for onset of coronain kV/cm, d is an air density factor that is near one at ambient conditions, and r is the radius of thewire in centimeters. For a metal wire with a radius of 30 μm the predicted electric field was around

200 kV/cm, about half the field calculated from the observed onset voltage for a jet of the samediameter.


Instability region


Solvent evaporation

Stephens and Bruce [Appl Spectrosc 55(10):1287–1290] demonstrated through Ramanspectroscopy that the solvent evaporation prior to the onset of instability could be

negligible.

It is clear that the velocity profile reproduced above demonstrating rapid unlimitedincrease in the longitudinal velocity, can be accepted as a satisfactory approximation only

in the beginning of the electrospun jet, before the instability region.The point is that the electrospinning process of a polymer solution is accompanied by

significant mass loss due to extremely rapid solvent evaporation occurring with

continued jet travel, so the solution properties are substantially modified along the jet.Due to solvent evaporation the solution viscosity sharply increases, and in the end the jet

solidifies. The stretching of the filament also affects the viscoelasticity of the polymersolution. As a result the increase in the longitudinal velocity (vz) slows down; and, finally,

it tends to a fixed value (vz→ v∞). In this case the radial velocity (vr) tends to zero,

whereas the filament radius stops decreasing and tends to its final value rJ∞. The effect ofthe evaporation rate on the final radius value of electrospun nanofibers and their

properties was studied experimentally.


Effects of solvent evaporationAs the solvent evaporates from the jet:

• a concentration gradient of thepolymer molecules in the jet iscreated leading to a morepronounced increase in viscosity;

• the jet temperature decreases,depending on the value of thelatent heat of the solvent and onheat exchange rate (given by theambient conditions), leading to afurther increase in viscosity.

It is not investigated if the reduction of mass of the jet during the jet flight can:• have an effect in changing the speed of the jet itself;• have an effect in reducing the charge density of the jet (probably depends

on the volatility of cherged species); however, corona discharges around thejet have been observed.

Acta Pharm. 62 (2012) 123–140


Case b. Molten polymer

Molten polymers generally have a higher viscosity than polymer solution.

The jet solidifies by heat exchange: the temperature decreases andconseguently viscosity increases.Often in melt-electrospinning, in order to give time to the jet to be stretchedenough the electrospinning chamber is heated too and it operates under

vacuum (reduction of heat exchance rate).

The mass of the jet doesn’t change. So, no diameter reduction due to solvent evaporation = thick fibres!

Melt-electrospinning can’t produce ultra-thin nanofibres.


Instability regionThe jet at the beginning of its life (in the instability region) is quite big (20-50μm) if compared with the nanofibre size (usually 200-400 nm).It is conjectured that a certain extent in diameter reduction in electrospinningis achieved also by further strecthing and acceleration of a fluid filament in theinstability region prior to solidification or impact on the collector.Also in this region stretching and acceleration is mainly ruled by the Coulom

repulsion of charges, but in the instability region the charges inside the jet playa crucial role in stretching and acceleration.

Important (see also Lecture 1, slide 74)The jet in the stable region is pushed(stretched and accelerated) by the external

electric field (capillary and Taylor conecharges dominate the process).As this electric field reduces (as the distanceincrease) internal charges predominantes(instability region).


Characteristics of whipping Beyond the stable zone the jet is subjected tobending deformations, i.e. it deviates thusfrom straight path directed towards thecounter electrode. It turns sideways anddisplays spiralling, looping motions at rates of200-1560 turns per second*. The envelope of

these loops resembles a cone with itsopening oriented towards the collector. Thistype of instability – called bending butfrequently also whipping instability – repeatsitself on a smaller and smaller scale obviouslyin a self-similar fashion as the jet diameter isreduced.

Phys . Fluids, Vol. 13, No. 8, August 2001

* Polymer (article in press). DOI: 10.1016/j.polymer.2012.06.009


Whipping looks like …

smoke honey

Straight

Spiralling

Straight

Spiralling


Electrospray

Whipping mode has been observedalso in electrospray on large waterdrop.


Description of bending instability

The bending instability of a jet does not

result in the fibre failure.After the jet flows away from the Taylorcone in a nearly straight line, it bendsinto a three-dimensional coil, passinginto the whip-like movement of a jet.After, several turns are formed, a newelectrical bending instability forms asmaller coil on a turn of the larger coil.This type of instability arises due torapid growth of bending perturbations

under the influence of the chargecarried with the jet and other forces.https://www.youtube.com/watch?v=AS6SHzeSgB4


Whipping observations

Optical observations of jets provide information useful for control ofelectrospinning of polymer solutions. Combinations of videography,stereography, and methods for illumination of the multiple coils of anelectrospinning jet path recorded quantitative information about the location,vector velocity, and rotation of selected segments of the jet.Several techniques were used:1. high-speed photography (static observation, whipping motion is freezed);2. high-speed video (dynamic observation, time depending evolution);3. bright glints of reflected light from the jet (dynamic observation, jet motioncan be followed and velocities calculated);


High-speed photographyWhen viewed with exposure timesdown to the ms range (1/250 s), th

instability zone has the appearance of a

conical region, which opens outward inthe direction of flow, suggesting an

envelope of multiple jets (Fig. A). Usinghigh-speed photography, it has been

observed that this conical envelope is

attributable to a single, rapidly,

whipping jet. The frequency of the

whipping motion is so fast thatconventional photography lends the

appearance of splitting into multiple

filaments. Illuminating the instabilityzone from behind with an 18 ns

exposure time a single filament was

observed (Fig. B).

