High contact angle hysteresis of superhydrophobic surfaces: Hydrophobic defectsFeng-Ming Chang, Siang-Jie Hong, Yu-Jane Sheng, and Heng-Kwong Tsao

Citation: Applied Physics Letters 95, 064102 (2009); doi: 10.1063/1.3204006 View online: http://dx.doi.org/10.1063/1.3204006 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effective three-dimensional superhydrophobic aerogel-coated channel for high efficiency water-droplettransport Appl. Phys. Lett. 104, 081601 (2014); 10.1063/1.4866262 An easy route to make superhydrophobic surface AIP Conf. Proc. 1447, 767 (2012); 10.1063/1.4710229 Using sharp transitions in contact angle hysteresis to move, deflect, and sort droplets on a superhydrophobicsurface Phys. Fluids 24, 062001 (2012); 10.1063/1.4723866 A simple method for measuring the superhydrophobic contact angle with high accuracy Rev. Sci. Instrum. 81, 065105 (2010); 10.1063/1.3449325 Nanodesign of superhydrophobic surfaces J. Appl. Phys. 106, 024305 (2009); 10.1063/1.3176484

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High contact angle hysteresis of superhydrophobic surfaces:Hydrophobic defects

Feng-Ming Chang,1 Siang-Jie Hong,1 Yu-Jane Sheng,2,a� and Heng-Kwong Tsao1,a�

1Department of Chemical and Materials Engineering, National Central University, Jhongli 320, TaiwanR.O.C.2Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan R.O.C.

�Received 30 May 2009; accepted 12 July 2009; published online 11 August 2009�

A typical superhydrophobic surface is essentially nonadhesive and exhibits very low water contactangle �CA� hysteresis, so-called Lotus effect. However, leaves of some plants such as scallion andgarlic with an advancing angle exceeding 150° show very serious CA hysteresis. Although surfaceroughness and epicuticular wax can explain the very high advancing CA, our analysis indicates thatthe unusual hydrophobic defect, diallyl disulfide, is the key element responsible for contact linepinning on allium leaves. After smearing diallyl disulfide on an extended polytetrafluoroethylene�PTFE� film, which is originally absent of CA hysteresis, the surface remains superhydrophobic butbecomes highly adhesive. © 2009 American Institute of Physics. �DOI: 10.1063/1.3204006�

The wetting of solid surfaces by water droplets is ubiq-uitous in our daily lives as well as in industrial processes.Wettability in terms of the contact angle �CA� between thegas-liquid and solid-liquid interfaces is one of the most im-portant properties associated with materials. The wettabilityof an ideal flat solid in terms of CA � is described by Youngequation, cos �= ��sg−�ls� /�lg, where �sg, �ls, and �lg repre-sent the interfacial tensions of solid-gas, liquid-solid, andliquid-gas, respectively. Typically, a surface is termed supe-rhydrophobic when water CA is very high, typically exceed-ing 150°.1 Recently, natural superhydrophobicity has at-tracted great interest owing to its importance in the inspiredmimetic attempts, such as the self-cleaning property of Lotusleaves, referred to as the Lotus effect.2

In general, the wetting behavior is governed by the twofollowing factors: the chemical composition and the geo-metrical microstructures of the solid surfaces. The mecha-nisms responsible for the influence of surface roughnesswere explained by Wenzel3 and later by Cassie and Baxter.4

Wenzel assumed that the liquid filled up the grooves on therough surface and generalized Young equation to have theapparent CA cos �a=r cos �, where r is termed the “rough-ness factor” and defined as the ratio of the actual area of arough surface to the projected area on the horizontal plane.Therefore, surface roughness can enhance hydrophobicity for��90°. On the other hand, according to the Cassie and Bax-ter model, the superhydrophobic surface is considered as asurface consisting of two types of homogeneous patches thathave different solid-fluid interfacial tensions.5 Since therough structure is mainly filled with air, the openings of thegrooves correspond to nonwetting patches with the CA �=180°. As a result, the apparent CA is given by cos �a= f cos �+ �1− f�cos 180°, where f represents the surfacearea fraction wetted by the liquid.

