Article pubs.acs.org/JPCC
Wetting Transitions on Hierarchical Surfaces K. L. Cho,† Alex H.-F. Wu,† Irving I. Liaw,† David Cookson,‡ and Robert N. Lamb*,† †
School of Chemistry, University of Melbourne, VIC 3010, Australia Australian Synchrotron, Clayton, VIC 3168, Australia
‡
ABSTRACT: The current study reports the fabrication and characterization of superhydrophobic surfaces with increasing nanoroughness by decreasing silica nanoparticle size in a sol−gel matrix. Using small-angle X-ray scattering (SAXS) measurements allowed for the direct quantification of air entrapped at the interface, revealing for the first time that significant air remains on hierarchical surfaces despite observed droplet pinning through hysteresis measurements. Combining contact angle hysteresis and SAXS measurements of the surfaces immersed in sodium dodecylsulfate (SDS) solutions with Cassie and Tadmor’s model, a series of predicted contact angles were generated, comparing wetting transition mechanisms based on wetting line advance, droplet adhesion/pinning, and interfacial air entrapment. The study provided confirmation of key theoretical assumptions on wetting of hierarchical surfaces: (i) Cassie wetting of the nanofeatures is the preferred wetting progression on hierarchical surfaces; and (ii) the presence of an intermediate petal state is dependent on the level of nanoroughness as compared to the microroughness.
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INTRODUCTION The wetting properties of surfaces are commonly described by a Young’s equilibrium contact angle1 and advancing and receding angles, which combine to give rise to contact angle hysteresis, a measure of the adhesion/pinning of the fluid droplet to the surface.2,3 The effect of surface roughness on contact angle is defined using the Wenzel model, which describes a fluid droplet pinned to the rough features of the surface, and the Cassie model, where air is entrapped in the rough texture beneath the droplet forming a composite interface of air and solid.4 In the Cassie model, the apparent contact angle (θ*), which is the equilibrium contact angle attained due to surface heterogeneity, is an area weighted average of contact angles (θ1,θ2) of the two composite regions (f1,f 2) as shown in eq 1,4,5 where the contact angle of air is 180°. cos θ* = f1 cos θ1 + f2 cos θ2
progression difficult to characterize. To better our understanding of the effect of hierarchical roughness in superhydrophobicity, it is important to study the wetting progression of such surfaces. However, as opposed to single scaled surfaces, wetting studies of multiscaled surfaces have been restricted to regularly patterned lithographic surfaces or theoretical models. Even on patterned surfaces, such wetting studies are characterized by difficulties in identifying the Wenzel state and significant variance in hysteresis measurements of transition points. Such issues are expected to be amplified in random hierarchical surfaces such as all natural8,17 and many fabricated surfaces.13 In the current study, the wetting progression of a series of superhydrophobic surfaces is characterized using a combination of experimental static contact angle, contact angle hysteresis, small-angle X-ray scattering (SAXS) measurements, and theoretical models. This work makes use of SAXS in transmission mode to quantify the characteristic size of the nanofeatures on a hierarchical surface and study the change in surface wetting under immersion conditions. The technique is based on the intensity of the SAXS profile (IAB) of a rough interface between two media as a difference in their average electron densities (ρA, ρB) (shown in eq 2).
(1)
On superhydrophobic surfaces, chemically hydrophobic surfaces with high levels of roughness, the composite interface is dominated by air, resulting in apparent contact angles θ > 150° and hysteresis < 10°.6−8 Such surfaces, readily observed in nature,9−11 are the focus of much research interest due to their potential applications.7,12,13 On single scaled rough surfaces, where roughness is readily characterized, the progression of wetting from the mobile Cassie state, with a composite interface, to the pinned Wenzel state is well established. However, it has been shown that dual and multiscaled roughness plays a significant role in enhancing superhydrophobic properties and giving rise to unique wetting states such as the intermediate petal state.14−16 Furthermore, the roughness profile of many superhydrophobic surfaces, both synthetic and natural, is of a random nature, making wetting © 2012 American Chemical Society
IAB ∝ (ρA − ρB )2 = (Δρ)2
(2)
Replacement of the air/water interface with a solid/water interface during wetting would result in a proportionate decrease in electron density contrast and hence the overall intensity of the SAXS profile. The use of SAXS, therefore, Received: September 5, 2012 Revised: October 18, 2012 Published: November 15, 2012 26810
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allows for a direct quantification of the wetting interface in situ, providing a complete picture of the wetting transition, from the Cassie state to the wetted Wenzel state, on random hierarchical surfaces.