A B

Polymer 42 (2001) 9955


High-speed video2000 frames/second (time between each frame was 0.5 ms), 0.025 ms exposure

Figure shows 30 successive

images of a region that

includes the onset of the

bending instability.In frame 6, a closed single

loop about 1 mm in

diameter is partly visible as

it extended from the lower

right part of the developinginstability. This small loop,

indicated by arrows in

frames 6, 12, and 18.

Higher magnification view

showed the nanofiberentangled with itself.

D.H. Reneker et al. Polymer 43 (2002) 6785


Glints of reflected lightGlints are seen when a beam of light is specularly reflected, particularly from a curvedsurface.

In (A), two intense glints are seen on the smooth surface of a metal ring in a beam ofparallel light. (B) is a computer model of jet paths with calculated glints. Coiled paths in

blue (1), red (2), and green (3) are the instantaneous positions of a jet path at three

sequential instants. White streaks are the traces of glints as the jet elongates along itslocal curved axis and moves from the blue (1) position to the green position (3) during

the exposure time.Polymer (article in press). DOI: 10.1016/j.polymer.2012.06.009


Arrangement for glints

A geographic coordinate system was used to describe the radial directions. The jet was centered inthe globe. The reservoir and the collector are in the direction of the north pole and the south pole,respectively. The camera defines the 0° longitude (equator). The axis of a beam light, from a 600W

xenon arc lamp, was at (90°W, 26°S). The axis of a broad, intense, strobe light was at (meridian 180°,40°N). The flash duration was about 180 μs. A black background, perpendicular to the camera axis,was placed behind the sample.


Glints at the onset of the bending instability

Two glint traces emerge from the region where the beginning of theelectrical bending ends the straight segment. This region is called thevertex of the envelope cone. An example was recorded in A.

The model in B, demonstrates the onset of the bending instability of ajet. As the bending perturbation grew, a single glint appeared near theend of the straight segment of the jet, indicated by the black arrow onthe leftmost jet in B. The segment producing the glint becameinvolved in the first turn of the bending coil, and the single glint split

into two glints (arrows on the rightmost jet). The two newly formedglints move apart as the coil expands, and simultaneously movetowards the grounded collector. These movements of the glintsappear as the bifurcation of a single glint trace in A (red arrow).

A

B

Polymer (article in press). DOI:

10.1016/j.polymer.2012.06.009


Glints at the whipping

Polymer (article in press). DOI: 10.1016/j.polymer.2012.06.009

Six glint traces were labeled. The X and Y components of each glint trace were calculatedfrom the stereographic image, while the Z component of the ends of a trace was

measured along the vertical axis. The velocity components are listed in Table. The highest

vertical velocity (vz) occurred near the vertex of the envelope cone, and then decreased.The radial velocities (vr) of glints decreased significantly as the turns of the coil moved

downward. This decrease is related to the increasing viscosity of the fluid jet resultingfrom the evaporation of the solvent, and to the decreasing charge per unit area on the

jet as the ratio of surface area to jet volume increased, it means that a fixed amount of

charge was distributed over a larger surface area.


Why whipping appens?

There are two main contributions (theories) on

bending perturbations:

1. non-axisimmetric Rayleight instability

2. visco-elastic relaxation process


1. Rayleigh instability familyFew cm far from the tip, charge repulsion is relatively stronger in contrast to electrical shear stressand surface tension, allowing one to neglect their contributions.At this point, as the jet thins, it ultimately succumbs to one or more fluid instabilities which deform

the jet as they grow. A family of Rayleigh instabilities exists and can be analyzed for differentconditions of the symmetry (axisymmetric or nonaxisymmetric) of the growing perturbations of thejet, using linear instability analysis. The most common mode of instability in electrospun jets

appears to be the growth of lateral excursions of the jet due to non-axisimmetric instability.

Schematic illustration of perturbations associated with several of the

lowest-order instabilities, distinguished by their azimuthal wave number (s).

Top views illustrate cross sections of the jet at maximum amplitudes of

(oscillatory) perturbation, with bold and dashed contours representingdifferent positions along the jet length. Bottom views illustrate changes in

shape and center line down the length of the jet. + and - are used to

indicate regions of positive or negative deviation from the unperturbed jet

shape.

(a) unperturbed cylindrical fluid element, (b) varicose (s=0) instability(axisimmetric), (c) bending (s=1) instability (non-axisimmetric), and (d)

splitting (s=2) instability. Growth of the varicose instability leads to equal-

sized droplets (beads); growth of bending instability leads to whipping

mode; growth of the splitting instability leads to two equal-sized sub-jets

(branching).

