Super Hydrophobic Surfaces

Science 11 (2006) 193–202www.elsevier.com/locate/cocis

Current Opinion in Colloid & Interface

Superhydrophobic surfaces

Minglin Ma, Randal M. Hill ⁎

Institute for Soldier Nanotechnologies and Department of Chemical Engineering, Massachusetts Institute of Technology,77 Massachusetts Avenue, Cambridge 01239, Massachusetts, USA

Dow Corning Corp., Midland, MI 48686, USA

Available online 11 October 2006

Abstract

Non-wettable surfaces with high water contact angles (WCAs) and facile sliding of drops, called superhydrophobic or ultrahydrophobic, havereceived tremendous attention in recent years. New publications have appeared in the last year documenting many new ways to prepare suchsurfaces—ranging from application driven work to make robust self-cleaning surfaces to careful model studies of patterned surfaces seeking tounderstand the relationship between surface morphology and wettability and droplet sliding. This review summarizes this recent work and looksahead to future developments. The emphasis of the review is on the diverse methods that have been developed to make such surfaces.© 2006 Published by Elsevier Ltd.

Keywords: Superhydrophobic; Water-repellent; Surface; Coating; Self-cleaning; Lotus effect

1. Introduction

Many surfaces in nature are highly hydrophobic and self-cleaning. Examples include the wings of butterflies [1] and theleaves of plants such as cabbage and Indian cress [2••]. Someundesirable plants such as gorse (Ulex europeaus) introducedinto New Zealand, and many common yard weeds have waxyleaf surfaces that make wetting them with water-based her-bicides very difficult. The trisiloxane superwetters [3] weredeveloped for their remarkable ability to wet such hard-to-wetsurfaces and enhance herbicide efficacy. The best knownexample of a hydrophobic self-cleaning surface is the leaves ofthe lotus plant (Nelumbo nucifera). Electron microscopy of thesurface of lotus leaves shows protruding nubs about 20–40 μmapart each covered with a smaller scale rough surface ofepicuticular wax crystalloids [4•]. Numerous studies have con-firmed that this combination of micrometer-scale and nanome-ter-scale roughness, along with a low surface energy materialleads to apparent WCAsN150°, a low sliding angle and the self-cleaning effect [5•]. Surfaces with these properties are called

⁎ Corresponding author. Dow Corning Corp. Visiting Scientist MIT Institute forSoldier Nanotechnologies 500 Technology Square Fourth Floor Cambridge, MA02140, USA. Tel.: +1 617 324 6446.

E-mail address: [email protected] (R.M. Hill).

1359-0294/$ - see front matter © 2006 Published by Elsevier Ltd.doi:10.1016/j.cocis.2006.06.002

“superhydrophobic”. However, some natural examples (thewingsof some insects) do not exhibit two length scales andmany studies(for example, [6••,7]) in which nanostructured surfaces wereprepared have found large contact angles and low sliding anglescalling into question the necessity for a double length scale.

Since the group at Kao [8••] first demonstrated artificialsuperhydrophobic surfaces in the mid-1990s, a very largenumber of clever ways to produce rough surfaces that exhibitsuperhydrophobicity have been reported. Besides water repel-lency, other properties such as transparency and color, an-isotropy, reversibility, flexibility and breathability have alsobeen incorporated into superhydrophobic surfaces. The inten-tion of this article is to provide readers the current status ofstudies on superhydrophobic surfaces, concentrating mainly onpublications appearing in the past year. Useful recent reviewshave been published by Nakajima et al. [5•] and Sun et al. [9•].Quere [10••] critically discussed the surface chemistry of non-sticking surfaces from the original papers by Cassie and Wenzelto the present.

Before we go into the details, it is worth pausing to recallthat the lotus plant achieves an apparent WCAN160° and nilsliding angle using paraffinic wax crystals containing predom-inantly –CH2– groups. Nature does not require the lowersurface energy of –CH3 groups or fluorocarbons to achievethese effects. This plainly demonstrates that extremely lowsurface energy is not necessary to achieve non-wetting. Rather,

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Fig. 1. SEM images of superhydrophobic surfaces made by rougheningfluorinated materials. (a) The honeycomb-patterned film (top image and crosssection) cast from a solution of the polymer shown in the inset under humidconditions [16•] (reproduced by permission of the American Chemical Society).(b) The PPy porous film made by electro- and chemical polymerization andtransition of hydrophobic states (inset) [17••] (reproduced by permission ofWiley-VCH).

194 M. Ma, R.M. Hill / Current Opinion in Colloid & Interface Science 11 (2006) 193–202

the ability to control the morphology of a surface on micron andnanometer length scales is the key. This decoupling of wettingfrom simple surface energy opens up many possibilities forengineering surfaces.

