Langmuir 2008, 24, 5411-5417
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Beyond the Lotus Effect: Roughness Influences on Wetting over a Wide Surface-Energy Range Doris M. Spori,† Tanja Drobek,† Stefan Zürcher,†,| Mirjam Ochsner,† Christoph Sprecher,‡ Andreas Mühlebach,§ and Nicholas D. Spencer*,† Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland, AO Research Institute, ClaVadelerstrasse, 7270 DaVos, Switzerland, Ciba Specialty Chemicals, Klybeckstrasse 141, 4002 Basel, Switzerland, SuSoS AG, Lagerstrasse 14, 8600 Dübendorf, Switzerland
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ReceiVed January 22, 2008. ReVised Manuscript ReceiVed February 24, 2008 To enhance our understanding of liquids in contact with rough surfaces, a systematic study has been carried out in which water contact angle measurements were performed on a wide variety of rough surfaces with precisely controlled surface chemistry. Surface morphologies consisted of sandblasted glass slides as well as replicas of acidetched, sandblasted titanium, lotus leaves, and photolithographically manufactured golf-tee shaped micropillars (GTMs). The GTMs display an extraordinarily stable, Cassie-type hydrophobicity, even in the presence of hydrophilic surface chemistry. Due to pinning effects, contact angles on hydrophilic rough surfaces are shifted to more hydrophobic values, unless roughness or surface energy are such that capillary forces become significant, leading to complete wetting. The observed hydrophobicity is thus not consistent with the well-known Wenzel equation. We have shown that the pinning strength of a surface is independent of the surface chemistry, provided that neither capillary forces nor air enclosure are involved. In addition, pinning strength can be described by the axis intercept of the cosine-cosine plot of contact angles for rough versus flat surfaces with the same surface chemistries.
Introduction For many practical applications, such as coating or fluid handling, the wettability of a surface plays a crucial role. The contact angle, θY, that a liquid drop makes with an ideally flat surface corresponds to a minimum in the energy of the liquid–solid-ambient gas system1 (see Figure 1). The prediction of contact angles for real surfaces presents a significant challenge, however, since it is well known that roughness exerts a significant influence over wetting phenomena.2–5 Only on ideally flat, uniform surfaces does θY have a unique value. On real surfaces, depending on how the drop is deposited, the contact angle θ can vary between the so-called advancing and receding contact angles. This hysteresis can be ascribed to inhomogeneities in the distribution of adsorbates or the presence of contaminants, to surface roughness, or to time-dependent surface rearrangements.2 On rough surfaces, the surface morphology strongly influences the value of θ. On rough, hydrophobic surfaces, the liquid can either follow the surface topography and show strong pinning or can bridge from asperity to asperity while enclosing air beneath and showing almost no hysteresis in the contact angle. For the first case, Wenzel6 introduced a roughness factor, r, to describe the roughness influence on θ (eq 1).
cos θW ) r · cos θY
(1)
* To whom correspondence should be addressed. E-mail: spencer@ mat.ethz.ch. † ETH Zurich. | SuSoS AG. ‡ AO Research Institute. § Ciba Specialty Chemicals. (1) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65–87. (2) Degennes, P. G. ReV. Mod. Phys. 1985, 57, 827–863. (3) Dettre, R. H.; Johnson, R. E. AdV. Chem. 1964, 43. (4) Genzer, J.; Efimenko, K. Biofouling 2006, 22, 339–360. (5) Quere, D. Physica A 2002, 313, 32–46.
Figure 1. Scheme of wetting phenomena: (a) definition of contact angle on an ideally flat surface; (b) Wenzel-type wetting; (c) Cassie-type wetting; (d) pinning, a growing drop pinned by one obstacle; and (e) hemiwicking.
where r is calculated by dividing the actual, roughness-enhanced surface area by its projection. This behavior is often referred to as Wenzel-type wetting. If the cosines in eq 1 are plotted versus each other, the effect of roughness is evident in the deviation from a straight line with slope 1. For the second case, Cassie and Baxter7 modified Wenzel’s equation by introducing the fractions f1 and f2, where f1 corresponds to the area in contact with the liquid divided by the projected area, and f2, to the area in contact with the air trapped
10.1021/la800215r CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
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beneath the drop, also divided by the projected area (Supporting Information discussion 1):
influence, it is suggested that the axis intercept be surfacemorphology-sensitive.