SpringerBriefs in Materials (2003), DOI: 10.1007/978-3-642-36427-3_1,


1. Non-axisimmetric Rayleigh instability

Non-axisymmetric instability appears due tobending forces and this phenomenon plays the keyrole in the mechanism of filament thinning andconsequently, polymer orientation.When the jet bends, the charge density (σ0) along

the jet is no longer uniform around thecircumference of the jet, but now also contains adipolar component oriented perpendicular to thejet axis, as the internal charges adjust to screen theexternal field. These dipoles set up a localized

torque that bends the jet, and oscillations of thebending instability account for the whippingmotion of the jet.

Polymer 42 (2001) 9955


2. Definition of viscoelastic relaxationViscoelastic relaxation is the time necessary for recovery of a stable structurestate after removal of external forces.

A dimensionless criterion is usually introduced, called Deborah number* (De) that is the ratiobetween time scale of a material and time of observation.

If De >> 1, material behaves like a liquid (water), which occurs when time scale of a material is smalland relaxation happens very quickly (in comparison with time of observation). In the opposite timescale (when De << 1), unrecoverable deformations cannot be detected and we cannot discover flow,the material is solid. For De ≈ 1 we have a viscoelastic material that behave like a liquid and a solid atour time scale.

*Rheologists like to cite a famous exclamation by the Bible’s Deborah: “The mountains melted from

before the Lord”, bearing in mind that in the scale of eternity, the Lord really can observe flow ofrocks (mountains).

The portion after the grey line at time x reflectbehaviour of a viscoelastic material afterremoval of the external force; the resultingdeformation is zero at time x+λ.

stress

deformation

x+λ


2. Visco-elastic relaxation processA hydrodynamic approach makes it possible also to take into account the viscoelasticity ofpolymer solutions. And the viscoelasticity of polymer solutions results in the bending

instability of the spinning jet.To account for a limited length of the straight part of the jet the assumption that has been

made is that no deviations occur as long as the conditions are such that no instabilities, inparticular the bending instability, can become dominant. The concept theoretically explored isthat such bending instabilities can become dominant only if the total tensile stress has

decayed sufficiently along the jet. Stress/longitudinal force first increases over time as thesegment is stretched, passes a maximum as function of time and then begins to decrease. The

decrease is attributed to the onset of the visco-elastic relaxation process merge with a fast

solvent evaporation.The interpretation proposed is as follows. The linear jet is stable for sufficiently high values of

force and stress. As these pass through a maximum and decay rather strongly at larger times,i.e. at locations further down the jet they become so small that instabilities, in particular the

bending instability, become dominant due to a finite length constraint. This causes the linear

jet to deviate from the original path at a length which is strongly controlled by the applied

voltage in agreement with experimental findings. This characteristic length is interpreted as

the length of the linear part of the jet observed experimentally.Nevertheless, up to now the effect of the polymer jet viscoelasticity under high stretching has

been insufficiently and incompletely studied.


Other forces acting on the jet

Together with the electrostatic forces, other forces act on the jet:

• Gravity

• Friction of air

• Electric wind

It is alredy demonstrated that gravity seems to be negliglible, at least as thejetting begins, but during whipping has been not yet demonstrated.While no information, data or studies were found in current literature aboutfriction of air on the jet. Up to now, literature neglected air friction onelectrospinning.Electric wind is the motion of ionized air molecules induced by strong electricalfield. Some researched observed the generation of electric wind duringelectrospinning, but no correlation between electric wind with the process was

reported.


Electric windElectric wind (or ion wind or coronal wind)is a stream of ionized air (or gas) produced

around electrically objects. It mainly

contains ions with a polarity opposite tothat of the object that generated the wind,

and neutral molecules of air (or gas) thatare pushed by the motion of ions.

An amusing application of the electric wind is the “ionocraft”, an ion-generated wind lift. It waspatented in the 1964 by Alexander P. de Seversky (Electronatom Corp.), US Patent 3,130,945.

Morphologies

Beads

Buckling

Branching

Generation of beads


Morphologies of beads

spherical

spindle-like bamboo-like

beads-on-a-string


Process condition

Process conditions, in particular polymer concentration (i.e. viscosity), affect themorphology os the beads.

Al low concentration the beads arealmost sphere (aspect ratio ~1). As theconcentration increases the aspectratio increases producing ellipticalbeads more and more stretched tillregular fibres.

Polymer 44 (2003) 4029

sphere spindle-like


Conditions for beaded nanofibresCharacterization of the formation of electrospun beadednanofibres shows that solution viscosity, net charge density

carried by the jet and surface tension of the solution are the

main factors.Higher viscosity favours formation of fibres without beads,higher net charge density not only favours formation of fibreswithout beads, but also favors the formation of thinner fibres.Surface tension drives towards the formation of beads, hence

reduced surface tension favours the formation of fiberswithout beads.Therefore, beaded nanofibres are preferred at an

equilibrium condition of low concentration polymer

solution, low charge density and high surface tension where

entanglements(*) are higher in number but not enough tostabilize the jet before solidification.In addition, voltage may play a role in the formation of beadssince it has been reported that spherical beads are formed

with electric discharge in high voltage region [Polym. Eng.

