instability mechanisms of electrically charged liquid jets in electrospinning vs. electrospraying

INSTABILITY MECHANISMS of ELECTRICALLY CHARGED LIQUID

JETS in ELECTROSPINNING vs. ELECTROSPRAYING

A.L. Yarin

Department of Mechanical Eng. UIC, Chicago

Acknowledgement

• D.H. Reneker• E. Zussman• A.Theron• S.N. Reznik• A.V. Bazilevsky• C.M. Megaridis• R. Srikar, S.Sinha Ray• Israel Science Foundation, Volkswagen Stiftung-

Germany, National Science Foundation through grants NSF-NIRT CBET 0609062 and NSF-NER-CBET 0708711-U.S.A.

Outline

1. Basic physics of the process: bending 2. Branching 3. Multiple jets 4. Needleless electrospinnning 5. Buckling 6. Self-assembly: Nanoropes and crossbars 7. CNT-containing nanofibers 8. Co-electrospinning: nanotubes&nanofluidics

Queen Elizabeth I was interested in electricity

William Gilbert made experiments for the Queen

In 1600 Gilbert published a book on his experiments

Modern Reproduction

Modern reproduction

Modern reproduction

G.I.Taylor’s Experiments with Glycerin

Electrospraying

Modern reproduction

Modern reproduction

Splaying

Nanofibers (Definition)

30,000 Volt

Basic Physics of Electrospinning

Electrospinning Setup

Process Initiation: Taylor Cone

Yarin A L, Reneker D H, Kombhongse S, J. App. Phys. 90, 2001

Theoretical Model of Theoretical Model of Jet InitiationJet Initiation

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

ExperimentExperimenton Jet Initiationon Jet Initiation

ExperimentExperimenton Jet Initiationon Jet Initiation

# 1

# 2 # 3

Experiment vs. Experiment vs. TheoryTheory

-1.0 -0.5 0 0.5 1.0

r

2.0

1.5

1.0

0.5

0

z

109

8

98

76

7

65

54

43

21

21

10

3

ExperimentExperimentvs. Theoryvs. Theory

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

The Reynolds number

The electrical Bond number

20

E

a EBo

0 0V aRe

The initial contact angle

Theoretical Model of JetTheoretical Model of JetInitiationInitiation

1.5

1.0

0.5

0.5

1.0

1.5

2.0

r

2

1

z

0

z

EBo 5.06

/ 3

The critical semi-angle: Taylor-49.3º,

The analytical model, numerics&experiment-33.5º

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

EBo 5.29

/ 3

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

EBo 3.24

/ 2

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

EBo 2.25

0.8

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

EBo 2.25

0.8

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

EBo 2.25

0.82

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

E

E

E

E

1 Bo 1.1

2 Bo 1.2

3 Bo 1.3

4 Bo 2.25

0.8

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

1 – radial velocity at the surface2 – vertical velocity at the surface

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

Critical electric Bond number vs. static contact angle

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

Predicted electric current vs. applied voltage

Theoretical ModelTheoretical Modelof Jet Initiationof Jet Initiation

Predicted convective and conductiveparts of the electric current

E

E

E

E

1 Bo 5.29

2 Bo 9

3 Bo 16

4 Bo 25

– Dielectric constant

– Electric conductivity

– Surface tension

a0 – Droplet diameter

– Viscosity

– Mass density

V0 – Characteristic fluid velocity in droplet

V* – Characteristic velocity in jet

l – Characteristic length scale

H – Hydrodynamic characteristic time

C – Characteristic charge relaxation time

Re – Reynolds number

Electrically-driven bending instability

A collection of point charges cannot be maintained at equilibrium: Earnshaw theorem

The Electrospinning Mechanism

1. Reneker D H, Yarin A L, Fong H, Koombhongse S, J. App. Phys. 87, 20002. Reznik S N, Yarin A L, Theron A, Zussman E, J. Fluid Mech. 516, 2004