Controlling the wetting of surfaces is an important problemrelevant to many areas of technology. The interest in self-cleaning surfaces is being driven by the desire to fabricate suchsurfaces for satellite dishes, solar energy panels, photovoltaics,exterior architectural glass and green houses, and heat transfersurfaces in air conditioning equipment. Non-wettable surfacesmay also impart the ability to prevent frost from forming oradhering to the surface. The fact that liquid in contact withsuch a surface slides with lowered friction suggests applica-tions such as microfluidics, piping and boat hulls. Most ofthese applications involve solid surfaces, but the emergence offlexible membrane forms should lead to uses in garments andbarrier-membranes—both Cassie and Wenzel were originallyinvolved in work on waterproofing textiles [11,12]. The non-wettable character has been claimed in biomedical applica-tions ranging from blood vessel replacement to woundmanagement. We assume that unexpected applications willemerge as the technology to make non-wettable surfacesmatures—nature uses this property in all known ecosystemsfrom polar bears to ducks to butterflies and water-walkers toplant leaves.

Techniques to make superhydrophobic surfaces can be sim-ply divided into two categories: making a rough surface from alow surface energy material and modifying a rough surface witha material of low surface energy.

2. Roughening a low surface energy material

2.1. Fluorocarbons

Fluorinated polymers are of particular interest due to theirextremely low surface energies. Roughening these polymers incertain ways leads to superhydrophobicity directly [13–15].For example, Zhang et al. [13] reported a simple and effectiveway to achieve a superhydrophobic film by stretching a poly(tetrafluoroethylene) (Teflon®) film. The extended film con-sisted of fibrous crystals with a large fraction of void space inthe surface which was believed responsible for the super-hydrophobicity. Shiu et al. [14] treated a Teflon® film withoxygen plasma and obtained a rough surface with a WCA of168°. Due to their limited solubility, many fluorinatedmaterials have not been used directly but linked [16•] orblended [17••] with other materials (which are often easy toroughen) to make superhydrophobic surfaces. Yabu andShimomura [16•] prepared a porous superhydrophobic mem-brane by casting a fluorinated block polymer solution underhumid environment (Fig. 1a). The membrane was alsotransparent due to the small pore size. Xu et al. [17••]fabricated a double-roughened perfluorooctanesulfonate(PFOS) doped conducting polypyrrole (PPy) film by acombination of electropolymerization and chemical polymer-ization (Fig. 1b). Interestingly, a reversible switching betweensuperhydrophobicity (doped or oxidized state) and super-

hydrophilicity (dedoped or neutral state) was obtained uponchanging the applied electrochemical potential.

2.2. Silicones

Another well-known material with low surface energy ispolydimethylsiloxane (PDMS). Because of its intrinsic deform-ability and hydrophobic property, PDMS can readily bemade into superhydrophobic surfaces using various methods[18–20]. For example, Khorasani et al. [18] treated PDMSusing a CO2-pulsed laser as an excitation source. The WCA forthe treated PDMS was as high as 175° which was believed to bedue to both the porosity and chain ordering on the PDMSsurface (Fig. 2a). Similarly, Jin et al. [19] used a laser etchingmethod to make a rough surface of PDMS elastomer containingmicro-, submicro- and nanocomposite structures. Such a surfaceexhibited a superhydrophobicity with WCA higher than 160°and sliding angle lower than 5°. Sun et al. [20] recently reporteda nanocasting method to make superhydrophobic PDMS sur-face. They first made a negative PDMS template using lotus leafas an original template and then used the negative template tomake a positive PDMS template—a replica of the original lotusleaf. The positive PDMS template (Fig. 2b) had the samesurface structures and superhydrophobicity as the lotus leaf.

Fig. 2. SEM images of superhydrophobic surfaces made by roughening silicone-based materials. (a) PDMS surface treated by CO2-pulsed laser [18] (reproduced bypermission of Elsevier); (b) lotus leaf-like PDMS surface by nanocasting [20]; (c) PS-PDMS/PS electrospun fiber mat and the droplets on it [21•]; (d) PS-PDMS surfacecast from a 5 mg/ml solution in dimethylformamide (DMF) in humid air [22]. (Fig. 2b, c and d are reproduced by permission of the American Chemical Society.)

195M. Ma, R.M. Hill / Current Opinion in Colloid & Interface Science 11 (2006) 193–202

Given the difference in composition and consequent surfaceenergy between the lotus leaf (paraffinic wax crystals, –CH2–,30–32 mN/m) and the PDMS replica (–CH3, 20 mN/m), thesimilarity of the hydrophobicity obtained is surprising.