cos θCB ) f1 · cos θY - f2
Methods
(2)
Since the introduction of these equations, wetting on rough surfaces has been the subject of intensive research,2,3 which was significantly boosted by the discovery of the “self-cleaning” properties of the superhydrophobic lotus leaves (“lotus effect”).8 Although the many publications on this topic have increased our knowledge of superhydrophobic behavior,4,9–13 showing, for example, that the most stable topographies to achieve superhydrophobicity are “undercut”,14–16 many aspects of this field still remain controversial.17–20 Structured surfaces that exhibit superhydrophobicity can also show an effect known as hemiwicking5 or superwetting if they are surface-chemically functionalized to be hydrophilic. Hemiwicking is complete wetting due to the presence of capillary forces in two dimensions.21,22 We have chosen to examine wetting-property changes upon significant variation in the surface chemistry of samples with four different surface topographies. This is of both fundamental and practical relevance, since many surface coatings change their surface chemistry over time due to contamination or oxidation. Surfaces have been analyzed by scanning electron microscopy and roughness factors evaluated by means of white-light profilometry. Static (θs) and dynamic (θa, θr) contact angles have been measured. To vary surface chemistry over a wide range of surface energies, the surfaces were coated with gold and subsequently functionalized by means of mixed, self-assembled monolayers of methyl- and hydroxyl-terminated alkane thiols. In this way, contact angles between 20° and 105° can be obtained on a flat surface. Surfaces examined were sandblasted glass microscope slides (SBG), as well as replicas of sandblasted (large grit), acid-etched titanium (SLA); lotus leaves (LLR); and golftee-shaped micropillars of photoresist (GTM) on a silicon wafer. The GTM pillars show an extraordinarily stable, Cassie-type hydrophobicity. All four surface topographies are uniformly rough, such that it does not make a difference where a drop is placed. This precondition, as emphasized by McHale,19 has to be met to be able to compare contact angle data with eqs 1 and 2. By applying the Wenzel equation over this wide range of surface chemistries, the roughness factor fails to predict the data. If the θ data are presented in a cosine-cosine plot of rough vs smooth θ for the same surface chemistries, the three major classes of behavior (“hemiwicking”, “pinned”, “Wenzel- or Cassie-type” wetting (see Figure 1)) can be readily distinguished. For the surface-energy range where pinning has the most profound (6) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (7) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 0546–0550. (8) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (9) Barbieri, L.; Wagner, E.; Hoffmann, P. Langmuir 2007, 23, 1723–1734. (10) Bhushan, B.; Chae Jung, Y. Ultramicroscopy 2007, 107, 1033–1041. (11) Blossey, R. Nat. Mater. 2003, 2, 301–306. (12) Callies, M.; Quere, D. Soft Matter 2005, 1, 55–61. (13) Patankar, N. A. Langmuir 2003, 19, 1249–1253. (14) Ahuja, A.; Taylor, J. A.; Lifton, V.; Sidorenko, A. A.; Salamon, T. R.; Lobaton, E. J.; Kolodner, P.; Krupenkin, T. N. Langmuir 2008, 24, 9–14. (15) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. ReV. 2007, 36, 1350–1368. (16) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618–1622. (17) Gao, L. C.; McCarthy, T. J. Langmuir 2007, 23, 3762–3765. (18) Gao, L. C.; McCarthy, T. J. Langmuir 2007, 23, 13243–13243. (19) McHale, G. Langmuir 2007, 23, 8200–8205. (20) Panchagnula, M. V.; Vedantam, S. Langmuir 2007, 23, 13242–13242. (21) Extrand, C. W.; Moon, S. I.; Hall, P.; Schmidt, D. Langmuir 2007, 23, 8882–8890. (22) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103.