Sci., 50:1788–1796, 2010].

SpringerBriefs in Materials (2003), DOI: 10.1007/978-3-642-36427-3_1,


EntanglementsIn the dilute regime, polymer molecules have a coil-likeconformation, and can be approximated by separated spheresformost descriptions of the solution behaviour. There aremany

fewer polymer chain entanglements between macromoleculesin a dilute solution. The viscosity of a dilute polymer solution istherefore low, approaching the viscosity of the solvent.Electrospinning in the semi-dilute regime produces thinnernanofibres, which may be beaded.

The regimes are defined by changes of the slope of specific

viscosity (ηsp) plotted against polymer concentration. Thespecific viscosity ηsp is defined as:

where η0 is the zero-shear rate viscosity of the solution(theroretical viscosity at shear rate of 0 s-1 by flow curve, seeplot a) and ηs is the solvent viscosity. The entanglementconcentration (Ce), which is defined as the transition from the

semi-dilute unentangled to the semi-dilute entangled regimes,was measured using the change in slope at the onset of theentangled regime (see plot b).

Flow curve

dilute

semi-dilute


Conditions for beaded nanofibres

An electrically driven jet of a low molecular weight liquid will form

droplets (electrospraying). The formation of these droplets is due to thecapillary breakup of the spinning jet by surface tension.

For polymer solutions, the pattern of the capillary breakup is changedradically. Instead of breaking rapidly, the filaments between the dropletsare stabilized and a stable beads-on-a-string structure is formed. Thereason for this is that the coiled macromolecules of the dissolvedpolymer are transformed by the elongational flow of the jet into

oriented, entangled networks that persist as the fibre solidifies. Thecontraction of the radius of the jet, which is driven by surface tension,causes the remaining solution to form beads.As the viscosity of the solution is increased, the beads become bigger,the average distance between beads longer, the fibre diameter larger,

and the shape of the beads changes from spherical to spindle-like.As the net charge density increases, the beads become smaller and

more spindle-like, while the diameter of fibers become smaller.Decreasing the surface tension make the beads disappear gradually.Neutralization of the charge carried by the jet favours the formation of

beads, because the tension in the fibre depends on the net chargerepulsion and the interaction of the net charge with the electric field.

Water

Polymer 40 (1999) 4585–4592


Beads-on-a-string

The instability of elastic fluids which looks like the formation of a “beads-on-a-string”structure, consisting of drops interconnected by thin and stable filaments, has been

described. However, these viscoelastic filaments do not break up into separate drops (as

opposed to a liquid). The polymer inside the drops remains relaxed, whereas polymer

macromolecules in connected cylindrical parts of the filaments between drops become

strongly oriented, leading to the high strength of these bridges.

Development of a “beads-on-a-string”

structure. An aqueous PEO solution.

Concentration is 0.2%.

Figures below images are time in ms.

Phys Fluids 2005;17, 0171704/1–4.


Axisimmetric Rayleigh instability

Phys. Fluids, Vol. 13, No. 8, August 2001

It is reported that the beaded fibers are produced when developing the axisimmetric Rayleigh

instability (also called varicose instability) due to electric charges.The presence of surface charge modifies the characteristics of both the Rayleigh and the conducting

modes in a fashion that is not (at first glance!) intuitive. Various instabilities are produced, namelyperturbation of the velocity (u), charge density (s), electric field and radius.It will be helpful for our intuitive exploration to discuss the forms of the first three perturbations(velocity, charge density, and electric field) in terms of the perturbation to the radius. In particular, itwas found that the velocity perturbation is π/2 out of phase with the radius perturbation, and for

a growing mode it pushes fluid into the bulging regions and draws fluid from the narrowing regions

(so beads enlarge).

Δ denotes perturbation on the

surface charge density (s).

Arrows indicate the perturbation

of the velocity (u)

The electric field response is morecomplicated, as well as the charge densityresponse.

Briefly, the perturbation in surface chargedensity causes an electric field perturba-tion inside the jet: the polarization charge(induced by perturbation surface densityit-self) tries to cancel out the penetration

of the applied field (E∞) and this causes anelectric field perturbation on the surface.


Summary of the instabilities

Bending or Whipping(Non-axisymmetric

Instability)

Classical Rayleigh instability

Axisymmetric instability

Regular nanofibres Beads Beaded-nanofibres

J. Appl . Phys. 114, 171301 (2013)


Advantages of beaded fibres

Authors have reported that a nonwoven which contains beaded fibres hashigher hydrophobic property than that made of bead-less fibres with a samediameter and polymer.Moreover, in the production of scaffolds for tissue engineering, cellular

adhesion can be promoted by the presence of beads along regular nanofibres.Finally, for some biomedical applications (e.g. drug delivery, wound healing)researchers developed a process called “dual electrospinning” which consists inthe use of two electrically cherged nozzles at the same time. One producesregular nanofibres by electrospinning, while the other actually work as anelectrospray.

More details in the lectures on “plant design” and “applications”.

