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University of Ljubljana Faculty of Mathematics and Physics Seminar I Superhydrophobic Surfaces Author: Borut Lampret Mentor: dr. Daniel Svenˇ sek Ljubljana, May 2017 Abstract Although we are creatures that require water to live we prefer our surfaces dry. Either to protect them from degradation, keep them clean or simply because we do not like the touch of wet fabric. Superhydrophobic surfaces fill all these needs and the recent advances in their preparation is why people are starting to show interest in this new technology. This seminar is about the basic behaviour of superhydrophobic surfaces, their variety and uses. It is meant to familiarize the reader with superhydrophobic surfaces in general.

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Page 1: Superhydrophobic Surfaces - University of Ljubljanamafija.fmf.uni-lj.si/.../Superhydrophobic_surfaces.pdf · Superhydrophobic surfaces ll all these needs and the recent advances in

University of LjubljanaFaculty of Mathematics and Physics

Seminar I

Superhydrophobic Surfaces

Author:Borut Lampret

Mentor:dr. Daniel Svensek

Ljubljana, May 2017

Abstract

Although we are creatures that require water to live we prefer our surfaces dry. Either to protectthem from degradation, keep them clean or simply because we do not like the touch of wet fabric.Superhydrophobic surfaces fill all these needs and the recent advances in their preparation is whypeople are starting to show interest in this new technology. This seminar is about the basic behaviourof superhydrophobic surfaces, their variety and uses. It is meant to familiarize the reader withsuperhydrophobic surfaces in general.

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Contents

1 Introduction 2

2 Wetting 22.1 Characteristic Angles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Contact Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Contact Angle Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Roll-Off Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Superhydrophobicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Superhydrophobic Regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Wenzel Regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Cassie-Baxter Regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Superhydrophobic Surfaces 53.1 Engineered Ordered and Disordered Superhydrophobic Surfaces . . . . . . . . . . . . . . . 53.2 Superhydrophobic Diatomaceous Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Volumetric Superhydrophobic coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.4 Omniphobic Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Practical use 74.1 Water Repellency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2 Self Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.3 Viscous Drag Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.4 Anti-icing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.5 Anti-corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.6 Anti-bio-fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5 Summary 9

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1 Introduction

We encounter numerous different surfaces everyday a lot of which are not hydrophobic (i.e. water fearing),much less superhydrophobic. Most surfaces are easily wet which leads to several problems such as corrosion,dirt and ice accumulation, mold, bacterial and plant growth and so on. The more a surface is hydrophobicthe more it is able to resist such degradation. This is the main reason that drives research and developmentof superhydrophobic coatings. With numerous new surfaces being developed every year it is importantto understand some basics of how liquids interact with said surfaces, how are such surfaces characterised,how they differ from one another and what could they be used for.

2 Wetting

Wetting is the ability of a liquid to stay in contact with a surface of a solid surrounded by a gas or animmiscible liquid. It arises from intermolecular interactions when the two materials are brought together.The extent of wetting is controlled by a balance between adhesive and cohesive forces. Adhesive forcesbetween the liquid and the solid force the liquid towards the solid’s surface whereas the cohesive forceswithin the liquid force the liquid towards a spherical shape.

2.1 Characteristic Angles

Figure 1: A diagram of forces acting on the triple-phasecontact line and the associated Young contact angle.

Characteristic angles are used to describe the re-sponse of a liquid drop on a solid surface. Namelythese are the contact angle, the contact angle hys-teresis and the roll-off angle

Contact Angle The contact angle is the anglebetween the liquid drop surface and the surface ofa solid. For an ideally smooth, rigid, planar andhomogeneous surface there exists only one contactangle. It is defined by the equilibrium of the three surface tensions: solid-vapor, solid-liquid, liquid-vapor.This equilibrium is described by the Young’s equation[1]:

γlvcosθ = γsv − γsl (1)

where θ is the Young contact angle and γlv, γsv and γsl are the corresponding surface tensions. Figure1 shows a representation of the equation. Liquids with a contact angle less than 90◦ tend to wett thesurface and those with a contact angle above 90◦ tend not to, if the liquid is water such surfaces are calledhydrophilic and hydrophobic respectively. The contact angle is thus an inverse measure of wettability.

