J. Phys. Chem. C 2010, 114, 12675–12680
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Influence of Nanoroughness on Contact Line Motion Renate Fetzer† and John Ralston* Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes Campus, Mawson Lakes, Adelaide, SA 5095, Australia ReceiVed: April 13, 2010; ReVised Manuscript ReceiVed: June 8, 2010
The dependence of wetting behavior on roughness is of great relevance in many practical applications. However, most studies concentrate on static wettability. In this study, the influence of nanoroughness on the spontaneous dewetting of water is investigated. For both smooth and nanorough surfaces, contact line motion is governed by two distinct regimes: hydrodynamic dissipation dominates for small water contact angles, while the molecular kinetic theory describes contact line motion at large contact angles. In a quantitative comparison of nanorough with chemically heterogeneous surfaces, the similarities in behavior suggest that there may be a universal dewetting mechanism. Introduction
Theoretical Models
Roughness influences the wetting properties of solid surfaces. Topographical features, both on the nanometer and micrometer scales affect the equilibrium wettability of a substrate, as well as the static contact angle hysteresis and pinning forces acting on the three phase contact line.1 Although a large body of work exists on the impact of roughness on static wetting and contact angle hysteresis, only a few studies concern its influence on wetting dynamics: nanometer scale pinning seems to control the dynamics of contact line motion at small speeds,2 while for micrometer-sized features, individual pinning and depinning events at the contact line can be experimentally observed.3,4 We focus our research on surfaces that are moderately rough, with nanometer scale undulations in height that do not involve sharp edges, corners, or overhangs that may give rise to strong contact line pinning or trapping of gas or liquid. Thus, there is no gas trapped at the solid-liquid interface, neither is any liquid left behind in the structures upon dewetting, and therefore, the surfaces studied are in the Wenzel state. As the contact line travels, it follows the topography of the solid surface. The question we ask is “how do dewetting dynamics depend on the nanoroughness of such a substrate?” To control substrate roughness on a nanometer scale, the surface of an SU8 coating, a commonly used negative photoresist, was etched using air plasma. The surfaces were then coated with a thin layer of plasma-polymerized n-heptylamine to retain a constant and moderately hydrophobic surface chemistry. In order to study contact line dynamics, the spontaneous dewetting process that occurs when a bubble contacts a nanorough hydrophobic surface submerged in water was investigated. By using high-speed video microscopy, the moving contact line was monitored as the dry patch between the bubble and the surface expanded until equilibrium was reached. The data of contact line speed as a function of the respective macroscopic water contact angles were then analyzed and compared for different types of surfaces.
Various models describe capillary driven contact line motion on smooth, homogeneous, ideal, and planar surfaces. The two main approaches are the hydrodynamic model developed by Voinov5,6 and Cox7 and the molecular kinetic theory introduced by Blake and Haynes.8 There are other models including the comprehensive approach of Shikhmurzaev.9-12 The hydrodynamic description assumes viscous shear within the liquid wedge to be the predominant dissipation mechanism during contact line motion. For liquid-gas systems with small capillary numbers Ca ) Vη/γ (with contact line velocity V, liquid viscosity η, and liquid-gas surface tension γ) and contact angles below 135°, this leads to the following equation for wetting (plus sign) and dewetting (minus sign) dynamics:
* To whom correspondence should be addressed. † Present address: Karlsruhe Institute of Technology, Institute for Pulsed Power and Microwave Technology, P.O. Box 3640, D-76021 Karlsruhe, Germany.