Substrates. Glass microscope slides were sandblasted with an air pressure of 8 bar. The sand jet was passed over the microscope slide twice for about 15 s in perpendicular directions to achieve a homogeneously roughened surface. Subsequently, these SBG substrates were ultrasonicated in ethanol for 10 min, rinsed with ethanol, and dried under a stream of nitrogen. GTM masters were produced by standard photolithography on 4-in. silicon wafers upon which, after cleaning, SU-8 2025 negative photoresist (Microchem) was spin-coated for 60 s at 2000 rpm to a thickness of 30 µm and soft-baked on a hotplate for 2 min at 65 °C and 3 min at 95 °C. Subsequently, the wafer was exposed to UV light (MA6, Karl Süss) with constant intensity (total energy 180 mJ/cm2) through a chromium mask having circular patterns of 20 µm diameter and 70 µm pitch distance (center to center). After postbaking for 2 min at 65 °C and 3 min at 95 °C, the wafers were developed for 5 min in SU-8 developer. Finally, the wafer was hard-baked at 190 °C for 10 min. The prepared wafer was used as a master to make replicas. Replicas of acid-etched, large-grit-sandblasted (SLA) titanium (Straumann, Switzerland), the SU-8 pillar-structured silicon wafer, and lotus leaves (Nelumbo nucifera) obtained from the Botanical Garden in Zürich were prepared according to Wieland et al.23 In short, a low-viscosity and fast-curing silicone (PROVILnovo, Light C.D. 2, fast set; Heraeus Kulzer GmbH, Germany) was cast in a mold affixed to the surface to be replicated. This silicone sample, exhibiting the original’s negative structure was then used in turn as a mold to cast positive replicas with an epoxy blend (EPO-TEK 302-3; Epoxy Technology, Billerica, MA). The epoxy was then cured for 5 h at 60 °C, removed from the mold, and subsequently postcured for 1 h at 150 °C. The lotus leaf replicas (LLR) were sawn into 5 × 15 mm2 pieces. The SLA replicas were cast in a circular mold of 12 mm diameter. All replicas were cleaned in a 2 vol % solution of Hellmanex (Hellma, Germany) in ultrapure water (resistance 18.2 MΩ, EASYpure by Barnstead, Dubuque, IA) and subsequently rinsed five times with ultrapure water. Single-side-polished silicon wafers (Si-Mat Silicon Materials, Germany) were cut into 10 × 10 mm2 pieces. To remove glue residues from the cutting step, they were sonicated for 10 min in toluene and 10 min in ethanol. Gold Coating. The rough surfaces and single-side-polished silicon wafers were cleaned for 2 min in air plasma and then coated by resistance evaporation (MED 020 coating system, BALTEC, Liechtenstein) with 10-15 nm of Cr and 50 nm of Au (purity >99.99%, Unaxis, Liechtenstein). The rough surfaces were rotated during evaporation, and the pillars were also tilted by 25° to achieve a homogeneous coating. The gold-coated silicon wafers were used in every experiment as a flat reference. Surface Modification. To achieve a wide surface-energy range, self-assembled monolayers (SAMs) of 11-mercaptoundecanol and dodecanethiol (Aldrich Chemicals, USA) were chemisorbed in both pure and mixed forms on freshly gold-coated samples. The samples were immersed in 0.1 mM ethanolic thiol solutions for 20 min. For the solution preparation, the total thiol concentration of 0.1 mM was held constant, and the composition of the two compounds was varied in terms of dodecylthiol molar ratio from 100, 70, 50, 45, 30, 15, 10 (GTM only) to 0%. After assembly, the samples were rinsed with ethanol (purity >99.8%, Merck, Germany) and dried under a stream of nitrogen. Contact Angle Measurements. Static water contact angle (θs) measurements were performed on a Ramé-Hart contact-angle goniometer on freshly prepared surfaces. A drop of 6-8 µL was produced and then gently placed on the surface. For superhydrophobic (23) Wieland, M.; Chehroudi, B.; Textor, M.; Brunette, D. M. J. Biomed. Mater. Res. 2002, 60, 434–444.