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EXPERIMENTAL METHODS Superhydrophobic Surfaces. Multiscale superhydrophobic surfaces were fabricated through the self-assembly of silica nanoparticles via sol−gel.18−20 Silica nanoparticles (Evonik, USA) dispersed in ethanol (Sigma-Aldrich, Australia), was gelled using methyl-trimethoxysilane (99%, Sigma-Aldrich) in acidic conditions while under sonication (30 min, 40 kHz). The resultant gel solution was cast onto glass and Kapton (American Durafilm, USA) substrates by spin coating and cured at 300 °C. Monodisperse latex spheres18 were added during the sol−gel process as a template to control micropore structure. During the curing process, the polymer latex spheres were fully degraded resulting in size-controlled micropores. Surface nanoroughness was altered by varying the size of the silica nanoparticle precursor. Smooth Surfaces. Smooth methylated surfaces were fabricated by immersing a glass slide in a solution of 5% w/w trimethylchlorosilane in n-hexane for 24 h, followed by curing at 100 °C.21 Wetting Measurements. Contact angle measurements were performed using a model 200 standard goniometer (Ramé-Hart, USA). Because of the nature of heterogeneity of all surfaces used in this study, all measured apparent contact angles will be henceforth referred to as “contact angle”. Static contact angles were measured with a 20 μL droplet of fluid, depositing using a microsyringe, at an ambient temperature (25 °C). The recorded contact angle was an average of five replicate measurements at different spots of the coated surface. Advancing and receding contact angles were measured, respectively, by depositing a 20 μL droplet of fluid onto the surface, then slowly increasing or decreasing the droplet volume until the droplet edge was observed to move. Measurements were repeated with water (Milli-Q). The surface tension of the sodium dodecylsulfate (SDS) solution was measured using a model K6800 tensiometer (Kruss, Germany) using a Du Nüoy ring. The surface tension and corresponding contact angle on a smooth methylated surface of SDS solutions of various concentrations are shown in Figure 1. Small-Angle X-ray Scattering (SAXS). In situ SAXS wetting experiments were performed on coated Kapton samples at an ambient temperature (29 °C) using a modified Omni-cell IR fluid cell (Specac, UK). The use of Kapton as opposed to glass as a substrate in the SAXS studies was due to the welldefined and significantly reduced scattering from the substrate. The samples were mounted in transmission mode at the SAXS/ WAXS beamline of the Australian Synchrotron. The wetting fluid was injected into the cell via a peristaltic pump and was allowed to stabilize for a short period of time. Test scans were conducted showing no observable changes in wetting occurred within the scanning time frame. Wetting was induced by flushing the cell with a sufficient quantity of the surfactant solution to ensure the fluid cell is filled with an aqueous solution of SDS at the desired concentration. An incident beam energy of 15 keV was used. The scattering pattern was collected at a medium camera length (3 m) using a Pilatrus-1 M CCD detector (Dectris, Switzerland). Each sample was scanned at 25 acquisitions spots evenly placed in a 2 μm × 2 μm grid, with an integration time of 2 s for each spot. The scattering images
Figure 1. (*) Contact angle of pure fluids of various surface tensions on a smooth methyl-terminated surface plotted from Fox et al.22 (●) Contact angle measurements of SDS solutions of various surface tensions on a smooth methyl-terminated silica surface.
were reduced using SAXS15id software provided by the Australian Synchrotron. Fluid scattering was subtracted relative to a blank Kapton sample wetted with the respective fluid. Scans of the coated Kapton surface before and after the wetting studies revealed no observable X-ray damage to the coating. Test scans were also conducted showing no observable changes in wetting occurred within the experimental time frame from fluid injection to completion of all of the scans. To ensure maximum dynamic range of wetting for analysis, the cluster of profiles with highest intensity, corresponding to the thickest part of the coating, was chosen to form a statistical average.