The “Taylor cone” droplet

Jet initiation

Modern reproduction

Modern reproduction

Basic Equations: Discretized Quasi-one-dimensional Equations

Electrically-driven Bending Instability

time

i

i= N

i+ 1

i - 1

i = 1

i =1

F0 ~ q.E

Fve ~ velocity difference

Fc ~ coulomb force

i =1

i =2

time

i = 1

i = 101

time

i + 1i

i - 1

Fcap ~ surface tension effects from local curvature and cross section

Electrospinning of Polymer Solutions

Reneker D H, Yarin A L, Fong H, Koombhongse S, J. App. Phys. 87, 2000

Yarin A L, Koombhongse S, Reneker D H, J. App. Phys. 89, 2001

Electrospinning of Polymer Solutions

Reneker, Yarin, Fong, Koombhogse

Electrospinning of Polymer Solutions

Reneker, Yarin, Fong, Koombhongse

0 ms 16.5 ms 18 ms 22 ms

24.5 ms 30.5 ms 31.5 ms 32 ms

37.5 ms 38.5 ms

Reneker D H, Yarin A L, Fong H, Koombhongse S, J. App. Phys. 87, 2000

Nanofiber Garlands

Reneker D H, Kataphinan W, Theron A, Zussman E, Yarin A L, Polymer 43, 2002

Electrospinning of PCL photographed at 2000fps (playback speed = 30fps)

2m

8m

200nm 20m

1m1m

As-spun Polymer Nanofibers

PEO

PCL

SiloxanePolyacrylic acid

PVA PPV

Branching in PCL Electrospinning

Yarin A L, W. Kataphinan, D.H. Reneker J. Appl. Phys. 98, 064501 (2005)

Branching in PCL Electrospinning

Branching in PCL Electrospinning

Experiment with a 3x3Experiment with a 3x3 SetupSetup

S.A.Theron, A.L. Yarin, E. Zussman, E. Kroll, Polymer, 46, 2005

Experiment with a 9x1 Experiment with a 9x1 SetupSetup

Multiple JetMultiple Jet ElectrospinningElectrospinning

Theoretical ModelTheoretical Modelof 3x3 Multiple Jetsof 3x3 Multiple Jets

Theoretical ModelTheoretical Modelof 3X3 Multiple Jetsof 3X3 Multiple Jets

Theoretical ModelTheoretical Modelof 3x3 Multiple Jetsof 3x3 Multiple Jets

Theoretical ModelTheoretical Modelof 3x3 Multiple Jetsof 3x3 Multiple Jets

Theoretical ModelTheoretical Modelof 9x1 Multiple Jetsof 9x1 Multiple Jets

Theoretical ModelTheoretical Modelof 9x1 Multiple Jetsof 9x1 Multiple Jets

Theoretical ModelTheoretical Modelof 9x1 Multiple Jetsof 9x1 Multiple Jets

a

b

c

f

d

e

H

Upward Needleless Electrospinning of Multiple Nanofibers

a- Layer of magnetic fluidb- Layer of polymer solution

Yarin&Zussman Polymer 45, 2977-2980 (2004)

Magnetic Fluid Cones

Perturbed Outer Surface of Polymer Solution

Electrospinning of Multiple Nanofibers

As-spun Nanofibers

Buckling of Electrified Jets

Han,Reneker,Yarin, Polymer 48, 6064-6076 (2007)

Buckling of Electrified Jets

Han,Reneker,Yarin, Polymer 48, 6064-6076 (2007)

Self-assembly: Nanoropes and Crossbars.A Sharpened Wheel – Electrsostatic Lens

Experimental setup

Plot of the electric field strength in the region of the wheel

Tip of

the wheel

Axis of

the wheel

Theron A, Zussman E, Yarin A L, Nanotechnology 12, 2001

Tip of wheel

Tip of syringe

3D Nano-structuresA rotating table on the wheel collector enables collection of multiple nanofiber layers at different angles.

Theron A, Zussman E, Yarin A L, Nanotechnology 12, 2001

Aligned Nanofibers: 2D Arrays

Fiber Diameter: 100nm - 500nm

Pitch: 1m - 1.5m

2m

2m

Nanoropes

5m

2m

Diameter: 50nm - 100nmPitch: 2 m - 3m.