Beyond the Lotus Effect surfaces, the drop had to be enlarged up to 12 µL for the drop to be able to detach from the syringe. The contact angle is then defined as depicted in Figure 1. Dynamic water contact angle (advancing (θa) and receding (θr)) measurements were performed on a Krüss contact-angle-measuring system (G2/G40 2.05-D, Krüss GmbH, Germany) with a speed of 15 µL/min. Two movies with 40 images were recorded for the advancing contact angle and only one for the receding. Analysis was carried out by means of the tangent method 2 routine of the Krüss Drop-Shape Analysis program (DSA version 1.80.0.2 for Windows 9x/NT4/2000, 1997–2002 Kruess). The movies were evaluated as follows: If the drop exhibited a stick-jump behavior, but moved over the entire period of recording, all contact angles were evaluated. In this way, the stick-jump led to a high standard deviation. If one side of the drop did not move at all during the recording, only the other, moving side was evaluated. In the superhydrophobic case, since the drop is confined between the syringe and the surface while being very strongly repelled, it becomes squeezed out at one side. In this case, only the mobile, squeezed-out side was evaluated. The tangent method 2 routine, a fourth-order polynomial function, has difficulty in fitting drops in the Cassietype wetting regime, leading to a systematic underestimation of a few degrees. For many experiments, the receding contact angle could not be defined, since the drop was so strongly pinned to the surface. A value of 0° was assumed for these cases. Several exemplary movies can be found in the Supporting Information. Scanning Electron Microcopy. Replicas and SBGs were analyzed in a Zeiss Gemini 1530 FEG SEM at 3-5 kV, at room temperature, gold-coated, as described above. White Light Profilometry. For the evaluation of the roughness factor, an optical profilometer was used (FRT MicroGlider, Fries Research & Technology GmbH, Germany). X and Y resolution were 1 µm, Z resolution was better than 10 nm. Areas of 2 × 2 mm2 were measured with a resolution of 1000 × 1000 pixels. The data points were assembled into adjacent, nonoverlapping triangles, and their areas were summed up to achieve the actual surface area. The roughness factor was then obtained by dividing this actual surface area by the analyzed area (2 mm2).
Results and Discussion Surface-Morphology Characterization. SEM analysis of the investigated surfaces reveals considerable differences in surface morphology (Figure 2). The SBG (Figure 2a and b) shows a very profound microroughness with very rough patches, nanosized features, and very flat conchoidal fracture areas. The SLA replica surface (Figure 2c and d) also contains two major roughness scales. The microscale roughness originates from the sandblasting step, leading to troughs. The superimposed nanoscale roughness was created by the acid-etching process.24–26 The LLR (Figure 2e and f) shows the wavy structure of the lotus leaf cells topped with papillae. The papillae exhibit a diameter of 9 ( 2 µm, a spacing of 21 ( 7 µm, and a height of around 20 µm. The nanoscale roughness is an approximate replica of the leaf’s wax structure, which was compressed during the replication process (Supporting Information 1). The GTM pillars (Figure 2g and f) measure 24 µm in diameter on top and thin out to a diameter of 15 µm toward the bottom. Their height slightly exceeds 30 µm, and the pitch distance (center to center) is 70 µm. The pillar tops constitute 9% of the projected geometric surface area. The pillar surface is highly ordered, whereas the other three samples show nonperiodic structures. (24) Kunzler, T. P.; Drobek, T.; Sprecher, C. M.; Schuler, M.; Spencer, N. D. Appl. Surf. Sci. 2006, 253, 2148–2153. (25) Tosatti, S.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537–3548. (26) Wieland, M.; Textor, M.; Spencer, N. D.; Brunette, D. M. Int. J. Oral Maxillofacial Implants 2001, 16, 163–181.
Langmuir, Vol. 24, No. 10, 2008 5413 Table 1. Relevant Factors for SBG (sand-blasted glass), SLA (replica of sand-blasted, acid-etched titanium), LLR (lotus leaf replica), GTM (replica of golf-tee shaped micropillars) Extracted From Data Analysis: Roughness Factors, r, From White-Light Profilometry (WLP) Data and SEM Imagea r k dS
SBG
SLA
LLR
GTM
2.3 (WLP) 1.18 ( 0.07 -0.03 ( 0.03
1.5 (WLP) 1.11 ( 0.08 -0.86 ( 0.04
1.7 (WLP) 1.09 ( 0.04 -0.31 ( 0.01
1.3 (SEM) -
a k and dS are the corresponding slopes and axis intercepts from the linear fits in Figure 4a.