To obtain the Young’s equation one must consider how the total interfacial free energy changes whena liquid drop comes in contact with the solid surface. Free energy of a drop in air is simply a product ofits surface and its surface tension. However when the drop comes in contact with a surface, adhesion workhas to be deducted as shown in equation (2):

Wtotal,liquid = γlv(Asl + Alv)− wa,slAsl (2)

Adhesion work per area (wa,sl) is the amount of energy required to separate the liquid from the solidin a vapor medium.

wa,sl = γsv + γlv − γsl (3)

Equilibrium of the drop is achieved when total free energy is minimal in regards to area:

0 = γlv(dAsl + dAlv)− wa,sldAsl (4)

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by inserting dAlv/dAsl = cosθ (figure 2) and the adhesion work in equation (4) the Young’s equationis derived.

Figure 2: Change of liquid-vapor area in regards tosolid-liquid area and contact angle.

For liquid drops placed on flat, smooth and ho-mogeneous surfaces the contact angle is a functionof surface tensions but an increased roughness ofthe surface can result in a significant decrease orincrease of the contact angle depending on philicor phobic nature of the surface. In this case theYoung’s equation no longer aptly describes the ap-parent contact angle.

Contact Angle Hysteresis The Young’s equa-tion implies that there is a single contact angle forany group consisting of a liquid, solid and a gas.In practice this is not true as real surfaces are notperfect, even the smoothest ones have irregularities.In reality there are two border angles (the dynamic contact angles), the advancing angle (θa) and the re-ceding angle (θr) each made by the advancing and receding liquid respectively. The advancing angle isthe maximum angle that the drop can achieve before it has to move its triple-phase contact line (the linewhere the liquid, solid and gas meet around the droplet) likewise the receding angle is the minimum anglepossible. Figure 3 shows two cases where these angles can be observed. The difference between the twoangles is known as the contact angle hysteresis[2]:

∆θ = θa − θr (5)

Common reasons for the hysteresis are either roughness or heterogeneity of a solid surface. Others includepenetration of the liquid into the solid, swelling of the solid by the liquid, chemical reactions etc.

The contact angle hysteresis is a good measure for stickiness of a surface, in other words it is a measureof motion resistance of a droplet on a surface.[6]

Figure 3: Two different scenarios depicting the dynamic contact angles.

Roll-Off Angle The roll-off angle is the tilt of the surface at which a liquid drop will roll off due togravity. The angle is determined by the balance between gravity and the resistance from contact anglehysteresis. The model predicting the roll-off angle can be written as:

sinω =2γlvDTCL(cosθr − cosθa)

πρgV(6)

where ω is the roll-off angle, γlv and ρ are the surface tension and density of the liquid, g gravitationalacceleration, V the volume of the droplet and DTCL the average diameter of the triple-phase contact line[3].

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Surfaces with a small roll-off angle are know for their self cleaning effect. To achieve this effect,according to equation (6), a smaller contact angle hysteresis is needed and usually this means a highcontact angle.

2.2 Superhydrophobicity

A superhydrophobic surface is such a surface upon which a water droplet exhibits a contact angle greaterthat 150◦. To achieve superhydrophobicity surface roughness in required as the highest contact angle on asmooth surface achieved due to chemistry is around 120◦. The contact angle on a rough surface observedlocally on the microstructre is the same as that on the smooth surface but the apparent contact anglebetween the drop and surface changes dramatically. For a hydrophobic material change in roughness canresult in change in contact angle from around 120◦to almost 180◦thus making it superhydrophobic.

On rough surfaces exhibiting superhydrophobicity two main states or regimes are observed: Wenzeland Cassie-Baxter regimes (others include Lotus state, a state between Wenzel and Cassie and the Geckostate). Notable difference between these two regimes is that the contact angle is smaller and hysteresis islarger for the Wenzel regime than that of the Cassie-Baxter regime.