3 θ3 ) θm ( 9Caln(L/LS)
(1)
where θm is the microscopic contact angle. Optically θm is not observable and thus is often assumed to be constant during contact line motion and equal to the equilibrium contact angle; L is a macroscopic length scale (of the order of the drop/bubble diameter or the capillary length), and LS denotes a microscopic cutoff length (of molecular scale) introduced to remove the singularity of shear stress at the contact line that arises as a consequence of the no-slip boundary condition. On the other hand, nonhydrodynamic friction at the contact line is considered in the molecular kinetic approach, the socalled MKT model.8,13 Based on Eyring’s activated rate theory, local displacements occur at the contact line due to the thermal energy kBT. At equilibrium, the rate of these displacements is given by K0 ) kBT/h · exp{-∆G/(kBT)}, where ∆G is the activation free energy of the displacements and h is Planck’s constant. For a system out of equilibrium, the equilibrium frequency K0 is shifted as a result of the unbalanced capillary force γ(cos θ0 - cos θ). Here, θ0 is the equilibrium, and θ is the instantaneous contact angle. For a displacement distance λ and an isotropic distribution of adsorption sites, this approach leads to a contact line velocity given by V ) 2K0λsin h{λ2γ(cos θ0 - cos θ)/(2kBT)}. For large arguments, the sin h function reduces to a single exponential:
10.1021/jp103279u 2010 American Chemical Society Published on Web 07/01/2010
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|V| ) K0λexp{λ2γ · |cos θ0 - cos θ|/(2kBT)}
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(2)
The hydrodynamic and MKT approaches can be combined. Often, there is very good agreement between experimental data and the combined theory; however, unreasonably small K0 values and slip lengths are frequently obtained.14 Generally, hydrodynamic dissipation dominates at low contact angles, while the MKT applies at large contact angles. In this study, both the hydrodynamic and molecular kinetic approaches are compared with data obtained on nanorough surfaces. Experimental Section Sample Preparation and Characterization. Polymeric substrates were used to produce surfaces with different surface roughness without substantial changes in the surface morphology. A 10 µm thick layer of the resin-based negative photoresist SU8 was prepared on top of a clean and dust free microscope glass slide following standard procedures for spin coating, baking, UV exposure, and hard bake, as provided by the manufacturer. SU8 surfaces were imaged without further treatment using tapping mode atomic force microscopy (AFM, Asylum Research) and found to be very smooth, without any significant topographical features. The root-mean-square (rms) roughness was below 1 nm over a scan size of 1 µm2. The samples were roughened by treatment with air plasma for up to 10 min, during which time the SU8 surfaces undergo a chemical and topographical rearrangement;15 the substrates became hydrophilic and increasingly rough. Samples at different exposure times were imaged using tapping mode AFM (Figure 1). Upon plasma etching, the SU8 surface developed randomly distributed nodular features, the average size of which increased with etching time up to an rms roughness of 4.5 nm. The topographical modification was uniform over the entire sample surface. In order to maintain constant surface chemistry with moderate hydrophobicity, the rough samples were then coated with a plasma-polymerized, 20 nm thick layer of n-heptylamine.16 In order to determine whether or not coating thickness influenced wetting dynamics, smooth glass slides were coated with polymer layer thicknesses from 10 to 50 nm. The n-heptylamine coating is stable in contact with water and covers the underlying substrate homogeneously without holes or discontinuities.16 AFM imaging of all samples after the plasma-polymerization step confirmed that the main topographical features are retained, with only a minor reduction in surface roughness. Values for the rms roughness as well as the roughness factor introduced by Wenzel (A/Ao, the ratio of real to projected area), both obtained from the AFM images following plasma deposition, are shown in Table 1. Static wettability of the samples was characterized using the sessile drop technique.3 The advancing water contact angle was 77° on the smooth surfaces, increasing up to 85° as the surface was progressively roughened (Table 1). At the same time, the receding contact angle decreased from 45° on smooth samples down to 33° on these nanorough surfaces. Dynamic Dewetting. The experimental apparatus used for dynamic dewetting experiments has been described in detail previously.17,18 A liquid container was used upon which the sample of interest was inverted. By using a microfluidic glass chip with a simple T-junction, small bubbles (of diameter typically between 150 and 350 µm) were formed in the liquid