Table 1 contains roughness factors for the studied surfaces, as well as corresponding k and dS factors, as described below in Figure 4 and eq 3. Water Contact Angle Data. Figure 3a) is an explanatory graph. It illustrates the effects of roughness on a surface chemistry defined by the contact angle found on the flat reference. The maximum θ that can be ordinarily achieved on a flat surface is 120°. Lower surface energies are thus excluded, and no data can be obtained beyond 120° on the flat surface (area A). With a hydrophobic coating (90-120° on the flat surface), the drop shows either Wenzel- or Cassie-type wetting (area B). Below 90° on a flat surface, even though the surface chemistry is intrinsically hydrophilic, the drop on rough surfaces shows a higher θ, and even hydrophobic values are possible (area C). In some cases, surface topography can be such that Cassie-type wetting persists in the hydrophilic regime27 (area F). In the high surface energy range (low θ), hemiwicking can occur (area D). Surface topography determines the surface energy at which capillary forces come into play. Presenting the data in a cosine-cosine plot facilitates comparison with the Wenzel predictions (area E), a linear function of surface energy (cosine of Young contact angle) with the roughness factor as slope and axis intercept of zero. Water contact angles were measured on both rough and flat surfaces that had been exposed to the same thiol mixtures. In Figure 3, the θ data are plotted such that the cosines of the static and dynamic contact angles on the rough surfaces are plotted versus the cosine of the static contact angles measured on flat surfaces with the corresponding surface chemistry. Thus, each data point corresponds to one comparative experiment. With these diagrams, a direct comparison with the Wenzel theory6 (eq 1) is possible. Figure 3b-e shows the data obtained on SBG, SLA, LLR, and the GTM surfaces. The static and the advancing θ values for SBG (Figure 3b) display two linear trends, changing from slope 1 to slope 2.3 (corresponding to a Wenzel roughness factor, Table 1) at 90°. The surface cannot be made superhydrophobic, but becomes superwetting when functionalized with a pure OHterminated SAM. As long as the surface chemistry remains hydrophilic, the receding contact angle is zero because the contact line is pinned to the surface. The static and advancing θ values on the SLA replica (Figure 3c) show an extremely strong shift toward hydrophobicity compared to the θ predicted by the Wenzel equation. All data points in the lower right-hand quadrant correspond to hydrophilic surface chemistry, whereas the apparent contact angle on the SLA surface is hydrophobic. This phenomenon can be explained by pinning of the contact line. If the intrinsic surface energy is hydrophobic, the surface is capable of achieving contact angles near 150° but remains essentially in a Wenzel-type wetting regime. (27) Liu, J. L.; Feng, X. Q.; Wang, G. F.; Yu, S. W. J. Phys.: Condens. Matter 2007, 19, 12.
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Figure 2. SEM images at two different magnifications of the examined surfaces. The scale bar on the left side corresponds to 100 µm, and the one on the right side, to 2 µm. (a, b) SBG (rough and flat regions enlarged), (c, d) SLA, (e, f) LLR, and (g, h) GTM.
The pinning strength is still sufficiently strong that the drop exhibits no roll-off, even though the receding θ is high. Similarly
to SBG, SLA surfaces can also be rendered hemiwicking when coated with OH-terminated thiols.
Beyond the Lotus Effect
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Figure 3. (a) Model graph clarifying the meaning of the axes in b-e and the effects that can be observed in the plot. (A) surface energies leading to contact angles above 120° on a flat surface are physically not possible, (B) hydrophobic region (Wenzel- or Cassie-type wetting occurs), (C) pinning effects cause the drop to adopt (considerably) higher contact angles as compared to the flat surface, (D) capillary forces occur, (E) series of Wenzel predictions, (F) contact angles above 150°, Cassie-type wetting. (b-e) Water contact angle data for four different surfaces; static (solid symbol), advancing (grey symbol), and receding (open symbol) θ. The x axis corresponds to the cosine of the static contact angles of the flat reference; the y values correspond to the cosine of the advancing, static, and receding contact angles on the corresponding rough surface. The dashed line corresponds to the prediction by Wenzel when calculated with the roughness factors derived from white light profilometry (displayed in Table 1): (b) SBG, (c) SLA, (d) LLR, (e) GTM.