2.3 Superhydrophobic Regimes

Figure 4: A schematic representation of a droplet on a smooth surface (left) and the two superhydrophobicregimes on a micro rough surface (center, right). It shows how the contact angle can change (however notcompletely accurate as the angle on the left should be larger than 90◦).

Wenzel Regime The Wenzel model describes the homogeneous wetting regime, where the liquid com-pletely penetrates the asperities. This model is can be obtained in the same way as the Young’s equation(eq. 2-4) except that surface roughness is taken into account when considering the change is solid-liquidinterface area (dAsl → rdAsl) and thus arriving to the following equation[4]:

cosθW = rcosθ (7)

where θW is the apparent contact angle which corresponds to the stable equilibrium state. The roughnessratio r is a measure of how roughness affects the area of surface. It is defined as the ratio of the area incontact with the liquid versus the apparent flat area. θ is the Young contact angle as defined for an idealsurface. It is seen from the equation that for a hydrophobic (θ > 90◦) surface the increased roughnessration will result in a larger contact angle. Much like the Young’s equation the Wenzel’s equation doesnot describe the contact angle hysteresis.

Cassie-Baxter Regime When a droplet is placed on rough surface the liquid may not fully penetrateinto the surface structure. In this regime there is at least some air between the liquid drop and thesolid’s surface. Superhydrophobic surfaces reaching contact angles of 180◦ will have a uniform layer of air

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between them and the liquid. Such surfaces also have contact hysteresis and roll-off angles approaching0◦. Provided that the size of asperities is much smaller than the size of the liquid droplet the apparentcontact angle can be obtained by the Cassie-Baxter’s equation[5]:

cosθCB =n∑i

rifi(γi,sv − γi,sl)/γlv =n∑i

rificosθi (8)

this equation predicts the contact angle for a surface made of n types of materials where θCB is the apparentcontact angle, θi is the Young’s contact angle for a specific material, fi is the fraction of the surface thata specific material occupies and ri is the coresponding roughness ratio. Equation (8) can be derived fromYoung’s equation by substituting γsv =

∑ni fiγi,sv, γsl =

∑ni fiγi,sl and taking roughness into account.

The most common case is when there are two materials and one is air. In this case, considering thatcosθair = −1 (air is not wet), the equation simplifies to:

cosθCB = rfcosθ + (1− f)cosθair = rfcosθ + f − 1 (9)

When f = 1, the Cassie–Baxter’s equation becomes the Wenzel’s equation.

3 Superhydrophobic Surfaces

As most things, superhydrophobic surfaces were first found in nature. The lotus leaf is the most famouswith its high contact angle and small hysteresis that results in the self-cleaning effect. Another notablemention is the rose petal where the contact angle hysteresis is large and as a result small droplets stick tothe petal.

Attempts to mimic the lotus leaf effect have been going on for decades and were largely successful,the lotus effect was even improved upon however there are still numerous challenges remaining. Surfaceswith great water repellency have been reported for the past decade but were not commercially availableuntil recently. Major reasons why include their cost, nano structure stability, durability, condensation,impingement and oil wetting issues[7].

3.1 Engineered Ordered and Disordered Superhydrophobic Surfaces

Figure 5: Micrographs of glass spikes (left) and silicon posts (right). Spikes are about 12 µm tall and 7 µmapart. The diameter of silicon posts is about 1 µm and the distance between them is about 4 µm.[8][9]

An example of engineered and ordered surfaces is an array of one micron diameter silicon rods createdusing microelectronics based photo-lithography. The created micro-structure is treated with a hydrophobicsolution to make the surface superhydrophobic. Another example is an array of glass cone spikes createdusing glass fiber drawing techniques. Surface chemistry is then changed to hydrophobic using a fluorinatedsilane solution creating a self-assembled monolayer on top of the cones. This array has the apparent contactangle of 179◦[8]. Both examples are shown in figure 5.