3D Nanocrossbars

Theron A, Zussman E, Yarin A L, App. Phys. Lett. 82, 2003

0 180

Sink flow

CNT

~200 nm

CNTs in Polymer Solution (PEO)

CNT Alignment During Electrospinning of Polymer Solutions

Dror Y, Salalha W, Khalfin R, CohenY,Yarin A L, Zussman E, Langmuir 19, 2003; Langmuir, 20, 2004.

CNTs Embedded and Aligned in Electrospun Nanofibers

50nm

50nm

Single-wall carbon nanotubes embedded in nanofiber

Multi-wall carbon nanotube

embedded in nanofiber

1st CNT

2st CNT

Overlap area

Co-electrospinning: Compound Nanofibers

Solution: PEO (1e6) 1% in ethanol/water

Inner solution contains 2% bromophenolOuter solution contains 0.2% bromophenol

Sun Z, Zussman E, Yarin A L, Wendorff J H, Greiner A, Advanced Materials 15, 2003

and Nanotubes

Co-electrospinning

Zussman E, Yarin A L, Bazilevsky A.V., R. Avrahami, M. Feldman, Advanced Materials 18, 2006

Core: PMMA

Shell: PAN

Carbonization

Core: PMMA

Shell: PAN

Turbostratic Carbon Nanotubes

Core: PMMA

Shell: PAN

Core Entrainment Problems

Core: PMMA

Shell: PAN

Numerical Simulation: Numerical Simulation: Core-shell JetCore-shell Jet

(a) (b)

S.N. Reznik, A.L. Yarin, E. Zussman, L. Bercovici. Phys. Fluids v. 18, 062101 (2006)

Co-electrospinning with Protrusion

Zussman E, Yarin A L, Bazilevsky A.V., R. Avrahami, M. Feldman, Advanced Materials v. 18, 348-353 (2006).

Stress level at the interface:~ 5000 dyne/(square cm)

Optical appearance of a PMMA/PAN emulsion about 1 dayafter mixing of a homogeneous blend containing 6 wt% PMMA + 6% PAN in DMF

Core-Shell Nanofibers from PMMA-PAN Emulsion

A.V.Bazilevsky,A.L. Yarin,C.M. MegaridisLangmuir v.23,2311-2314 (2007).

Experimental set-up and hollow carbon tubes

Schematic of the modeled PAN/DMF flow around a sphericalPMMA/DMF droplet trapped over the core-shell Taylor cone

2

r

2

2

r

p v0

1 p v0

R

v 1 (v sin )0

R sin

The Stokes equations inLubrication approximation

'

r r

r

R R R, h( ) ecos

From the continuity eq : v O(Rv / ) v

From the momentum eq :[ p / ]/[ p / R ]

v / v / R

Then, p p( )

Integrating the momentum balance eq using the no-slip&free surface conditions, we find

2

2

h dp dpv

R d 2 R d

and the average velocity in the gap is

h dpV

3 R d

Then intergrating the continuity eq over the gap

3

we arrive at the Reynolds eq

d dp[h sin ] 0

d d

3 3

r

Integrating the Reynolds eq, find pressure

3 Qp (p 0)

2 [1 (e / )]

Also, / p O( / R) 1

2

3 1/ 2

The tip is stretched by the traction

and resists elastically :

2G(L / ) P

which yields

L / [( Q) / ]

For Q 1 mL / h, 0.1s, 0.005 cm,

we deduce L / 10.

Fine emulsion should result in multi-core fibers

Conclusion

(i) Sophisticated nanofibers and nanotubes are relatively easily

achievable.

(ii) In situ self-assembly is possible.

(iii) Jet bending is the leading mechanism. Branching is secondary. No splaying.

(iv) Modeling is quite reliable for jet initiation and bending stages in

both single- and multiple-jets cases.

(v) Core-shell nanofibers and hollow nanotubes can be made.

(vi) Co-electrospun nanofluidics is possible.

(vii) Bio-medical applications are tempting and challenging.