below the substrate. Single bubbles detached from the microfluidic chip and rose freely, due to buoyancy, until they contacted the substrate. After film thinning, rupture and dewetting of the liquid film between the bubble and the surface took place. The dewetting process was monitored from above using an optical microscope (Olympus BXFM) with a high-speed camera (Photron 1024 PCI, 512 × 512 pixels at 3 000 frames per second) attached. From the optical images, the radius of the circular dewetting patch (contact area) between the bubble and the surface was obtained. For each sample, the experiment was repeated several times, and the dewetting behavior was monitored for at least four bubbles on different spots. After relaxation, the diameter of each bubble was measured individually. Assuming a spherical shape, the volume of each bubble, VB, was determined from the final, relaxed diameter and contact radius. Assuming further that the bubble volume stays constant and its shape remains spherical throughout the dewetting process, the geometric relationship r3 ) 3VB/π · sin3 θ/(2 + 3cos θ - cos3 θ) allows us to calculate the instantaneous contact angle, with respect to the liquid phase, for each snapshot. As noted previously, this calculated angle was verified experimentally using the camera in side view mode.17,18 The dewetting velocity was obtained by numerical derivation of the radius data (polynomial fit over five data points) with respect to time. To reduce the scatter in the low velocity data, a rational function of degree 3 was fit to the data, and its time derivative was taken (data not shown). Plotting the contact line velocity versus the instantaneous water contact angle allows direct comparison between the various experiments as well as with theoretical model functions, independent of the individual bubble size. Results In preliminary experiments, the influence of the n-heptylamine layer thickness on dewetting dynamics was studied. Both static wetting properties as well as dewetting dynamics were investigated. It was observed that the sample with the thinnest coating (10 nm) was slightly more hydrophilic in nature (with a receding water contact angle of 28°), while there was no detectable difference in static wettability between the 20 and 50 nm thick polymer layers (receding contact angles of 34 ( 1°, respectively). Furthermore, within experimental error, contact line dynamics was found to be the same on all three samples, independent of the layer thickness. Thus, in order to avoid any fluctuations in surface wettability due to deviations in coating thickness and to keep the nheptylamine coating as thin as possible to retain the surface topography, a 20 nm layer thickness was chosen for the nanorough SU8 samples. Figure 2 shows typical dewetting data on smooth and nanorough SU8 samples, coated with a 20 nm thick layer of n-heptylamine. The first noticeable feature is that the contact area, characterized by its diameter, does not reach a plateau value within the time scale studied in the experiments. This becomes even more obvious in Figure 2b, where the time (with its origin at film rupture) is shown on a logarithmic scale. Note that the time scale of the experiments ranges over more than 2 orders of magnitude. This suggests that there are two different dewetting regimes: the initial rapid growth in contact diameter with a relatively large but varying slope and a final regime that exhibits a constant slope for contact diameter versus log t. The same qualitative behavior is found for both smooth and nanorough surfaces. In Figure 3, contact line velocity data are plotted against the respective instantaneous water contact angle for the data sets
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Figure 1. AFM images of SU8 surfaces, roughened by air plasma for 0, 6, and 10 min, respectively, and plasma coated with n-heptylamine. Scan size, 1 µm2; height scale, 18 nm; linear gray scale.
TABLE 1: Roughness Parameters and Static Water Contact Angles on Nanorough SU8 Samples with n-Heptylamine Coatinga plasma etching
rms [nm]
A/A0
θa [deg]
θr [deg]
θ0 [deg] via cos θ0 ) (cos θa + cos θr)/2
n/a 6 min 10 min
0.7 3.2 3.9
1.01 1.06 1.08
77 ( 3 82 ( 2 85 ( 2
45 ( 6 35 ( 5 33 ( 8
62 ( 4 61 ( 3 62 ( 4
a
A/A0 is the Wenzel roughness factor (see text).
shown in Figure 2. Dewetting progresses fastest (up to 4 cm/s initial speed) on samples that were not treated with air plasma prior to the n-heptylamine coating step. On these smooth samples, the final receding water contact angle is largest, at about 35°. Roughening of SU8 by treatment with air plasma for 6 and 10 min, respectively, causes both a decrease in dewetting velocity and in the final contact angle. Now, the question arises as to which theoretical models best describe dewetting dynamics on nanorough surfaces and how the respective model parameters correlate with the roughness. As observed in Figures 2b and 3b, the dewetting dynamics involves two distinct regimes. For all data sets, the velocity exhibits exponential behavior over a wide range but deviates from this exponential behavior at high speeds and small angles, that is, in the initial stage of the dewetting process. This deviation is less pronounced for the surfaces with the largest roughness. The hydrodynamic model eq 1 was found to capture the qualitative run of the data in the initial, high velocity small contact angle regime quite well (Figure 3a). The free fitting parameters gained from the best fits to all experimental data sets obtained are shown in Figure 4a, where data for the microscopic parameter ln(L/LS) are plotted against the respective values for θm. The microscopic contact angle data θm are systematically smaller than the static contact angle and may well be velocity dependent. Further, θm was found to depend on surface roughness: the rougher the samples, the smaller was θm. However, there is no obvious trend observed in the results for ln(L/LS) as a function of roughness nor of the microscopic contact angle θm. As mentioned above, the predominant part of the dewetting process (i.e., except the initial stage) shows an exponential behavior of the velocity as function of cos θ. This behavior is predicted by the molecular kinetic approach for large arguments, eq 2. Thus, a straight line was fitted to the data (excluding high
Figure 2. Evolution of contact diameter on nanorough surfaces.