Branching/Splitting/Splaying

Generation of branches/ramifications


Definition

Radial charge repulsion sometimes results in splitting of the primary jet into

multiple filaments, in a process known as splaying, branching or splitting.In this view, the final fibre size is determined also by the number of subsidiaryjets formed. When the filaments dry and solidify, electrically charged secondaryfibres remain as ramifications.

In the past, when high-speed camera were not available it was believed that thiswas the main mechanism to produce ultra-fine fibres by electrospinning.Nowaday, some people erroneously still consider electrospinning as a splitting-fibre process.


Videoing branching

A frame of a nice video named “Birth of Nanofibers”(https://www.youtube.com/watch?v=Hr9qWgFmii0)


Branching looks like…

Turin 07/07/2014

…branched lightning.Moreover, both are electrically generated.


Observing the branching

J. Appl. Phys. 98, 064501 (2005)

Observations of branching of the jet were carried out by a high-speed camera.The observedazimuthal directions of the jet branches vary (Fig. A). Adjacent branches can lower their electrostaticinteraction energy by extending in different azimuthal directions. Interactions between the branches

and the charges on nearby loops of the primary jet may also affect the direction of a branch.Bending and branching may occur together, as Fig. B shows. The branches did not occur

continuously, but repeated about 10 times/s. The reasons for this repeating sequence are notpresently known.

A B


Characteristics of the branching

The bending instability and the occurrence of

branching coexist with only minor interactions,

even when both instabilities are fully developed.

Branching can be abundant, with many long,closely spaced, and rapidly growing branches. Jets

with larger diameters, associated with higher

voltages, tend to have more branches.

An imperfect spiral is suggested by observation.

The jet and the branches are tapered.

J. Appl. Phys. 98, 064501 (2005)

Electrospinning of 15 wt. %

polycaprolactone in acetone at voltage of

4–10 kV and distances between electrodes

of the order of several centimeters.


Theory for branchingAn electrohydrodynamical theory proposed to describe this phenomenon showed thatthe surface of a conducting fluid jet can acquire complicated static equilibrium

undulations under the combined effects of the electric stresses and surface tension as

the electrical stresses increase. A perfectly smooth cylindrical jet develops staticundulations with complex shapes in a cylindrical electric field. It was shown that such

undulating surfaces could become unstable at the sites of the highest local surfacecurvature on the tops of the longest wavelength undulations. This instability leads to the

emanation of lateral branches from the primary jet.

J. Appl. Phys. 98, 064501 (2005)


Resulting nanofibresBranched fibres are usually flat.(30% PS in dimethylformamide)

Highly-branched nanofibres producing a network of fibrils

having diameter of 10 nm.

(15% keratin in formic acid)

Buckling process

Generation of ribbons/belts


Definition

In addition to round nanofibres, electrospinning a polymer solution can producefibres with bean-shaped cross-sections. The transverse dimensions of thesefibres were typically 1 or 2 μm, measured in the widest direction.

J. Phys. D: Appl. Phys. 42 (2009) 015507Materials Letters 58 (2004) 493–497


Theory of buckling processPolymer molecules in the electrospun jet can be inhomogeneously distributed in the

cross section of the jet forming a compact surface, shell or skin. Observations indicate

that the presence of a thin and mechanically distinct polymer skin on the liquid jet can

lead to the bucking. After the skin formed, the solvent inside escaped and atmospheric

pressure tended to collapse the tube formed by the skin as the solvent evaporated. The

circular cross section became elliptical and then flat, forming a ribbon with a cross-sectional perimeter nearly the same as the perimeter of the jet, see (a–d). Sometimes,

small tubes formed at each edge of the ribbon. A web made from the skin connected the

two tubes (e).

J Polym Sci: Part B: Polymer Physics 39 (2001) 2598-2606


Mechanism of buckling

The buckling mechanism involves the evaporation of the core solution through

a solidified shell resulting in a pressure difference across the fibre shell.Exceeding a critical value, this pressure difference enables fibre buckling;however, the final form of the as-spun core-shell nanofibres is controlled by therate of evaporation of the solvent that remains within the microtubes after theelectrospinning process. The process has been physically modeled.

Physical Review E 76, 056303 (2007) 056303


Results

J Polym Sci: Part B: Polymer Physics 39 (2001) 2598-2606

The ribbon often bent, either in aregular way as a result of the electricallydriven bending instability or morechaotically as a result of the forces thatoccurred when the ribbon was stoppedon the collector.


Specific surface

where α is the shape factor (ts/P), P is cross-sectional perimeter, ts is skin thickness, ρ is the

polymer density.

As the ribbons flatten, the specific surface increases. A theoretical relationships betweenthe specific surface area (Sf) of an electrospun polymer network and the cross-sectional

morphology has been found:

J. R. Soc. Interface (2010) 7, 641–649

1/(2π) ≈ 0.159

Effects of the corona discharge


Corona discharge (resume)

After the jet formed, a corona discharge was observed when both a negativeand positive electrical potential was applied. The corona typically began about

1 or 2 mm below the tip and extended for about 1 mm along the jet. Thecorona was faintly visible in a dark room. The light from the corona became

more intense when the electrical potential, of either polarity, increased.