The LLR (Figure 3d) shows a shift to hydrophobic contact angles due to pinning effects, which is greater than that of the SBG, but less than the case of the SLA. In contrast to SBG and SLA, the LLR functionalized with a pure CH3-terminated SAM shows extremely high dynamic and static contact angles, an indication of Cassie-type wetting. The papillae are able to support the drop, even in the absence of the tubular wax of the original leaf. As soon as there is a hydrophilic contribution present in the SAM, Wenzel-type wetting occurs. Note the high standard deviations, which arise from the fact that in some measurements, the Cassie state still persisted, whereas in others, the drop penetrated into the structures. Both types of behavior were sometimes even observed on the same sample, which is an indication of the presence of metastable states. An explanation for this metastable Cassie regime
might be found in the naturally grown leaf topography, which is inhomogeneous in pillar density, top perimeter, and height. On some areas of the leaf, the papillae can support the drop, whereas on others, they cannot. Again, the LLR shows superwetting when coated with a pure OH-terminated SAM. The GTM surface (Figure 3e) behaves somewhat differently from the others. Thanks to the golf-tee, undercut shape, Cassietype wetting is moderately stable, even at higher surface energies, as predicted by Liu et al.27 Since the cross section of the pillar becomes smaller toward the bottom, the drop would need to form a larger liquid–air interface to follow the topography, which would be energetically unfavorable. Therefore, the surface energy of the solid must be quite high to overcome this energy barrier. Tilting the sample also helps to overcome this energy barrier.
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empirical description to characterize nonperiodical surface topographies in the intermediate hydrophilic regime (θY, ranging approximately from 40° to 90°). θS is the contact angle measured on the rough surface, k ≈ 1 is the slope, and dS is the axis intercept found in the linear regression through the data.
cos θS ) k · cos θY - dS
Figure 4. Water contact angle data and the derivation of an empirical description. (a) Water contact angle data of the three nonperiodical topographies. Here, the x axis consists of not only the static but also the dynamic values on the flat surface. Open symbols correspond to receding; solid, to static; and grey, to advancing contact angles. SBG, 1; LLR, 9; SLA, b. The circles with a cross are advancing contact angle data found by Tosatti et al.25 by applying mixed SAMs of OH- and CH3terminated alkylphosphates on SLA titanium surfaces. The dashed line has slope 1. (b) Empirical description of the mechanisms found in the intermediate hydrophilic regime where pinning effects are predominant.
The hysteresis of dynamic measurements on the GTM is quite high, as compared to that of the LLR with the pure hydrophobic coating, for example. This is commonly observed for micrometersized pillars28 in the absence of a second, smaller-scale structure. If the GTM surface is coated with a pure OH-terminated SAM, water undergoes spreading until it completely fills the volume between the pillars. At this point, it forms a drop coexisting with a liquid film within the structure. Although the water film fully spreads over the whole patterned area and is visible by eye, a contact angle of around 6° can still be measured (Supporting Information 2). If static θflat vs static θrough and advancing θflat vs advancing θrough are presented in the same cosine-cosine plot, as shown in Figure 4a, differences between advancing and static measurements are canceled out. The difference between the advancing and the static contact angles on the rough surface is equal to that on the flat surface. This means that it does not matter whether θs or θa is measured, since the effects observed are governed by the environment at the contact line and describe the same tendency. In the intermediate surface-energy region, additional effects, such as air enclosure or capillary forces, can be excluded. Thus, the slope for all three nonperiodical surfaces tested (SBG, SLA, LLR) is close to unity (see k, Table 1) and no significant effect on the slope, originating from surface roughness, can be distinguished. Roughness mainly influences the axis intercept (see dS, Table 1), which is an indication of the magnitude of the energy barrier pinning the contact line. We propose the following (28) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220–226.
