Receding Contact Line Motion on Nanopatterned and Micropatterned

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Receding Contact Line Motion on Nanoand Micro-patterned Polymer Surfaces Nan Gao, Ming Chiu, and Chiara Neto Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03100 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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Receding Contact Line Motion on Nano- and Micro-patterned Polymer Surfaces Nan Gao,1,2 Ming Chiu,2 Chiara Neto2,* 1 Future Industries Institute, University of South Australia, Mawson Lakes Campus, Adelaide, SA 5095, Australia 2 School of Chemistry and Australian Institute for Nanoscale Science and Technology, The University of Sydney, NSW 2006, Australia * Corresponding author: [email protected] Abstract Surface properties such as topography and chemistry affect the motion of the three-phase contact line (solid/liquid/air), which in turn affects the contact angle of a liquid moving on a solid surface. In this work, the motion of the receding water contact line was studied on chemically and topographically patterned surfaces obtained from the dewetting of thin polymer films. The patterned surfaces consisted of hydrophilic poly(4-vinylpyridine) (P4VP) bumps, which were either micro-sized and sparse or nano-sized and dense, on top of a hydrophobic polystyrene (PS) background layer. These patterns are designed for atmospheric water capture, for which the easy roll-off of water droplets is crucial to their efficient performance. The dynamic receding water contact angle and contact line height of the patterned surfaces were measured by withdrawing vertically the surfaces from a water bath and compared to those of a flat P4VP substrate. For both the micro-patterned and nano-patterned surfaces, the height of the dynamic contact lines normalised by the capillary length were characterised by the equilibrium limit that was predicted from static states. The nano-patterned surface had faster increase in the normalised height as the capillary number increased. The dynamic receding contact angles on all surfaces studied decreased with increasing withdrawing velocity. Surprisingly, even for these patterned surfaces with high hysteresis, the dynamic receding contact angle followed the Cox– Voinov relation, at capillary numbers between 1-5 ·10-5. Introduction The surface structure on the exoskeleton of the Namib desert beetle is optimised to collect atmospheric water:1 it is made of an array of hydrophilic bumps 0.5 - 1.5 mm apart, each about 0.5 mm in diameter, over a hydrophobic background. The hydrophilic bumps act as nucleation points for water droplets in fog-laden winds, and at these locations water droplets grow until they are sufficiently large for gravity to carry them down to the beetle’s mouth, serving as a viable supply for drinking water. The surrounding hydrophobic areas facilitate the formation of droplets, i.e. drop-wise condensation, which is a more efficient method of water collection than a uniform film of water (filmwise condensation).2 In this regard, the Namib Desert beetle has been considered to have “nature’s version of a drop-wise condensing surface.”3 Neto and co-workers have shown that surface patterns consisting of raised hydrophilic bumps on a hydrophobic background can be prepared by the dewetting of thin polymer bilayers.4-6 Dewetting is the spontaneous break-up of a liquid film on a surface, driven by intermolecular forces at the interface between the two materials, and results in the transformation of the liquid films into isolated droplets (typical film thickness around 100 nm).7-9 In the case of polymer films, the instability can be initiated by thermal or solvent annealing above the polymer’s glass transition temperature. The most common dewetting mechanism, nucleation dewetting, leads to a series of isolated droplets that are random in 1 ACS Paragon Plus Environment

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position (typical droplet diameter of the order of a few micrometers).4 Thus, a stable pattern of microscopic polymer bumps (droplets) can be created, with tuneable diameter, height, contact angle of the bump and distribution, dependent on the initial film thickness and on the annealing method. Recent work in the Neto group has shown that by annealing polymer films in mixtures of good and poor quality solvent vapors, much thicker films, up to 600 nm thick, may be dewetted, which increases the range of possible bump diameters by 2 orders of magnitude.10-11 A bilayer of polymer films may be made to dewet into surface patterns with chemical and topographical contrasts;6 in the water capture application, a hydrophilic polymer poly(4vinylpyridine) (P4VP) film dewets from a hydrophobic polystyrene (PS) film substrate, creating a topography that mimics the back of the Namib desert beetle.4 The micropatterned surfaces have improved performance in atmospheric water collection than the corresponding flat films.4, 11 In particular, the P4VP/PS micropatterned surfaces had a lower critical tilt angle (roll-off angle) for water droplet detachment than that of a flat P4VP surface, which instead encourages film-wise condensation, slowing down water collection.12 A smaller roll-off angle means that condensed water droplets detach from the surface at lower volumes, leading to faster water collection rates.13-16 On the other hand, the presence of hydrophilic P4VP patches is beneficial as it decreases the barrier for water droplet nucleation, so the P4VP/PS patterns enable condensation at lower humidity and at lower surface sub-cooling compared to a flat PS film.17 However, the raised P4VP bumps in the pattern pin the contact line and delay the roll-off of the condensed water droplets compared to flat hydrophobic surfaces, as revealed by relatively high contact angle hysteresis values on the patterned surfaces (30° 40°).4, 12 The motion of the contact line over wide areas containing random patterns is crucial to optimise the performance of surface coatings in atmospheric water collection. The motion of the contact line on rough or structured surfaces is also important in many industrial processes, such as coatings, microfluidic devices, printing and lithography, and the validity of existing theoretical models for these structured surfaces has not been sufficiently tested. The alternating hydrophilic domains across the patterned surfaces can influence contact line motion (spreading) of the condensing water. Motion of the liquid/solid contact line is a complex dynamic process, which depends on combined effects of interfacial energy balance, energy dissipation, and geometrical or chemical heterogeneities of the solid surface.18-19 It has been studied by examining contact line dynamics during interfacial wetting (or dewetting) phenomena, for example, by vertically withdrawing a solid surface from a liquid reservoir at a constant velocity U.20-23 Most studies have investigated either smooth uniform surfaces or regular patterns, and especially surfaces with very low hysteresis;24 hardly any work has been dedicated to exploring the shape (meniscus) of dynamic interfaces near the contact line on patterned bilayers, or surfaces with high hysteresis. In contrast to smooth surfaces, where the apparent contact line advances continuously, on patterned surfaces, such as those with ultra-liquid-repellent properties, the advancing liquid can gradually bend or touch down, so that a contact angle of 180° or greater can be achieved, regardless of the intrinsic surface wettability.25 On the receding side, contact line pinning takes place due to variations in surface wettability associated with surface patterns, and determines the apparent receding contact angle. Consequently, the apparent receding contact angle is more useful than the apparent advancing contact angle, and even contact angle hysteresis (particularly for ultra-liquid-repellent surface patterns), in the characterization of surface patterns.