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Figure 6: Micrograps of nano-textured silica (left) and nano-textured fluorinated polymer strands (right).[7][10]

Nano-structured and nano-porous silica produced by differential etching phase separated borosilicateglass is an example of an engineered and disordered surface. Initially it is superhydrophilic until treatedwith a self-assembled monolayer making it superhydrophobic with the contact angle reaching 178◦. Nanos-trands of fluorinated polymers are also a good example of disordered surfaces with contact angles reaching145◦. However these can be matted down and this can reduce the angle to 130◦.[7]

3.2 Superhydrophobic Diatomaceous Earth

Figure 7: A micrograph of a group of diatomskeletons.[7]

Diatomaceous earth (DE) is a natural silica based materialwhich is plentiful all around the world. It consists of mi-croscopic skeletal remains of diatoms, a class of phytoplank-ton. These skeletons possess both micro and nano-porosityand nano-roughness which greatly increases their ability toeither attract or repel water. The nano structure on diatomsurfaces cna be clearly seen in figure 7. Their natural sur-face is hydrophilic which makes diatomaceous earth super-hydrophilic. To get the superhydrophobic effect it has tobe treated with hydrophobic silane. The result is a super-hydrophobic diatomaceous earth (SHDE). A surface coveredwith SHDE powder can reach contact angles as high as 175◦.[7]

3.3 Volumetric Superhydrophobic coatings

Figure 8: A micrograph of a volumetric su-perhydrophobic paint.[7]

A volumetric superhydrophobic coating is such a coating thatis superhydrophobic throughout its entire volume. The mostimportant quality of this coating is that it remains superhy-drophobic even if its surface is scratched away (but not thewhole coating). This basicly solves the durability problem ofsuperhydrophobic surfaces. One way of making such a coat-ing is using SHDE. Simply mixing SHDE with a binding agent(example paint) would not result in the desired effect becausethe nano-pores would be completely wet and lose their hy-drophobic effect once the mixture dries. This problem canbe circumvented if SHDE is first wet by an inert low surfacetension solvent. This would partially protect the SHDE fromthe binding agent. Therefore when the mixture dries and theinert solvent has evaporated some of the SHDE remains superhydrophobic. Figure 8 shows a volumetricpaint surface created using such a method.[7]

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3.4 Omniphobic Surfaces

A slippery liquid-infused porous surface (SLIPS) is a surface with nano-porosity that has been infusedwith a low surface tension liquid as can be seen in figure 9. Essentially the liquid is immobilized inthe nanostrucutre and the surface created on top is super smooth. Such a surface exhibits exceptionalwater and ice repellency, pressure stability, enhanced optical transparency and additionally it repels mostliquids that have a higher surface tension than that of the infused liquid (omniphobic). These surfaces arevolumetric and also capable of self-healing (liquid fills the damaged surface).[7][11][12]

Figure 9: Immobilization of a liquid in a solid and the creation of a super smooth surface.[12]

4 Practical use

4.1 Water Repellency

Water repellency is the most obvious use of superhydrophobic surfaces. A complete list of possible usesis extensive. Some of these uses would be water repelling and breathable clothing, umbrellas, buildingmaterials, paints, epoxies and silicones.

4.2 Self Cleaning

The self cleaning effect allows surfaces to stay clean or simply be cleaned by spraying water. Whileself cleaning is useful for opaque materials its true potential would be reached on objects made out oftransparent materials such as glases, optical lenses and windows.