(3)
The explanation of why SBG, the surface with the highest roughness factor, has the lowest hydrophobic shift, can be ascribed to the presence of conchoidal fracture areas on the surface, which present the energetically most favorable route for the contact line to move forward.2 As soon as a step forward is made, the energy barrier is overcome, and the rest of the contact line will follow. This behavior resembles that found in dislocation movement in metals and in the special case of kink pairs (Seeger’s dislocation mechanism).29 Pinning of the contact line is therefore not very effective in this case. The LLR has no flat patches but a pillarlike structure, which presents fewer pinning sites than the dimpled SLA replica surface. It therefore seems that, for an understanding of roughness effects, the energy barrier is a more fruitful avenue to pursue than the effect of increased surface area. The receding contact angle remains close to zero, as long as the corresponding angle on the chemically equivalent flat surface remains below 90°. The detachment of the contact line is facilitated by a hydrophobic coating. The energy barrier governing this detachment is significantly lowered by the presence of air on Cassie-type wetted surfaces, leading to extremely low hysteresis. The advancing contact angle data of Tosatti et al.,25 measured on SLA titanium with SAMs of mixed OH- and CH3terminated alkylphosphate monolayers, coincide extremely well with the mixed thiol data taken on the SLA replica in the present study. This demonstrates that the approach is valid, independent of the specific adsorbate–substrate system. White-Light Profilometry. The white-light profilometry data was used to calculate the actual surface area, in order to be able to define the roughness factor, r. The roughness factor for the GTM surface was calculated by measuring the relevant lengths of a detached pillar in a SEM image (see Table 1) Additionally, “roughness factors” k from Figure 4a were extracted by applying a linear regression through the data found in the surface energy region corresponding to the contact angles from 40° to 90° on the flat surface. Contrary to Wenzel’s predictions, all tested surfaces show at least a small axis intercept. Additionally, slopes for SBG, SLA, and LLR in the region corresponding to 40-90° on the flat surface lie between 1 and 1.2; that is, far from the roughness values extracted from the white-light profilometry data in Table 1. Due to the pillar surface’s strong and persistent Cassie-type wetting, the data show a transition from composite behavior directly to hemiwicking, leaving little room for roughness-dependent pinning.
Conclusions Four different, heavily structured surfaces have been analyzed over a wide range of surface energies via water contact angle measurements. The data show three wetting regimes: If the surface energy is high, wettability is indeed enhanced by the surface roughness, causing hemiwicking in many cases due to capillary forces. At lower surface energies, pinning of the contact line results in a shift to more hydrophobic θ values. It was found that the surface topography defines the pinning strength and with it (29) Hull, D. Introduction to Dislocations, 2nd ed.; Pergamon Press: Oxford, 1975; Vol. 16.
Beyond the Lotus Effect
the energy barrier counteracting the wetting behavior of the drop. An indication of this barrier is the axis intercept seen in the cosine-cosine plot (see Figure 4). This plot is surface-topographysensitive, and the behavior of all tested surfaces can be readily distinguished. The topographical influence on θ cannot simply be predicted via a roughness factor. With the exception of the hydrophobic data in the case of the SBG surface, none of the measured contact angles could have been predicted by the Wenzel equation. With regard to superhydrophobic surfaces, the golf-tee-shaped (GTM) pillars show stable superhydrophobicity over a wide range of surface energies. This topography seems to be a very effective design for microstructured, superhydrophobic surfaces. Acknowledgment. The authors thank Ciba Speciality Chemicals for their generous financial assistance; the Botanical Garden
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of Zurich for providing the lotus leaves; and Electron Microscopy ETH Zurich, EMEZ, for their support. Supporting Information Available: Supporting discussion 1 emphasizes that in the initial equation (eq 2) by Cassie and Baxter, f1 + f2 g 1, and only in special cases (e.g., flat-top pillars) does it equal unity. Discussion 2 shows that eq 2 can also be applied to calculate the contact angle in the very hydrophilic wetting regime. As soon as all cavities are filled with water, a finite contact angle can be observed, as predicted by eq 2. Additionally, a SEM analysis shows the different steps of the lotus leaf replica process, and five illustrative movies are shown for the evaluation of the dynamic contact angles: advancing and receding in the supherhydrophobic case, the stick-jump behavior, and a one-side pinned drop, as well as a movie of a receding drop showing no finite contact-angle value. This information is available free of charge via the Internet at http://pubs.acs.org. LA800215R