Another way to exploit the low surface energy of PDMS is touse a block copolymer such as poly(styrene-b-dimethylsilox-ane) (PS-PDMS). For instance, Ma et al. [21•] made a super-hydrophobic membrane in the form of a nonwoven fiber mat byelectrospinning a PS-PDMS block copolymer blended with PShomopolymer (Fig. 2c). The superhydrophobicity with WCA of163° was attributed to the combination of enrichment of PDMScomponent on fiber surfaces and the surface roughness due tosmall fiber diameters (150 nm to 400 nm). The flexibility,breathability and free-standing feature of the membrane are ofparticular interest in areas such as textile and biomedical appli-cations. More recently, Zhao et al. [22] prepared a super-hydrophobic surface by casting a micellar solution of PS-PDMSin humid air based on the cooperation of vapor-induced phaseseparation and surface enrichment of PDMS block (Fig. 2d).

2.3. Organic materials

Although fluorocarbons and silicones are known as hydro-phobic materials, nature achieves non-wetting and self-cleaningusing paraffinic hydrocarbons. Recently, several groups havedemonstrated superhydrophobic surfaces made from organicmaterials. Lu et al. [23•] proposed a simple and inexpensive

method to produce a highly porous superhydrophobic surface ofpolyethylene (PE) by controlling its crystallization behavior. WCAup to 173° was obtained by adding nonsolvent (cyclohexanone) tothe PE/xylene solution to form nanostructured floral-like crystalstructures (Fig. 3a). Jiang et al. [24••] showed that by electrostaticspinning and spraying a PS solution in dimethylformamide (DMF)they obtained a superhydrophobic film composed of porousmicroparticles and nanofibers as shown in Fig. 3b. Lee et al. [25]produced vertically aligned PS nanofibers by using nanoporousanodic aluminum oxide as a replication template in a heat- andpressure-driven nanoimprint pattern transfer process. As the aspectratio of the PS nanofibers increased, the nanofibers could not standupright but formed twisted bundles resulting in a three-dimension-ally rough surface (Fig. 3c) with advancing and receding WCA of155.8° and 147.6°, respectively.

Other organic materials such as polyamide [26], polycar-bonate [27] and alkylketene dimer [28] have also recently beenmade into superhydrophobic surfaces. Yan et al. [29•] syn-thesized a poly(alkylpyrrole) film by electrochemical polymer-ization—the needle-like poly(alkylpyrrole) structures grownperpendicularly to the surface of the electrode yielded anenvironmentally stable superhydrophobicity (Fig. 3d).

2.4. Inorganic materials

Certain inorganic materials have also been made into super-hydrophobic surfaces. For example, superhydrophobic surfaces

Fig. 3. SEM images of superhydrophobic surfaces by roughening organic materials. (a) Floral-like crystal structures of PE [23•]; (b) PS surface made by electrostaticspinning and spraying [24••]; (c) aligned PS nanofibers replicated from nanoporous anodic aluminum oxide [25] (reproduced by permission of the American ChemicalSociety); (d) double-roughened poly(alkylpyrrole) film made by electrochemical polymerization. Scare bar: 15 μm [29•]. (Fig. 3a, b and d are reproduced bypermission of Wiley-VCH).


have been produced from ZnO [30••,31] and TiO2 [32]. Fig. 4ashows SEM image of the ZnO nanorods Feng et al. [30••]synthesized via a two-step solution method. The ZnO nanorodfilms were superhydrophobic due to the surface roughness and thelow surface energy of the (001) plane of the nanorods on the surfaceof the film as confirmed by X-ray diffraction (see the inset of Fig.4a). Interestingly, the superhydrophobicity was transformed tosuperhydrophilicity upon UV irradiation which generated electron-hole pairs and resulted in hydroxyl adsorption in the ZnO surface.Dark storage of the UV irradiated film for 7 days made itsuperhydrophobic again. Similarly, they also obtained TiO2

nanorod films with reversibly switchable wettability (Fig. 4b).

3. Making a rough substrate and modifying it with lowsurface energy materials

Methods to make superhydrophobic surfaces by rougheninglow surface energy materials are mostly one-step processes andhave the advantage of simplicity. But they are always limited to asmall set of materials. Making superhydrophobic surfaces by a

Fig. 4. SEM of superhydrophobic surfaces by roughening inorganic materials(a) Aligned ZnO nanorods prepared by a two-step solution approach. The insetsshow the XRD pattern and hydrophobicity transition of the nanorod film [30••

(reproduced by permission of the American Chemical Society). (b) TiO2

nanorod film and a single papilla at high magnification [32] (reproduced bypermission of Wiley-VCH).

.

]


totally different strategy, i.e., making a rough substrate first andthen modifying it with a low surface energy material, decouplesthe surface wettability from the bulk properties of the material andenlarges potential applications of superhydrophobic surfaces.