4.3 Viscous Drag Reduction

Superhydrophobic paints and epoxies could greatly reduce water drag on the hull of a ship (or any waterfairing vehicle) and consequently reduce the cost of transport. The existing technology of drag reductionusing riblets combined with superhydrophobic surfaces can result in a significant drag reduction.[13]

4.4 Anti-icing

Superhydrophobic and liquid-infused superhydrophobic coatings can greatly reduce or even eliminate form-ing of ice on power lines or prevent aircraft and wind turbine icing.[11]

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4.5 Anti-corrosion

Volumetric superhydrophobic coatings are highly porous and it would be expected that they would notprovide adequate protection against corrosion. However testing has shown the opposite (shown in figure10). Liquid-infused superhydrophobic coatings prevent corrosion as well and even do a better job.[7]

Figure 10: Two plates subjected to 1000 hours of salt fog testing. Shown left is the epoxy coated plate while onthe right is a plate coated in a volumetric porous superhydrophobic epoxy paint.[7]

Corrosive effects of salt are the main reason why evaporative desalination has been abandoned, main-tenance costs are just too great. Since volumetric superhydrophobic coatings are salt-water corrosionresistant it is possible that their large scale use could make evaporative desalination commercially viable.Figure 11 shows how salt climbs and binds with the container walls after water evaporates but in the caseof superhydrophobicly coated container the salt does not stick to the wall and piles up in large crystalsthat can be easily removed.[7]

Figure 11: Experiment of salt water evaporation using an aluminium container (left) and a superhydrophobiclytreated aluminium container (right).[7]

4.6 Anti-bio-fouling

Liquid-infused superhydrophobic coatings would be able to reduce or eliminate bio-fouling of ship hulls,piers, water intake systems and other surfaces permanently submerged in water. Figure 12 shows howeffective a liquid-infused coating can be. Such surfaces would also be very useful in medical applicationsas they also prevent biofilms from forming.[11]

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Figure 12: These two Teflon epoxy plates were submerged in an aggressive bio-fauling environment for threemonths. The one on the left was untreated while the one on the right was treated with a liquid-infused superhy-drophobic coating.[7]

5 Summary

This seminar started of with basic behaviour of liquids on surfaces and how is that characterised usingcontact angles, contact angle hysteresis and roll-off angles. It shows the basic idea of how to form superhy-drophobic surfaces and two basic regimes of superhydrophobicity. It describes some examples of differentnatural and artificial surfaces and their characteristics. And finally provides examples of possible uses forcertain superhydrophobic surfaces.

It is very likely that in the future most surfaces that we will encounter outside and likely inside will besuperhydrophobic. Which is logical as we are economically driven and such surfaces would greatly reducecosts in an overwhelming amount of different fields of industry and home use.

References[1] Young T. 1805 Phil. Trans. 95 84; Works, edit by Peacock, 1, 432[2] Good R. J. 1952 A thermodynamic derivation of Wenzel’s modification of Young’s equation for contact

angles; together with a theory of hysteresis Am. Chem. Soc. 74 5041–2[3] C.W. Extrand and A.N. Gent 1990 J. Colloid Interface Sci. 138 (2) 431–442.[4] Wenzel R. N. 1936 Ind. Eng. Chem. 28 988[5] Cassie A. B. D. and Baxter S. 1944 Trans. Faraday Soc. 40 546[6] Choi W. et al 2009 A modified Cassie–Baxter relationship to explain contact angle hysteresis and

anisotropy on non-wetting textured surfaces J. Colloid Interface Sci. 339 208–16[7] J.T. Simpson, S.R. Hunter and T. Aytug 2015 Superhydrophobic materials and coatings:a review,

Rep. Prog. Phys. 78 086501[8] D’Urso B. and Simpson J. T. 2007 Emergence of superhydrophobic behavior on vertically aligned

nanocone arrays Appl. Phys. Lett. 90 044102[9] Krupenkin T. et al 2005 Electrically tunable superhydrophobic nanostructured surfaces Bell Labs

Tech. J. 10 161–70[10] Bormashenko E. et al 2006 Wetting properties of the multiscaled nanostructured polymer and metallic

superhydrophobic surfaces Langmuir 22 9982–5[11] Epstein A. K. et al 2012 Liquid-infused structured surfaces with exceptional anti-biofouling perfor-

mance Proc. Natl Acad. Sci. USA 109 13182–7[12] Wong T. S. et al 2011 Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity

Nature 477 443[13] Barbier C. et al Large drag reduction over superhydrophobic riblets arXiv:1406.0787 [physics.flu-dyn]

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