Microscopically, real solids are actually rough and con-tain hydrophilic blemishes. It is often found that the CA isnot unique and depends on whether the liquid is advancingover the surface or receding. This is known as CA hysteresis,which is generally expressed in terms of the difference be-

tween the advancing and receding angle ���=�A−�R�. Theadvancing and receding angles are typically obtained by dy-namic sessile drop method, i.e., inflating and deflating thedroplet volume, respectively. The advancing angle refers tothe maximum angle associated with adding volume while thereceding angle corresponds to the smallest possible angleupon removing volume. The CA hysteresis is accompaniedwith the pinning of the contact line as the volume of a drop-let is withdrawn. In general, it is believed that the CA hys-teresis originates from hydrophilic chemical defects.6 Duringretraction, the contact line tends to get trapped at certainposition on the surface because solid surfaces are usually nothomogeneous on a microscopic scale.

On the superhydrophobic surface, most of the area isoccupied by air due to surface roughness and a small fractionof area that supports a water droplet is made of hydrophobicmaterials, such as polytetrafluoroethylene �PTFE� and paraf-fin. As a result, the CA hysteresis is insignificant. A typicalexample is given in Fig. 1 for an extended Teflon film, whichis a rather simple but yet effective way to obtain a superhy-

a�Electronic addresses: yjsheng@ntu.edu.tw and hktsao@cc.ncu.edu.tw.

FIG. 1. �Color online� The CA and BD are plotted against the volume as thedroplet is inflated and then deflated on an extended Teflon film �superhydro-phobic surface without hysteresis�.

APPLIED PHYSICS LETTERS 95, 064102 �2009�

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drophobic surface.7 The CAs are plotted against the dropletvolume when the drop is inflated and then deflated. Theshapes of sessile drops were recorded at an elapse time of 90s and analyzed at room temperature on a Krüss DSA10 CAmeasuring system. On extended Teflon films, the advancingand receding CAs are essentially the same, about 150°. Thecontact line pinning is not obvious since the base diameter�BD� is altered with changing the volume.

Recently, it has been reported that unlike lotus leaf, rosepetal exhibits superhydrophobicity with CA about 152.4° butsignificant hysteresis.8 While a water droplet with size lessthan capillary length can roll off the surface of a lotus leafeffortlessly, it stays pinned to the surface of a red rose petal.In fact, the droplet maintains the shape of spherical cap whenit is turned upside down. The different behavior of wettingbetween lotus leaf and rose petal has been explained by thediverse design in the surface microstructure. Since the rosepetal’s microstructures are larger in scale than the lotus leaf,the liquid is allowed to impregnate the larger scale grooves,but cannot enter into the smaller grooves. This is referred toas the Cassie impregnating wetting regime, in which the wet-ted surface area is less than that in the Wenzel regime butgreater than that in the Cassie–Baxter regime. Such an ex-planation implies that the extent of hysteresis rises with in-creasing wetted surface area, which varies with surface mi-crostructure.

In this letter the CA hysteresis of scallion and garlicleaves has been investigated to demonstrate the importanceof the surface chemistry of the wetted materials in additionto surface microstructure. Scallion and garlic are genus alli-ums and frequently used as fundamental components inAsian food recipes. As shown in Fig. 2�a�, the CAs of waterdroplets with volume about 2 �L on a horizontal leaf ofscallion �Allium fistulosum L.� or garlic �Allium sativum L.�are very high, exceeding 150°, and thus their surfaces aresuperhydrophobic. Similar to the lotus leaf, their high waterrepellency can be attributed to micro- and nanoscopic archi-tecture of the leaf surface, which is covered with epicuticularwax. The typical scanning electronic micrographs �SEMs� ofscallion and garlic leaves are shown in Fig. 3 to illustratetheir micro- and nanostructures. As the leaf is tilted verti-cally, however, the droplet remains staying on the leaf, asdemonstrated in Fig. 2�b�. This consequence reveals a sig-nificant CA hysteresis associated with superhydrophobicscallion and garlic leaves. Figure 4 shows the variation of the

CAs with the droplet volume on a scallion leaf upon inflationand deflation. Although the advancing angle can be as highas 160°, the angle continues decreasing to about 80° as thevolume is deflated to less than 0.5 �L. Evidently, this CAhysteresis is associated with the contact line pinning demon-strated by constant BD during deflation. Even though we areunable to determine the receding �lowest� angle, one can stillconclude that the CA hysteresis is very significant, ���80°.