There are many ways to make rough surfaces, includingthose mentioned above such as mechanical stretching, laser/plasma/chemical etching, lithography, sol–gel processing andsolution casting, layer-by-layer and colloidal assembling,electrical/chemical reaction and deposition, electrospinningand chemical vapor deposition. There are also several methodscommonly used to modify the chemistry of a surface. Forexample, covalent bonds can be formed between gold and alkylthiols. Silanes are often used to decrease the surface energy.Physical binding, adsorption and coating can also change thesurface chemistry. In this section, we will focus on the importanttechniques reported in the past two years to make rough surfaces(not necessarily from low surface energy materials) andsubsequent modifications of the surface chemistry. Althoughmany of these studies are directed towards making practicalsurfaces, some important model studies are included here also.

3.1. Etching and lithography

Etching is a straightforward and effective way to make roughsurfaces. Different etching methods including plasma etching

Fig. 5. Superhydrophobic surfaces produced by etching and lithography. (a) AFM ima[33] (reproduced by permission of Elsevier). (b) SEM image of the aluminum surfacshape of a droplet on the surface after fluoroalkylsilane coating (inset) [35•]. (c) SEM[39]. (d) SEM image of the nanopillars after hydrophobization the base diameter of ththe American Chemical Society.)

[14,33], laser etching [19,34] and chemical etching [35•,36]have all been used in the past year to fabricate superhydrophobicsurfaces. For example, Teshima et al. [33] obtained a transparentsuperhydrophobic surface from a poly(ethylene terephthalate)(PET) substrate via selective oxygen plasma etching followed byplasma-enhanced chemical vapor deposition using tetramethyl-silane (TMS) as the precursor (Fig. 5a). Qian and Shen [35•]described a simple surface roughening method by dislocation-selective chemical etching on polycrystalline metals such asaluminum (Fig. 5b). After treatment with fluoroalkylsilane, theetched metallic surfaces exhibited superhydrophobicity.

Lithography (e.g. photolithography, electron beam lithogra-phy, X-ray lithography, soft lithography, nanosphere lithogra-phy and so on) is a well-established technique for creating large-area periodic micro-/nanopatterns [37••,38]. Abdelsalam et al.[39] conducted a systematic study of the wetting of structuredgold surfaces formed by electrodeposition through a template ofsubmicrometer spheres and discussed the role of the pore sizeand shape in controlling wetting (Fig. 5c). Martines et al. [6••]fabricated ordered arrays of nanopits and nanopillars by usingelectron beam lithography and plasma etching. They obtained asuperhydrophobicity with WCA of 164° and hysteresis of 1° fora surface consisting of tall pillars with cusped tops (see Fig. 5dfor the SEM image) after a hydrophobization with octadecyl-tricholorosilane. Composite surfaces with superhydrophobic

ge of the PET surfaces coated with TMS layer after the oxygen plasma treatmentes etched with a Beck's dislocation etchant for 15 s at room temperature and theimage of a gold film electrodeposited through a submicrometer sphere templatee pillars is about 120 nm [6••]. (Fig. 5b, c and d are reproduced by permission of

Fig. 6. Superhydrophobic surfaces prepared by sol–gel process. (a) SEM imageof the methyltriethoxysilane (MTEOS) sol–gel foam. The insets: phenolphtha-lein in water on the sol–gel foam heated to 390 °C (left) and 400 °C (right)[41••] (reproduced by permission of the Royal Society of Chemistry). (b) AFMimage of sol–gel film containing 30 wt.% colloidal silica. Image area: 5×5 μm2;the inset shows surface condensation of water vapor on this film [42](reproduced by permission of the American Chemical Society).

Fig. 7. Double-roughened superhydrophobic surfaces made by (a) layer-by-layerassembly [49••] and (b) colloidal assembly [52••] (reproduced by permission ofthe American Chemical Society).


and superhydrophilic patterns have also been reported by Notsuet al. [40] using photolithography.

3.2. Sol–gel processing

Sol–gel processes have been used to fabricate superhydro-phobic surfaces from a variety ofmaterials [30••,32,41••,42–44].In most of these investigations, no post-process hydrophobizationwas used for the achievement of superhydrophobicity since thelow surface energy materials were already included in the sol–gelprocess. For example, Shirtcliffe et al. [41••] prepared poroussol–gel foams from organo-triethoxysilanes which exhibitedbinary switching between superhydrophobicity and superhydro-philicity when exposed to different temperatures (Fig. 6a). Hikitaet al. [42] used colloidal silica particles and fluoroalkylsilane asthe starting materials and prepared a sol–gel film with super-liquid-repellency by hydrolysis and condensation of alkoxysilanecompounds (Fig. 6b). Instead of blending low surface energymaterials in the sols, Shang et al. [43] described a procedure tomake transparent superhydrophobic surface by modifying silica-based gel films with a fluorinated silane. On a similar note, Wuet al. [44] made a microstructured ZnO-based surface via a wet-chemical process and obtained the superhydrophobicity aftercoating the surface with long-chain alkanoic acids.