velocities) shown in Figure 3b, and from the respective gradient, the displacement distance λ was obtained. The intercept of the fitted line is a function of both the equilibrium displacement rate K0 and the equilibrium contact angle θ0. Thus, neither K0 nor θ0 can be gained from the fit only; some further independent value is required. An obvious way to obtain such an independent number is to fix the value of the equilibrium contact angle experimentally. The problem with this approach is that, due to substantial contact angle hysteresis in our system, only an upper and lower limit to the equilibrium contact angle, that is, the advancing and receding contact angles θa and θr, are accessible
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Figure 3. Dewetting dynamics on nanorough surfaces. The solid lines in (a) indicate the best fits of the hydrodynamic model; the straight lines in (b) represent the best fits of the molecular kinetic approach for large arguments.
via static contact angle measurements. The equilibrium contact angle θ0 may roughly be estimated via cos θ0 ) (cos θa + cos θr)/2 (cf. Table 1 for respective data). Fixing this value, the equilibrium displacement rate can be determined from the fitting results. Values for K0 in the order of 10-6 to 10-2 Hz are found for the smooth samples, decreasing for increasing roughness down to values between 10-42 and 10-38 Hz for the 10 min plasma treated nanorough sample. Because the θ0 data shown in Table 1 are only a very rough estimate, also these values for K0 should also be understood as an approximate guide.1 Beside static contact angle measurements, dynamic measurements also do not yield a true static value, to which the contact angle relaxes. The contact line velocity decays exponentially as the instantaneous contact angle approaches its static limit, but zero velocity is never reached, at least not within the experimental time frame studied here, and thus, the equilibrium contact angle is not accessible. To resolve this dilemma, a cutoff velocity V0 was chosen, at which the “static” receding contact angle θ0r was taken, θ(V0) ) θ0r. We set the cutoff velocity at an extrapolated speed of V0 1 The reason for such low equilibrium displacement rates might be the large contact angle hysteresis in our system, combined with the fact that K0 is evaluated at or close to the equilibrium contact angle. Most studies dealing with large hysteresis do not use the equilibrium contact angle to obtain K0 but rather the static advancing or receding contact angle, respectively, see below.
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Figure 4. Fitting parameter values for the hydrodynamic regime (a) and the exponential regime (b) for all n-heptylamine coated SU8 samples.
) 10-5 m/s consistently for all data sets, which allowed us to obtain the corresponding values of θr0 and the displacement rate at this angle, K0r, from the best fit. Note that K0r differs significantly from the equilibrium displacement rate K0, which is not accessible. In Figure 4b, the parameter values for λ gained from fitting all experimental data sets are shown versus the respective receding contact angle θ0r. The displacement rate at velocity V0 is given by K0r ) V0/λ (data not shown). As expected, the receding contact angle θ0r decreases for increasingly rough surfaces. Further, some significant variation in the displacement distance λ is found for nanorough surfaces: λ increases with increasing roughness and decreasing θ0r, that is, decreasing apparent hydrophobicity. Discussion For both smooth and nanorough SU8 surfaces, coated with n-heptylamine, two distinct dynamic regimes are observed: the initial, high speed, low contact angle dynamics appears to be governed by hydrodynamics while the final high contact angle regime is described by a thermally activated process.2 These qualitative results are in line with our previous observations on the dewetting dynamics of titania surfaces, partly coated 2 Note that, for advancing contact lines, hydrodynamic behavior is also found for small contact angles, which in that case corresponds to the final, low velocity regime.