Fig. (a) shows a negative jet at a potential of 12kV. The position along the jet at which thecorona became visible is marked with a whitearrow. The measured radius of the jet at thisposition was 30±5 μm.

In Fig. (b) the negative potential was 20 kV.



Fibres with lobes

Fibres with 2 or 3 lobes that split into 2 or 3 fibres are abundant in the samplesproduced during corona discarge, but not observed elsewere.The corona discharge current reduces both the charge per unit area on the

surface of the jet to values that can cancel the mutual repulsion between

enveloping jet. So, during the travel between the tip to the collector, almostsolidified nanofibrescan glue together producing this morphology.


At a first glance it seems like a ribbon, but don’t.

Collecting nanofibres


Description

The collector is where the jet stopped. The polymer fibre that remains after thesolvent evaporates may be collected on a metal screen. For polymers dissolvedin non-volatile solvents, water or other appropriate liquids can be used tocollect the jet, remove the solvent, and coagulate the polymer fibre. Mechanicalreels or shaped moving strutures can also be used for collection. If the jetarrives with a high velocity at a stationary collector, the jet tends to coil or fold.Since the jet is charged, a fibre lying on the collector tends to repel fibres that

arrive later. The amount of charge on the fibres can be changed by ions createdin a corona discharge and carried to the collection region by air currents. Thecharge may also be removed by charge migration through the fibre to the

conducting substrate, although for dry fibres with low electrical conductivity,this charge migration may be quite slow.

Reneker and Chun, Nanotechnology 7 (1996) 216–223


Results of the whippingThe whipping motion greatly affects the nanofibre collection in two main ways:1) nanofibres usually are collected in a disordered fashion, i.e. anisotrope non-

woven structure;2) the zone of the collector where the nanofibres lay down (deposition zone)

can be large, the radius of the envelope cone depends on the workingdistance.

J. Appl. Phys., Vol. 89, No. 5, 1 March 2001


Don't try this at home!

http://www.pltwohio.org/news_Polymer06.php

Note the level of detail which shows through the material.

Electrospun nanofibres can be deposited on many collector, even a hand!


Flat collector

It is the most common electrospinning collector.Nanofibres deposite as a randomly oriented non-woven structure.

Static flat metallic collector


Variations of rotating cylinders


Rotating cylinder

Rotating cylinder.

At low speed it collects randomly

oriented nanofibres as a flat collector.


High speed rotating cylinder

High speed rotating cylinder is to mechanically stretch the fibres, thus helping itto align along the periphery of the mandrel. The mandrel can transverse alongits axis in order to obtain aligned fiber mat.Rotation usually exceeds 1000 rpm to obtain good alignment degree ofnanofibre depending on the cylinder diameter.

Aligned nanofibres


Small rotating drums

Nanofibres can be collected around a small rotating mandrel to produce tubes. This 3D-shaped structure is particularly important in biomedical application (e.g. vascular

scaffolds, guidances for nerve reparation).

A Teflon tube of diameter of few mm isusually used as drum collector for this

purpose. Teflon is used because of the low

friction. Unfortunately, it is not conducting,therefore a charged counter-electrode

must be used behind the drum to attractthe nanofibres.

Nanotechnology 16 (2005) 918–924


Rotating disk

Alignment of the nanofibres is reached by the rotation and the shape of the edge of

the disk.

It was noted that alignment of the fibers

improved with an increase in the rotation speed of the disc from 800 to 2000 rpm


Longitudinally wired drum

conductor

conductor

nanofibres

Aligned fibres al low rotation.

Wires are conducting, the drum is insulator.


Rotating open frame collectors

Rotation and shape of the frame help the nanofibres to be orientated.


Counter electrodesAppreciable degree of alignment

was obtained from these setups.

Collector configurations are

mainly based upon a group ofcounter electrodes placed in

certain configuration:

(a) parallel electrodes;

(b) array of counter electrodes;

(c) two steel blades placed with agap;

(d) parallel electrode system

consisting of aluminum and gold

electrodes;

(e) two pieces of stainless steelelectrodes with the provision to

selectively connect to HVDC

power supply;

(f) collector system consisted of

dual vertical stainless steel wiresas the secondary electrodes with

grounded aluminum foil as the

primary electrode;

(g) external magnets as auxiliary

electrodes.


Parallel electrodes

Nanofibres were aligned only in the narrow gap

between the electrodes.

gap

On the electrodes nanofibres were

randomly oriented.


Pairs of counter-electrodes

With array of pairs of counter-electrodes it is possible obtain 90° aligned nanofibres.


Static shaped frameconductor

insulatorNanofibres orientated in different ways depending on

the shape of the insulator

(patterned deposition).


Patterned collectionOrdinarily, the electrospinning process generates one-dimensional fibers which assemble into randomly-oriented nonwoven membrane structures due to

instabilities in the fluid jet.Many papers utilized patterned collectors to produce

aligned membranes with designed complex

topological structures.The template-assisted electrospinning approach is

demonstrated to produce patterns includingalphanumeric characters and a printed electroniccircuit chip, with feature sizes on the order of severalhundred microns. The process has a significant impact

on micro-manufacturing, and provides the capability

for incorporation of oriented fiber materials in

patterned micro-composites.