3.3. Layer-by-layer (LBL) and colloidal assembly

LBL self-assembly is a rich process to fabricate conformalthin film coatings with molecular level control over film thick-ness and chemistry using electrostatic interaction and hydrogenbonding. Recently, LBL process has been used by several groupsto make rough surfaces for superhydrophobicity [45–48]. Forexample, Zhai et al. [49••] used an LBL technique to create apH-sensitive poly(allylamine hydrochloride)/poly(acrylic acid)(PAH/PAA) multilayer which formed a honeycomb-like struc-ture on the surface after an appropriate combination of acidictreatments. After cross-linking the structure, they depositedsilica nanoparticles on the surface via alternating dipping of thesubstrates into an aqueous suspension of the negatively chargednanoparticles and an aqueous PAH solution, followed by a finaldipping into the nanoparticle suspension. Fig. 7a shows the SEMimage of the resultant hierarchically roughened structures.Superhydrophobicity was obtained after the surface was modifiedby a chemical vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane followed by a 2 h thermalannealing. LBL self-assembly could also be combined withelectrochemical deposition to prepare superhydrophobic surfacesas shown by Zhang et al. [45,46,50•] during the past 2 years.

Assembly from colloidal systems has also been demonstrat-ed to provide right surface roughness for superhydrophobicity[51•,52••,53]. Ming et al. [52••] prepared a double roughened

Fig. 8. SEM images of the superhydrophobic surfaces made by electrochemical reaction and deposition or chemical bath deposition. (a) Dendritic gold cluster formedon an ITO electrode modified with a polyelectrolyte multilayer [50•]. (b) Copper surface with dual-scale roughness and the droplet on it (inset) [54•] (reproduced bypermission of Wiley-VCH). (c) The copper surface after electrochemical reaction with sulfur gas [55] (reproduced by permission of the Royal Society of Chemistry).(d) The BCH-LA nanopin films and the contact angle (inset) [56••]. (Fig. 8a and d are reproduced by permission of the American Chemical Society.)


surface consisting of raspberry-like particles which were madeby covalently grafting amine-functionalized silica particles of70 nm to epoxy-functionalized silica particles of 700 nm via thereaction between epoxy and amine groups (Fig. 7b). The surfacebecame superhydrophobic after being modified with PDMS.

3.4. Electrochemical reaction and deposition

Electrochemical reaction and deposition has been extensivelyused to prepare superhydrophobic surfaces. For instance, Zhanget al. [50•] demonstrated that the surface covered with dendriticgold clusters, which was formed by electrochemical depositiononto indium tin oxide (ITO) electrode modified with a poly-electrolyte multilayer, showed superhydrophobic propertiesafter further deposition of a n-dodecanethiol monolayer. (SeeFig. 8a for typical structures of the gold clusters.) Shirtcliffe et al.[54•] prepared a double-roughened copper surface whichresembled “chocolate chip cookies” by electrodeposition andpatterning technique. Further hydrophobization with fluorocar-bons yielded a superhydrophobicity with WCA of 160°(Fig. 8b). Cho's group [55] recently described the fabricationof lotus leaf-like superhydrophobic metal surfaces by using

electrochemical reaction of Cu or Cu–Sn alloy plated on steelsheets with sulfur gas, and subsequent perfluorosilane treatment(Fig. 8c). It is worth mentioning that chemical bath deposition(CBD) has also been used to make nanostructured surfaces. Forexample, Hosono et al. [56••] used this technique to fabricate ananopin film of brucite-type cobalt hydroxide (BCH) andobtained a WCA as high as 178° after further modification oflauric acid (LA) (Fig. 8d).

3.5. Other methods

Electrospinning is powerful technique to make ultrafine fibersand has been found by several groups to provide sufficient surfaceroughnesses for superhydrophobicity [15,21•,24••,57–59••,60].Electrospinning a hydrophobic material led to superhydrophobi-city in one step. Ma et al. [59••] showed that an even highersuperhydrophobicity with WCAs up to 175° could be obtainedby applying a thin layer of conformal coating of a fluorinatedpolymer to electrospun mats by initiated chemical vapor de-position (iCVD). This paper contained also a useful discussion ofthe impact of fiber morphology on superhydrophobicity (seeFig. 9a).

Fig. 9. (a) Superhydrophobic surfaces by modifying electrospun fiber mats witha fluorinated polymer. The plot shows the effect of fiber morphology on thecontact angles [59••]; (b) SEM image of ZnO-coated CNTs. The insets are aTEM of a single coated CNT and the contact angle on the surface [63](reproduced by permission of the American Chemical Society).

• Of special interest.•• Of outstanding interest.


Chemical (or physical) vapor deposition (CVD or PVD) hasbeenwidely used for themodification of surface chemistry aswellas the synthesis of nanostructured surfaces [61–63]. Huang et al.[63] reported a stable superhydrophobic surface composed ofaligned carbon nanotubes (CNTs) synthesized by chemical vapordeposition on an Fe–N catalyst layer. The superhydrophobicitywas achieved after a thin layer coating of ZnO (see Fig. 9b).