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RESULTS AND DISCUSSION Surface Characterization. By varying the precursor nanoparticle size in a templated sol−gel matrix, superhydrophobic surfaces were fabricated to exhibit varying nanoscale but similar microscale features. This is evidenced by the fact that the surface exhibited similar, with a slight increase, surface roughness as characterized using AFM, and, with a slight increase, similar wettability characteristics as shown in Figure 2. The SAXS profiles of the superhydrophobic surfaces are displayed in Figure 3a as a function of intensity with respect to scattering momentum (q), where q = 4π sin(φ/2)/λ, φ is the scattering angle, and λ is the incident X-ray wavelength. Displayed in Figure 3a, the scattering profiles reveal a distinct shifting of the Guinier knee, indicative of a prominent feature both in the bulk and in the surface structure of the coating. The shift toward lower q-space of the Guinier knee with increasing silica nanoparticle size signifies an increase in the characteristic feature size. In addition, it is possible to derive a B parameter from the high-q portion (linear Porod region, Figure 3a) from the power law equation.23 I(q) = Bq−4
(3)
For well-defined interfaces, such is the case for all of our surfaces, the B parameter is proportional to the amount of 26811
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Figure 2. AFM images of superhydrophobic coatings fabricated with (a) 7 nm (Ra = 100 nm), (b) 12 nm (Ra = 120 nm), (c) 20 nm (Ra = 200 nm), and (d) 40 nm (Ra = 240 nm) silica nanoparticles with their average roughness (Ra). Vertical color scale bar = 2 μm on the left represents height profiles of the surfaces.
Figure 3. (a) The SAXS profiles of superhydrophobic coatings highlighting the shift in Guinier knee, the point at which the profile drops into the linear Porod region (black line). (b) Porod B parameter of superhydrophobic surfaces as a function of silica nanoparticle size.
Figure 4. (left) A cosine−cosine plot, mapping the apparent contact angle (θ*) of SDS fluids on 7 nm (●), 12 nm (▲), and 40 nm (○) superhydrophobic surfaces against the corresponding contact angle (θ) on a smooth hydrophobic surface. (Right) Contact angle hysteresis of SDS fluids on 7 nm (●), 12 nm (▲), and 40 nm (○) superhydrophobic surfaces.
solid/air, solid/liquid, and liquid/air interfaces as demonstrated in previous work.24 A reduction in B parameter can be correlated with the substitution of the solid/air interface with solid/liquid during wetting transitions. The B parameter shows that, with increasing silica nanoparticle size, despite an increase in AFM surface roughness, a distinct reduction in solid/air
interfacial area is observed, in agreement with the hypothesis that increasing nanoparticle size results in decreased nanoroughness. Contact Angle and Hysteresis. To study the wetting progression of the surfaces, static contact angle and contact angle hysteresis were measured for each surface in contact with 26812
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water and SDS solutions of increasing concentration to simulate decreasing surface tension. The measured contact angles, plotted as a cos θ − cos θ plot in Figure 4, are in good agreement with similar studies done on hierarchical superhydrophobic surfaces in literature.8 All surfaces exhibit superhydrophobicity and, as expected, show a reduction in contact angle and a corresponding increase in hysteresis as fluid surface tension reduces, promoting wetting of the pores. Surfaces fabricated using smaller nanoparticles as precursors, which generated smaller nanofeatures, observed an increased static contact angle on the surface even with low surface tension fluids. Such an observation is consistent with the capillary theory, and the ability of a hydrophobic capillary to inhibit wetting is inversely related to its diameter. In the cos θ − cos θ plots, although it is relatively simple to observe the beginning of the Cassie−Wenzel transition, the