Polymer 51 (2010) 3244-3248


Mesh substrates (1)In biomedical application, the use of patterned collectors(stainless steel wire meshes) to increase the pore size of

electrospun scaffolds can be useful for enhanced cell

infiltration. The morphology of the patterned scaffolds wasinvestigated by scanning electron microscopy, which showed

that the collector pattern was accurately mimicked by theelectrospun fibres.

An enlargement in the pore size and in the pore size

distribution compared with conventional electrospinning wereobserved. The pore size was dependent on the size of the

non-conductive gap of the patterned collectors. As a result ofthis enlargement of the pore size, fibroblasts were able to

infiltrate up to 250 μm into the scaffolds, demonstrating the

benefit of an increased pore size at designated locations forcolonisation of these tailored electrospun scaffolds.

Acta Biomaterialia 7 (2011) 2544–2557


Mesh substrates (2)

SEM image of electrospun nylon fibers collected with

corona ion assistance onto different substrates

SEM images of electrospun nylon fibers collected in

absence of ion assistance onto different substrates:

Inhomogeneous fiber coatings were observed on mesh substrates: neven fiber distributions oninsulating and on conducting substrates with a non-flat, porous morphology (‘structured materials’),such as felts and meshes.

To overtake the problem, researcher proposed to apply an additional electrode for the productionof corona ions around the needle. Ions reduce the charge on the substrate from the saturation levelthat would be attained without ion collection, and that such charge reduction results in attractionof airborne electrospun fibers towards the substrate region.

Polymer 51 (2010) 5221-5228


Complex grids

Patterned nanofibrous meshes consist of regions of high fibre density; the potential and fibre density were lower in insulated regions


Micro-patterned collector

SEM images of the micropatterned nanofibrous layer. The relief has been

copied to the fibrous scaffold, the periodic pattern is clearly visible. Scalebars represent 300 μm (a) and 100 μm (b).

SEM images of the relief on the collector. Engraved topography of the

collector (a) and (b) top view, (c) cross sectional view. The pattern had aperiodicity of 90 μm and a depth of 14 μm. Scale bar on panel (a)represents 300 μm.

Engineered muscle constructs provide a promising perspective on the regeneration or substitution of irreversibly

damaged skeletal muscle. However, the highly ordered structure of native muscle tissue necessitates special

consideration during scaffold development. A study reports the guidance effect of a scaffold that combines both

approaches, oriented fibres and a grooved topography. By electrospinning onto a topographically structuredcollector, matrices of parallel-oriented poly(ε-caprolactone) fibres with an imprinted wavy topography of 90 μm

periodicity were produced. Interestingly, pattern induced an orientation of myotubes at an angle of 24°

(statistical median) relative to fibre orientation.

Biomed. Mater. 8 (2013) 021001


Porous micro-tentsIn vascular endothelium, endothelial cells often reside on a curvedsurface and are dynamically loaded upon vessel contraction anddilation. Such curved surface mimics in vivo curved morphology

and regulates recruitment of specific cell populations.Electrospun polymer thin films with a porous and curved surfacewere fabricated by combining electrospinning andmicrofabrication, a microstructured substrate was used to collectthe polymer fibers.Human vascular endothelialcells were cultured on theconcave surface of the porous

thin nanofibre film, whichmimic the structuralcharacteristics of naturalendothelium.

J. Micromech. Microeng. 22

(2012) 085001


Self-assembed structure (1)

Periodic feature of surface roughness, such asdiamond-shaped structure naturally occurred asthe deposition time increased during

electrospinning of some polymers, such aspoly(trimethylene terephthalate).Some parts of nanofibrous mats exhibited adifferent morphology with relatively more fiberbundles and junctions indicating high fiber

density. Diamond-shaped structure, with themajor axis and the minor one, was shown at thedeposition time of 60 min.Three reasons can be introduced to understandthis phenomenon. One is the high chain mobility

of the polymer, the second is a short bond-to-

bond distances between fibers, and the third isrelated to the low boiling point of the solvent.Briefly, fibres are wet for a long time and tend tobond together with high bending.

Polymer 45 (2004) 295–301

Patterned structures are sometimes produced by electrospun nanofibre due to self-

assembly processes even if the collector in flat and homogeneous.


Self-assembed structure (2)

A phenomenon of self-assembly was observed in polyurethane electrospun nanofibers. Self-assembled nanofibers align themselves into unique three-dimensional (3-D) patterns such ashoneycomb meshes on a flat metal collector.

Residual charges on the collected fibers and the electrical property of the collector screen seem toinfluence the self-alignment of fibers.

However, researchers are unable toprovide a conclusive interpretation of theinfluence of the electrical properties of

the collector on the self-assemblingprocess.Self-assembled nanowebs will be ofenormous value in chemical counter-measure substrates such as face masks

and chemical protective clothing linersbecause of their enhanced trapping andfiltration capabilities.