4. Conclusions and perspectives

This is an active research field with many publicationsappearing each month dealing with methods to make super-hydrophobic surfaces and theoretical understanding of the re-lationship between surface morphology and wettability/sliding.Theoretical understanding is maturing with models appearingcapable of dealing with more complex forms of surface mor-phology. Techniques to make non-wettable solid surfaces usingpolymers or sol–gel chemistry have been widely documented.Other types of surfaces, including flexible membrane forms arereceiving more attention. Making such surfaces in a microfluidicdevice will probably be demonstrated shortly. Studies that presentresults for other aspects such as drag reduction are just beginningto appear.

Many studies in this area present only superficial results forwettability and slip—frequently only a contact angle is cited

without contact angle hysteresis or sliding angle values. Details ofmethods used tomeasure these quantities are often notmentioned.Though central to this topic, the concept of “roughness” isdifficult to reduce to a model that includes all the diverse formsfound in nature ormade in the laboratory.While the simple Cassieand Wenzel models provide a useful framework to understandhigh contact angles, a more detailed model will be necessary torelate hysteresis to roughness. A recent attempt to broaden thescope of “roughness” has been made by Nosonovsky andBhushan [64]. Contact angles N170° have been reported, forwhich the Cassie model calculates about 1% contact between thewater and the solid surface [65]. This seems problematic anddemands direct experimental validation. It is worth noticing thatan alternative view of superhydrophobicity or hysteresis from theperspective of contact line rather than contact area has beenextensively reported in particular by Oner and McCarthy [66••].

A fundamental question about superhydrophobic surfacesespecially for potential applications is the robustness of theeffect. Theoretical modeling of the transition between the het-erogeneous wetting (Cassie) state and the homogeneous wetting(Wenzel) state [67,68], hydraulic pressure experiments [69,70]and water condensation experiments [71] have been reported inthe past year to provide new insight into this problem. BothCheng et al. [71] and Jin et al. [72] found high contact anglesand sticky drops, which challenges the perception that roughsurfaces lead to Cassie states.

Acknowledgments

This research was supported in part by the U.S. Armythrough the Institute for Soldier Nanotechnologies, underContract DAAD-19-02-D-0002 with the U.S. Army ResearchOffice. The content does not necessarily reflect the position ofthe Government, and no official endorsement should beinferred. The authors would like to acknowledge the supportof Dow Corning Corporation and many useful discussions withGregory Rutledge.

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[23•] Lu XY, Zhang CC, Han YC. Low-density polyethylene superhydro-phobic surface by control of its crystallization behavior. MacromolRapid Commun 2004;25:1606–10.This paper described a simplemethod to make superhydrophobic surfaces from a simple polymer.

[24••] Jiang L, Zhao Y, Zhai J. A lotus-leaf-like superhydrophobic surface: aporous microsphere/nanofiber composite film prepared by electrohydro-dynamics. AngewChem Int Ed 2004;43:4338–41.Thiswas the first reportof superhydrophobic surface by electrostatic spinning and spraying.

[25] Lee W, Jin M-K, Yoo W-C, Lee J-K. Nanostructuring of a polymericsubstrate with well-defined nanometer-scale topography and tailoredsurface wettability. Langmuir 2004;20:7665–9.

[26] Zhang J, Lu X, Huang W, Han Y. Reversible superhydrophobicity tosuperhydrophilicity transition by extending and unloading an elasticpolyamide film. Macromol Rapid Commun 2005;26:477–80.

[27] Zhao N, Xu J, Xie QD,Weng LH, Guo XL, Zhang XL, et al. Fabricationof biomimetic superhydrophobic coating with a micro-nano-binarystructure. Macromol Rapid Commun 2005;26:1075–80.

[28] Mohammadi R, Wassink J, Amirfazli A. Effect of surfactants on wettingof super-hydrophobic surfaces. Langmuir 2004;20:9657–62.

[29•] Yan H, Kurogi K, Mayama H, Tsujii K. Environmentally stable superwater-repellent poly(alkylpyrrole) films. Angew Chem Int Ed2005;44:3453–6. This paper reported super water-repellent filmswhich also showed excellent environmental stability to both temper-ature and organic solvents and oils.

[30••] Feng XJ, Feng L, Jin MH, Zhai J, Jiang L, Zhu DB. Reversible super-hydrophobicity to super-hydrophilicity transition of aligned ZnOnanorod films. J Am Chem Soc 2004;126:62–3.This was the firstreport of reversible switching between superhydrophobicity andsuperhydrophilicity made from inorganic materials.

[31] Yang YH, Li ZY, Wang B, Wang CX, Chen DH, Yang GW. Self-assembled ZnO agave-like nanowires and anomalous superhydropho-bicity. J Phys, Condens Matter 2005;17:5441–6.