Influence of Nanoroughness on Contact Line Motion with a hydrophobic layer of octadecyltrihydrosilane (OTHS).17 In these previous experiments, the intrinsic hydrophobicity of the substrates was varied by tuning the OTHS coverage, while the roughness was kept constant at an rms value below 1 nm. The emergence of the two dynamic regimes in the present system suggests that this qualitative behavior may be universal for all dewetting processes of water on moderately hydrophobic surfaces. The distinct dynamic dewetting regimes are apparently not specific to a certain surface chemistry, at least for the two cases examined here. The microscopic fitting parameters obtained for both dynamic regimes may be compared with theoretical expectations and experimental observations. In the hydrodynamic regime, the ratio of the macroscopic cutoff length scale to the microscopic one determines the viscous friction that resists contact line motion. Reasonable values for ln(L/LS) are expected to be around 12, which was indeed found for the OTHS titania system.17 On smooth and nanorough SU8 surfaces coated with n-heptylamine, values of about 20 are found, independent of the surface roughness. These consistently larger values may reflect some mechanical effect caused by the n-heptylamine coating. Minor viscoelastic deformation of the surface coating due to the unbalanced surface tension perpendicular to the substrate might cause the additional (apparently viscous) resistance.25-27 In spite of this systematic deviation from expectations, we conclude that hydrodynamic friction is indeed dominant in the initial stage of the dewetting process, both for smooth and nanorough surfaces. Remarkably, ln(L/LS) is independent of surface roughness in the hydrodynamic regime. The dewetting velocity is significantly reduced (by 1 order of magnitude) for samples with an rms roughness of only 4 nm compared with smooth samples with 0.7 nm rms roughness. In the hydrodynamic model, this reduction in dewetting speed is accounted for by the decreased values for the microscopic contact angle θm. It seems that nanoroughness influences contact line dynamics in a similar manner to a change in surface hydrophobicity. The major portion of the dewetting dynamic behavior on both smooth and nanorough SU8 surfaces followed an exponential decay of the contact line velocity as function of cos θ. Such behavior is expected for thermally activated processes as described, for instance, by the molecular kinetic theory for large arguments λ2γ · |cos θ0 - cos θ|/(2kBT).8,13 Estimation of the smallest arguments in our system yields values above 10 for all data sets. Thus, even for the data points at the lowest velocity investigated (V ≈ 10-5m/s), the approximation of the MKT by a single exponential is justified. In Figure 5a, the displacement distance λ is compared with results found for OTHS covered titania surfaces. In both systems, λ increases with decreasing receding contact angle θ0r, and the data follow the same master curve. Thus, it appears that changes in intrinsic surface hydrophobicity in the OTHS-titania system and variations of the “apparent” hydrophobicity due to a change in surface nanoroughness in the present system have the same influence on the microscopic model parameter λ. This correlation between static wetting properties and wetting dynamics seems to follow a universal behavior, no matter whether the substrate is chemically or physically heterogeneous, at least for the situations examined here and previously.17,18 On the other hand, the final receding contact angle is of course directly determined by both chemistry and topography. Finally, to explore what the universal connection between static wetting and dynamic dewetting processes might be, the activation free energy of local contact line displacements per
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Figure 5. Comparison of fitting parameters on OTHS coated titania (varying intrinsic wettability) and on n-heptylamine coated SU8 (varying roughness at constant surface chemistry).
unit area, ∆G/λ2, was determined from the fitting parameters and is shown in Figure 5b for both the nanorough n-heptylamine surfaces and the chemically heterogeneous OTHS-titania substrates studied previously.17 For advancing contact line motion, it was found previously for various experimental19-22 and numerical23,24 investigations that the specific activation free energy correlates with the work of adhesion of the liquid, Wad ) γ(1 + cos θ0), where the equilibrium contact angle θ0 is commonly replaced by the experimentally accessible static advancing contact angle θa. This correlation is clearly not observed for dewetting dynamics. However, the reverse, which (v) ) γ(1 - cos is strictly the work of adhesion of the vapor, Wad θ0), where θ0 is replaced by the experimentally accessible static receding water contact angle θr, correlates rather nicely with the data for both chemically heterogeneous OTHS-titania surfaces17 and the topographically structured n-heptylamine coated SU8 samples studied here. For chemical heterogeneity, we argued previously17 that the thermally activated contact line displacements correspond to local pinning and depinning events, controlled by a microscopic pinning energy on chemical defects, r - cos θr). For hydrophilic defects given by P ) γ(cos θdefect r ) 0, this equation cannot be with receding contact angle θdefect distinguished from the work of adhesion of the vapor. Conclusion Dewetting dynamic behavior for both smooth and nanorough surfaces was investigated. The experiments confirmed that contact line motion is governed by two distinct dynamic regimes: initially, at high speeds and small contact angles,
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hydrodynamic friction dominates dewetting, while the slow relaxation toward equilibrium is governed by a thermally activated process of local contact line displacements. In this final regime, dewetting velocity decayed exponentially as a function of cos θ. From the exponent, the displacement distance λ was obtained directly and independently of any other parameters, including the displacement rate K0. A quantitative comparison of the results obtained for dewetting dynamics on nanorough surfaces in the present study with data obtained previously on OTHS-titania surfaces, where the intrinsic hydrophobicity was changed, was undertaken. This comparison suggests that, for these two systems, dewetting dynamics follows a universal pattern that depends only on the final static contact angle of the respective system. For the contact line, this final contact angle may be controlled by the surface chemistry or the nanoroughnessstheir influence appears to be the same. Acknowledgment. The authors thank Krasimir Vasilev for coating our samples with n-heptylamine. The financial support of the Australian Research Council Linkage Scheme, AMIRA International, and State Governments of South Australia and Victoria is gratefully acknowledged. References and Notes (1) Quere, D. Annu. ReV. Mater. Res. 2008, 38, 71. (2) Rolley, E.; Guthmann, C. Phys. ReV. Lett. 2007, 98, 166105. (3) Forsberg, P. S. H.; Priest, C.; Brinkmann, M.; Sedev, R.; Ralston, J. Langmuir 2010, 26, 860.