point at which cos θ* is observed to deviate significantly from −1 (θ* = 180°), it is difficult to discern the position at which a complete Wenzel state, fully wetted interface, is achieved. This is due to two reasons both as a result of the hierarchical nature of the surface: (i) the hierarchical nature of the surface means transition of the wetting does not follow a linear trend as expected from the wetting models and observed on single scaled rough surfaces, and (ii) it is difficult to accurately quantify the surface roughness factor, which is needed to predict the critical Wenzel contact angle.8 A possible solution to identify wetting transitions has been the use of contact angle hysteresis measurements as an indication of wetting. However, similar studies have highlighted that, although a hysteresis increase was observed as wetting progressed, there is significant error involved in such large hysteresis values such that it is difficult to measure with accuracy.8 Nevertheless, contact angle hysteresis measurements, presented in Figure 4, demonstrate a trend similar to that of the static contact angle measurements, in that wetting was observed to progress more rapidly on surfaces with larger nanofeatures. However, contact angle hysteresis measurements highlight that pinning of the droplet occurs at much higher surface tensions, an indication that wetting is occurring. Furthermore, hysteresis values were shown to reach a maximum of 160°−170° on both 12 and 40 nm surfaces, suggesting that the interface is fully wetted. It is of interest to note that significant error was observed in the hysteresis measurements during the transition between the superhydrophobic and wetted state. Theoretical Models. By equating the line energy associated with the advancing contact angle (θA) with that of receding contact angle (θR), Tadmor25 demonstrated, in agreement with experimental data, the calculation of Young’s equilibrium contact angle (θ0) on surfaces with local blemishes according to eq 4. ⎛ Γ cos θA + ΓR cos θR ⎞ θ0 = arcos⎜ A ⎟ ΓA + ΓR ⎝ ⎠
While Tadmor’s model was predominantly developed for smooth surfaces, the local blemishes considered within the model can be assumed to be caused by surface roughness, from which advancing and receding contact angles arise. This is similar to how Cassie’s model averages the interfacial regions of a rough surface, where the area between surface asperity is considered as a “surface” with an intrinsic contact angle of 180°. Tadmor’s model could therefore be applied to predict the overall wetting behavior of rough surfaces in a similar manner by considering the contact points between fluid and as blemishes upon which pinning occurs. Of particular interest is the fact that the deviation of the advancing contact angle and receding contact angle from equilibrium is not equal. In fact, as the equilibrium contact angle of a surface deviates from 90°, Tadmor’s equation predicts that the equilibrium contact angle would be closer to the receding contact angle as compared to the advancing. Nevertheless, the equilibrium contact angle calculated from Tadmor’s equation, based upon advancing and receding angle, would therefore be heavily influenced by adhesion/pinning of the fluid droplet on the surface. A surface with a relatively high hysteresis, hence a large advancing and small receding angle, would observe a greater drop in its Tadmor’s equilibrium contact angle. The results, presented in Figure 5, reveal that the Tadmor’s equilibrium contact angle is significantly different from that observed in static contact angle measurements. This is indicative of the pinning effects of the surface roughness as wetting progresses, thereby preventing the wetting line of the
(4)
where ⎞1/3 ⎛ sin 3 θA ⎟ ΓA = ⎜ 3 ⎝ 2 − 3 cos θA + cos θA ⎠
and Figure 5. Wetting progression of superhydrophobic surfaces as characterized by observed static contact angle, Cassie predicted contact angle using SAXS data, and Tadmor’s predicted contact angle using advancing and receding angles.