[32] Feng XJ, Zhai J, Jiang L. The fabrication and switchable superhydropho-bicity of TiO2 nanorod films. Angew Chem Int Ed 2005;44:5115–8.

[33] Teshima K, Sugimura H, Inoue Y, Takai O, Takano A. Transparent ultrawater-repellent poly(ethylene terephthalate) substrates fabricated byoxygen plasma treatment and subsequent hydrophobic coating. ApplSurf Sci 2005;244:619–22.

[34] Song XY, Zhai J, Wang YL, Jiang L. Fabrication of superhydrophobicsurfaces by self-assembly and their water-adhesion properties. J PhysChem, B 2005;109:4048–52.

[35•] Qian BT, Shen ZQ. Fabrication of superhydrophobic surfaces bydislocation-selective chemical etching on aluminum, copper, and zincsubstrates. Langmuir 2005;21:9007–9.This paper described a simplemethod to make superhydrophobic metal surfaces.

[36] Guo ZG, Zhou F, Hao JC, LiuWM. Stable biomimetic super-hydrophobicengineering materials. J Am Chem Soc 2005;127:15670–1.

[37••] Furstner R, Barthlott W, Neinhuis C, Walzel P. Wetting and self-cleaning properties of artificial superhydrophobic surfaces. Langmuir2005;21:956–61.This paper investigated the wetting and self-cleaningproperties of various artificial superhydrophobic surfaces.

[38] Callies M, Chen Y, Marty F, Pepin A, Quere D. Microfabricatedtextured surfaces for super-hydrophobicity investigations. Microelec-tron Eng 2005;78–79:100–5.

[39] Abdelsalam ME, Bartlett PN, Kelf T, Baumberg J. Wetting of regularlystructured gold surfaces. Langmuir 2005;21:1753–7.

[40] Notsu H, Kubo W, Shitanda I, Tatsuma T. Super-hydrophobic/super-hydrophilic patterning of gold surfaces by photocatalytic lithography.J Mater Chem 2005;15:1523–7.

[41••] Shirtcliffe NJ, McHale G, Newton MI, Perry CC, Roach P. Porousmaterials show superhydrophobic to superhydrophilic switching. ChemCommun 2005:3135–7.This paper demonstrated porous materialswhich were either completely filled with liquid or not wetted at allwhen exposed to different temperatures.

[42] Hikita M, Tanaka K, Nakamura T, Kajiyama T, Takahara A. Super-liquid-repellent surfaces prepared by colloidal silica nanoparticlescovered with fluoroalkyl groups. Langmuir 2005;21:7299–302.

[43] Shang HM, Wang Y, Limmer SJ, Chou TP, Takahashi K, Cao GZ.Optically transparent superhydrophobic silica-based films. Thin SolidFilms 2005;472:37–43.

[44] Wu X, Zheng L, Wu D. Fabrication of superhydrophobic surfaces frommicrostructured ZnO-based surfaces via a wet-chemical route. Lang-muir 2005;21:2665–7.

[45] Shi F, Wang ZQ, Zhang X. Combining a layer-by-layer assemblingtechnique with electrochemical deposition of gold aggregates to mimicthe legs of water striders. Adv Mater 2005;17:1005–9.

[46] Zhao N, Shi F, Wang ZQ, Zhang X. Combining layer-by-layer assemblywith electrodeposition of silver aggregates for fabricating super-hydrophobic surfaces. Langmuir 2005;21:4713–6.

[47] Jisr RM, Rmaile HH, Schlenoff JB. Hydrophobic and ultrahydrophobicmultilayer thin films from perfluorinated polyelectrolytes. AngewChem Int Ed 2005;44:782–5.

[48] Han JT, Zheng Y, Cho JH, Xu X, Cho K. Stable superhydrophobicorganic–inorganic hybrid films by electrostatic self-assembly. J PhysChem, B 2005;109:20773–8.


[49••] Zhai L, Cebeci FC, Cohen RE, Rubner MF. Stable superhydrophobiccoatings from polyelectrolyte multilayers. Nano Lett 2004;4:1349–53.This paper demonstrated a relatively simple process for creating highlystable superhydrophobic coatings with structures that conceptuallymimicked the lotus leaf.

[50•] Zhang X, Shi F, Yu X, Liu H, Fu Y, Wang ZQ, et al. Polyelectrolytemultilayer as matrix for electrochemical deposition of gold clusters:toward super-hydrophobic surface. J Am Chem Soc 2004;126:3064–5.This paper proposed a new way for the fabrication of superhydrophobicsurfaces by combining LBL technique and electrochemical deposition.