Fetzer and Ralston (4) Raphael, R. E.; Joanny, J. F. Europhys. Lett. 1993, 21, 483. (5) Voinov, O. V. Mech. Liquids Gas 1975, 5, 76. (6) Voinov, O. V. Fluid Dyn. 1976, 11, 714. (7) Cox, R. G. J. Fluid Mech. 1986, 168, 169. (8) Blake, T. D.; Haynes, J. M. J. Colloid Interface Sci. 1969, 30, 421. (9) Petrov, P.; Petrov, I. Langmuir 1992, 8, 1762. (10) Shikhmurzaev, Y. D. Int. J. Multiphase Flow 1993, 19, 589. (11) de Ruijter, M. J.; De Coninck, J.; Oshanin, G. Langmuir 1999, 15, 2209. (12) Shikhmurzaev, Y. D. J. Fluid Mech. 2000, 334, 211. (13) Ralston, J.; Popescu, M.; Sedev, R. Annu. ReV. Mater. Res. 2008, 38, 23. (14) Petrov, J.; Ralston, J.; Schneemilch, M.; Hayes, R. A. J. Phys. Chem. B 2003, 107, 1634. (15) Walther, F.; Davydovskaya, P.; Zurcher, S.; Kaiser, M.; Herberg, H. l.; Gigler, A. M.; Stark, R. W. J. Micromech. Microeng. 2007, 17, 524. (16) Vasilev, K.; Britcher, L.; Casanal, A.; Griesser, H. J. J. Phys. Chem. B 2008, 112, 10915. (17) Fetzer, R.; Ralston, J. J. Phys. Chem. C 2009, 113, 8888. (18) Fetzer, R.; Ramiasa, M.; Ralston, J. Langmuir 2009, 25, 8069. (19) Semal, S.; Bauthier, C.; Voue, M.; Vanden Eynde, J. J.; Gouttebaron, R.; De Coninck, J. J. Phys. Chem. B 2000, 104, 6225. (20) Petrov, J. G.; Ralston, J.; Schneemilch, M.; Hayes, R. A. Langmuir 2003, 19, 2795. (21) Vega, M. J.; Gouttiere, C.; Seveno, D.; Blake, T. D.; Voue, M.; De Coninck, J.; Clarke, A. Langmuir 2007, 23, 10628. (22) Ray, S.; Sedev, R.; Priest, C.; Ralston, J. Langmuir 2008, 24, 13007. (23) Adao, M. H.; de Ruijter, M.; Voue, M.; De Coninck, J. Phys. ReV. E 1999, 59, 746. (24) Martic, G.; Blake, T. D.; De Coninck, J. Langmuir 2005, 21, 11201. (25) Shanahan, M. E. R.; Carre, A. Langmuir 1995, 11, 1396. (26) Voue, M.; Rioboo, R.; Bauthier, C.; Conti, J.; Charlot, M.; De Coninck, J. J. Eur. Ceram. Soc. 2003, 23, 2769. (27) Sedeva, I. G.; Fetzer, R.; Fornasiero, D.; Ralston, J.; Beattie, D. A. J. Colloid Interface Sci. 2010, 345, 417.
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