⎞1/3 ⎛ sin 3 θR ⎟ ⎜ ΓR = 3 ⎝ 2 − 3 cos θR + cos θR ⎠ 26813
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fluid droplet from moving and maintaining a higher than equilibrium, static contact angle. Wetting progression can be quantified using SAXS as an overall reduction in scattering intensity resultant from the replacement of a proportionate amount of the air/solid interface with a solid/liquid interface. In this scenario, the B parameter, proportional to the total amount of solid/air, solid/ liquid, and liquid/air interface, provides a direct quantification of the wetted interface in relation to reducing fluid surface tension, as is represented by the percentile change in B, according to eq 5. percentage surface unwetted =
Bsample − Bwetted Bdry − Bwetted
The wetting progressions of the surfaces were studied under fluids of reducing surface tension. The use of SAXS allowed for the direct quantification of air entrapment in the nanoscale features of the surface as wetting progresses from the mobile Cassie state to the fully wetted Wenzel state. Combined with experimental contact angle, hysteresis measurements, and Cassie and Tadmor’s mathematical modeling, a series of predicted contact angles were generated, allowing for the direct cross-comparison of wetting transition mechanism based on wetting line advance, droplet adhesion/pinning, and interfacial air entrapment. The results highlight a deviation in wetting on hierarchical superhydrophobic surfaces where substantial levels of nanoroughness result in significant air entrapment at the interface despite pinning of the wetting fluid. As the size of nanofeatures increases, in line with capillary theory, the wetting of the surface through the removal of entrapped air converges with Tadmor’s model based upon the adhesion of a liquid droplet on the surface. This observation provided direct experimental evidence to the existence of a petal state, which could only be inferred previously using contact angle and hysteresis measurements. The SAXS analysis confirms experimentally that wetting of multiscaled rough surfaces proceeds with Cassie wetting at the nanoscale, a commonly used assumption in theoretical models.27,28 Furthermore, SAXS measurements of all surfaces demonstrated a clear indication of reaching the wetted Wenzel state.
*100 (5)
Applying the percentage of the surface unwetted into Cassie’s equation, it is possible to derive a Cassie predicted contact angle. Presented in Figure 5, the Cassie predicted contact angle shows, as expected, a reduction with decreasing surface tension of the wetting fluid. However, a deviation was observed between the observed contact angle and the predicted contact angles using both Cassie and Tadmor’s models. This significant deviation is attributed to the pinning effect of the droplet where the wetting line remains pinned and thus cannot advance or recede in response to changes in fluid surface tension. In this case, wetting predominantly occurs vertically, entrenched within the roughness of the coating. This is reflected by the Cassie predicted contact angle, which is derived from the direct quantification of the solid/air interface. The figure also revealed that on surfaces with smaller nanofeatures (i.e., small nanoparticle sizes), the Cassie predicted contact angle deviates significantly from Tadmor’s predicted contact angle, in particular highlighting significant wetting inhibition at the initial stages. This deviation between Cassie’s and Tadmor’s prediction provides significant insight into the wetting transition of hierarchical surfaces. It indicates that wetting of multiscaled surfaces proceeds with Cassie wetting of the nanofeatures, and considerable air is entrapped in the nanoroughness as the microroughness is wetting, resulting in high contact angles, pinning of the fluid droplet, and a majority of the surface remaining dry. Such wetting progression is in agreement with that of a petal state, postulated in the literature.14,26 The deviation vanishes as the size of the nanofeatures increased; that is, as the ratio of nanoroughness to microroughness size reduces, the petal state is no longer observed, and the air entrapped at the interface is proportional to the level of adhesion/pinning of the water droplet. With all surfaces, as surface tension reduces, the rapid reduction in the Cassie predicted contact angle observed initially on all surfaces diminishes to approach a flat gradient, indicating the Wenzel state is achieved. The convergence of Tadmor and Cassie’s contact angle further signifies that the surface is fully wetted, as the influence of air entrapment in elevating Cassie’s contact angle is eliminated.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +61 3 8344 6492. Fax: +61 3 9347 5180. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, Victoria, Australia. We would like to thank Dr. Nigel Kirby from the Australian Synchrotron for his assistance in SAXS experimentation and subsequent data analysis. The financial support of the Australian Research Council’s Discovery Projects (Project DP120104536) is gratefully acknowledged.
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REFERENCES
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CONCLUSIONS Superhydrophobic surfaces of varying nanoroughness were fabricated by changing the precursor silica nanoparticle size in a sol−gel matrix. The B parameter extrapolated from the SAXS profile highlighted an overall reduction in interfacial area with increasing nanoparticle size associated with a reduction in nanoroughness. 26814
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