[51•] Shiu JY, Kuo CW, Chen PL, Mou CY. Fabrication of tunable su-perhydrophobic surfaces by nanosphere lithography. Chem Mater2004;16:561–4.This paper described a fabrication method for creatingwell-ordered nanostructured surfaces whose hydrophobicity could bemodeled and tuned.

[52••] Ming W, Wu D, van Benthem R, de With G. Superhydrophobic filmsfrom raspberry-like particles. Nano Lett 2005;5:2298–301.This paperreported a procedure for preparing robust superhydrophobic surfaceswith hierarchical roughness.

[53] Zhang G, Wang D, Gu ZZ, Mohwald H. Fabrication of superhydrophobicsurfaces from binary colloidal assembly. Langmuir 2005;21:9143–8.

[54•] Shirtcliffe NJ, McHale G, Newton MI, Chabrol G, Perry CC. Dual-scaleroughness produces unusually water-repellent surfaces. Adv Mater2004;16:1929–32.This paper had a useful discussion on the effect ofdual-scale roughness on superhydrophobicty.

[55] Han JT, Jang Y, Lee DY, Park JH, Song SH, Ban DY, et al. Fabricationof a bionic superhydrophobic metal surface by sulfur-inducedmorphological development. J Mater Chem 2005;15:3089–92.

[56••] Hosono E, Fujihara S, Honma I, Zhou HS. Superhydrophobicperpendicular nanopin film by the bottom-up process. J Am Chem Soc2005;127:13458–9.This paper reported the highest contact angle so fardocumented.

[57] Acatay K, Simsek E, Ow-Yang C, Menceloglu YZ. Tunable, super-hydrophobically stable polymeric surfaces by electrospinning. AngewChem Int Ed 2004;43:5210–3.

[58] Gu Z-Z, Wei H-M, Zhang R-Q, Han G-Z, Pan C, Zhang H, et al.Artificial silver ragwort surface. Appl Phys Lett 2005;86:201915/1–3.

[59••] Ma M, Mao Y, Gupta M, Gleason KK, Rutledge GC. Superhydrophobicfabrics produced by electrospinning and chemical vapor deposition.

Macromolecules 2005;38:9742–8.This paper provided a useful insightinto the role of nanofibermorphology in thewettability of nonwovenmats.

[60] ShangHM,WangY, Takahashi K, CaoGZ, LiD,Xia YN. Nanostructuredsuperhydrophobic surfaces. J Mater Sci 2005;40:3587–91.

[61] Liu H, Feng L, Zhai J, Jiang L, Zhu DB. Reversible wettability of achemical vapor deposition prepared ZnO film between superhydropho-bicity and superhydrophilicity. Langmuir 2004;20:5659–61.

[62] Zhu LB, Xiu YH, Xu JW, Tamirisa PA, Hess DW, Wong CP.Superhydrophobicity on two-tier rough surfaces fabricated by con-trolled growth of aligned carbon nanotube arrays coated withfluorocarbon. Langmuir 2005;21:11208–12.

[63] Huang L, Lau SP, Yang HY, Leong ESP, Yu SF, Prawer S. Stablesuperhydrophobic surface via carbon nanotubes coated with a ZnO thinflm. J Phys Chem, B 2005;109:7746–8.

[64] Nosonovsky M, Bhushan B. Roughness optimization for biomimeticsuperhydrophobic surfaces. Microsyst Technol 2005;11:535–49.

[65] Cassie ABD. Contact angles. Discuss Faraday Soc 1948;3:11–6.[66••] Oner D, McCarthy TJ. Ultrahydrophobic surfaces. Effects of topogra-

phy length scales on wettability. Langmuir 2000;16:7777–82.Thispaper contains an important discussion on three-phase contact lines.

[67] Li W, Amirfazli A. A thermodynamic approach for determining thecontact angle hysteresis for superhydrophobic surfaces. J ColloidInterface Sci 2005;292:195–201.

[68] Krasovitski B, Marmur A. Drops down the hill: theoretical study oflimiting contact angles and the hysteresis range on a tilted plate.Langmuir 2005;21:3881–5.

[69] Zheng QS, Yu Y, Zhao ZH. Effects of hydraulic pressure on the stabilityand transition of wetting modes of superhydrophobic surfaces.Langmuir 2005;21:12207–12.

[70] Journet C, Moulinet S, Ybert C, Purcell ST, Bocquet L. Contact anglemeasurements on superhydrophobic carbon nanotube forests: effect offluid pressure. Europhys Lett 2005;71:104–9.

[71] Cheng YT, Rodak DE, Angelopoulos A, Gacek T. Microscopicobservations of condensation of water on lotus leaves. Appl Phys Lett2005;87:194112.

[72] Jin MH, Feng XJ, Feng L, Sun TL, Zhai J, Li TJ, et al. Super-hydrophobic aligned polystyrene nanotube films with high adhesiveforce. Adv Mater 2005;17:1977–81.

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Super Hydrophobic Surfaces