Journal of Applied Polymer Science, Vol. 101, 3121–3124 (2006)


Bundle of aligned nanofibres

This category deals with the setups in which

dispensing system of the electrospinning designs is

modified to obtain aligned fibers.

In (a) the dispensing system consists of a triangularaluminum tip without any solution supply system.

The grounded upright coin act as the collector

placed a certain distance away from the triangular

tip. The triangular tip is dipped in electrospinning

solution to form a small droplet on its tip. When ahigh voltage is applied to the tip, a bundle of

electrospun fibers were then formed between the

tip and the collector. This system successfully

fabricated ultralong highly oriented fiber bundles.

Nevertheless, as the bending instabilities weretotally suppressed, the fiber diameter obtained was

in micron range.

The system (b) is a collector system comprising a tip

collector, and a support plate. Tip collector is

assembled from the grounded wire electrode withwooden holder. The needle system was placed at an

angle to the tip collector. By controlling the flow

rate, the system was able to align individual

nanofibers.


Control deposition area and densitySetups to control the deposited area and density of electrospun fibres:

(a) regular hexagon multineedle system enclosed inside an iron ring;(b) insulating tube around the needle system;

(c) group of focusing and steering electrodes placed close to collecting substrate;(d) dual rings;(e) three rings;

(f) 2-needle system;(g) wavy-shaped collector;(h) porous-walled cylindrical tube as the dispensing system;

(i) sawlike-patterned dispensing system.


Collectors for yarns productionThese setups are employed to produce nanofibres yarns:(a) self-bundling

electrospinning, the external electrode initiates self-bundling of the nanofibres, which is later collected on the rotating drum;

(b) electrospinning in a liquidcontainer to neutralize the free charges and collect nanofibre bundle;(c) four auxiliary electrodes

electrically activated in sequence to allow the 360°rotation of the jet, the yarn was later collected on the grounded surface.;

(d) dual ring collector, a static ring and a rotating ring;(e) two horizontal needles systems opposite charged.


Liquid container collector

Alignment obtained mechanically by the take-up roller.


Yarns of nanofibres


Dynamic liquid collector

Polym. Eng. Sci. 51 (2011) 323

Nanofibres deposits on a liquid reservoirwith a vortex.

Instead of allowing the nanofibers to

flow down the vortex, the nanofibermesh can be drawn off the water

surface with the vortex twisting theresultant yarn from below as it is being

collected.


Examples of nanofibre yarns


Bundle productionElectrospinning system for fabrication of

a l igned nanofibrous bundles.(a) Schematic of the electrospinning systemconsisting of two grounded conductive pin

electrodes separated by an electrode gap.(b) Schematic representation of a

nanofibrous bundle structure produced bylayer-by-layer fiber deposition.(c−f) SEM images of the fibrous bundle at

di fferent magni fications, where the scalebars indicate 200 μm, 50 μm, 2 μm, and

200 nm from c to f, respectively.(g) Photograph of an aligned nanofibrousbundle formed between the pin electrodes

(white arrows) after electrospinning a 12wt % PCL solution for 30 min at a

spinneret−electrode distance of 12 cm.(h) Photograph of a ∼7 cm long PCL fibrousconstruct. (i ) Photograph of a PCL fibrous

bundle with a diameter of∼2 mm.(j) A typical s tandalone PCL nanofibrous

bundle collected from the pin electrodes.

ACS Appl. Mater. Interfaces 2012, 4,


Inclined water flow

A single layer of nanofibres can becollected on the water surface.

The floating single nanofibre layer

will collapse into a 3D scaffoldwhen lifted off the water surface.

Sci. Technol. Adv. Mater. 12 (2011) 013002


Deposition on cellulose paper

Nanofibre layer weight densitya) 0.02 g/m2

b) 0.1 g/m2

c) 0.5 g/m2


Deposition on textiles

Nanofibre layer is melt-bound to the substrate.


Nonwoven filters

Glass microfibre nonwoven PET microfibre nonwoven


Woven fabrics

Polyamide nanofibres on polyamide plain weave fabric during peeling test.Low adhesion between nanofibres and fabrics is still a problem.

J. Appl. Polym. Sci. 2014, DOI: 10.1002/APP.39766


Charging the collector

Charging the collector is not often used and still not in-depth studied

because requires two high-voltage generator (expensive), but there areadvantages in increasing the nanofibre layer density (deposition zone

becomes small). It is useful (mandatory) in multi-nozzle configurations

(because of the mutual repulsion of jets) and/or using a non-conducting

material to gather nanofibres.In a work, we deposited nanofibre by means of a multi-nozzle (9-nozzle)

electrospinning plant on a textile substrate. Images resulting from the

image-processing procedure applied to photographs of the collectingzones of the textile substrate show that a decrease in the areas of thedeposition zones, an increase in the gaps between the deposition zones,an increase in the distances between the rows of the deposition zonesoccurs. This allows one to suppose that the presence of the non-

conductive element strengthened the mutual Coulomb repulsion. Apossible explanation for this could be that the jets bearing electricalcharges are unable to discharge themselves on a non-conductivesubstrate. The effects of the textile substrate were limited increasing the

charge at the collector.

Polym Int 2010; 59: 1606–1615

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