The presence of surface wax among terrestrial plant spe-cies is ubiquitous. Epicuticular wax functions mainly to shedwater and prevent nonstomatal water loss.9 The combinationof epicuticular wax and surface microstructure leads to supe-rhydrophobicity of plant leaf. Although impregnation mayresult in a certain extent of CA hysteresis, it may not be theonly factor. In fact, it cannot explain the lack of hysteresisfor a significantly impregnated wax substrate without nano-structure, such as Parafilm. Consequently, chemical defectsthat display somewhat hydrophilic characteristics may be re-sponsible for such severe hysteresis. In addition to epicuticu-lar wax, a significant amount of organosulfur compounds,such as diallyl disulfide �DADS�, is found in the leaf ofplants of the genus allium. Intuitively, those compounds

FIG. 2. �Color online� �a� Water droplets on horizontal leaves of garlic andscallion �CA exceeding 150°�. �b� Water droplets on vertically tilted leaves�CA hysteresis�. FIG. 3. �Color online� The typical SEM images of scallion and garlic leaves

illustrate the micro and nanostructures.

FIG. 4. �Color online� CA and BD are plotted against the volume as thedroplet is inflated and then deflated on a scallion leaf �superhydrophobicsurface with serious hysteresis�.

064102-2 Chang et al. Appl. Phys. Lett. 95, 064102 �2009�

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might play the role of chemical defects and be responsiblefor the CA hysteresis. Indeed, along with diallyl trisulfideand diallyl tetrasulfide, DADS is one of the principal com-ponents of the distilled oil of garlic.

In order to verify the role of DADS, one can examine thewetting behavior of a superhydrophobic surface doping withDADS �Alfa Aesar�. DADS is a yellow liquid with a stronggarlic order at room temperature. We smear it on the ex-tended Teflon film, which shows no hysteresis originally asdepicted in Fig. 1. After the surface is dried, SEM imageshows no change in surface microstructure and the water CAon such a composite surface is measured. As shown in Fig. 5,the variation of the CA with the droplet volume demonstratethat CA hysteresis on the Teflon film becomes very seriousdue to the presence of DADS, ���60°. From the compari-son between Figs. 1 and 5, doping of diallyl disulfide onsuperhydrophobic surfaces alter the advancing angle slightly��A�140°� but reduces the receding angle very seriously.The small reduction in advancing CA may be caused by theblockage of the pores on the Teflon film10 by DADS grease.This consequence confirms that diallyl disulfide functions aschemical defects on superhydrophobic surfaces.

DADS is a small molecule �C6H10S2� with the density1.03 g /cm3. Its solubility in water is only 0.006% at 25 °Cand thus is typically regarded as a hydrophobic substance.Accordingly, it is somewhat surprising that DADS can func-tion as chemical defects. However, if one places a drop ofDADS on the water surface, it will spread over the surfaceas thin yellow film, instead of forming an oil lens. The wet-

ting behavior of liquid �o� on water surface is determined bythe spreading coefficient S=�wg− ��og+�ow�. When S�0,spreading occurs and the surface is considered wettable. Inthis case, �wg�70 mJ /m2 and �og�30 mJ /m2. DADS pre-fers wetting water surface because the interfacial energybetween DADS and water is not high at all ��ow

�40 mJ /m2�. Evidently, the free energy of isolated DADSmolecules in water is distinctly different from that of DADSfilm on water. In the latter case, DADS molecules can ar-range themselves together to a structure so that DADS mol-ecules become to favor in contact with water surface. Thispicture can be used to explain the high CA hysteresis on asuperhydrophobic surface. The structure of surface DADSmolecules may depend on the phase they are exposed to, i.e.,whether water or air. When the surface is in contact withwater, the rearrangements of DADS make themselves hydro-philic and lead to contact line pinning, i.e., serious hyster-esis.

Scallion and garlic leaves possess superhydrophobic sur-faces but high CA hysteresis. The superhydrophobicitycomes from epicuticular wax on surface microstructures andthe high CA hysteresis originates from unusual hydrophobicdefects. Although DADS molecules are essentially insolublein water, they spread spontaneously on water surface to forma film. This consequence reveals that a superhydrophobicsurface smeared with DADS becomes highly adhesive owingto the relatively low surface energy between water andDADS blemishes. Like lotus effect, the understanding of themechanism of the high hysteresis on a superhydrophobic sur-face may provide us a biomimetic possibility.

This research work is financially supported by BASFElectronic Materials Taiwan Ltd. and National ScienceCouncil of Taiwan under Grant Nos. 98-2221-E-008-051-MY3 and 98-2221-E-002-085.

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FIG. 5. �Color online� The variation of CA and BD with the volume as thedroplet is inflated and then deflated on an extended Teflon film smeared withDADS �superhydrophobic surface with serious hysteresis�.

064102-3 Chang et al. Appl. Phys. Lett. 95, 064102 �2009�

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