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Some recent studies focused on the motion of single droplets instead, such as spreading,26-28 evaporation,29-30 and condensation31-32. A single topographical defect can pin and warp the contact line of the droplet locally as a macroscopic force on the contact line per unit length, F = γ(cosθ - cosθE), will be present due to the difference between the local contact angle θ and the equilibrium contact angle θE from Young’s equation.33 Here γ is the surface tension. Thus the contact line will be stopped from moving while the local contact angle decreases over time, until a threshold force is overcome.34 This mechanism is responsible for contact line behaviour of droplets on rough substrates. In this work, the dynamic receding water contact angle and contact line motion were studied on topographically and chemically patterned surfaces of high hysteresis, by withdrawing the surface out of a water bath at constant velocities. The contact line motion on the structured surfaces was compared with that of highly hysteretic, but uniform hydrophilic surfaces. The effect of the size and distribution of the patterns on contact angle development as well as contact line motion was also explored. The experimentally extracted dynamic contact angle and height of capillary rise were compared with theory.

Theoretical background When a plate is immersed in a bath of liquid at rest, the height Z0 of the three-phase contact line at which the liquid rises on the plate is related to θ0, the apparent equilibrium contact angle of the meniscus at the interface between the liquid, the wall and the air, by the following relation:35-36  = 2(1 − sin  )

Eq. 1

where L is the capillary length defined as  = /, with γ the surface tension, ρ the density and g the gravitational acceleration (L ≈ 2.7 mm for water). That is, the static equilibrium shape of a meniscus that climbs up a wall results from the balance between the surface tension and gravity.33 When the plate is withdrawn from the liquid at a constant velocity, U, the apparent contact angle will decrease and find a dynamic equilibrium. According to the Cox-Voinov model, the relation between the dynamic apparent contact angle and the rescaled contact line velocity can be established using the following equation:37-38

 =  ± 9 

 

Eq. 2

where θd is the dynamic contact angle, θY is the Young’s contact angle, R is the macroscopic length (which can be the capillary length L or the size of a spreading drop), l is a microscopic length that represents the molecular processes around the moving contact line, and Ca is the capillary number, representing the rescaled contact line velocity which takes into account the effect of the viscosity, η, and the surface tension, γ,  =

 !

Eq. 3

Dynamic wetting can be described using several models.39 Here we take into consideration the CoxVoinov model from the hydrodynamic theory, which assumes a balance of viscous and capillary forces near the contact line, characterised by an intermediate region separating the inner (microscopic) and outer (macroscopic) zones.40-41 To use of the Cox-Voinov relation, inertial effect need to be 3 ACS Paragon Plus Environment

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negligible. In practice this means that the Cox-Voinov relation applies at low Reynolds number, "$& "# = % , which represents the competition between inertial force and viscous force within a 37 liquid. The hydrodynamics for the moving contact line has been described by a corner flow in the absence of any intrinsic length scale.42-43 This can also justify neglecting inertia, where a local Reynolds number based on a small distance to the contact line must be small. The Cox-Voinov relation has been successfully applied to describing forced wetting,39, 44-45 but it has also been seen to fail at very low capillary numbers smaller than the order of 10-4 - 10-5.44 On the one hand, even though the Cox-Voinov relation was derived for small advancing contact angles only, it has been also reported to be valid for large advancing contact angles up to 100° - 150°,40, 46-47 as well as receding contact angles.44 The sign within the Cox-Voinov relation is positive for advancing contact angles and negative for receding contact angles. It should be noted that this theory may not be applied to the spreading of a droplet associated with rapid changes in contact angles caused by randomly strong surface heterogeneity.37 In the present work, the receding contact angle on partially wetted surfaces is considered. On the other hand, at high velocities, an entrained liquid film will be formed on the surface, corresponding to a vanishing dynamic contact angle.33, 48 When the capillary number is large enough to cause an entrained thin film, the ground on which Cox-Voinov relation is based will be invalid. Particularly, for smooth surfaces that can be partially wetted, it has been reported that a critical capillary number determined by the microscopic contact angle will lead to the formation of a capillary ridge that can cause the disappearance of stationary menisci.23 However, the case of partial wetting has not been investigated thoroughly to correlate with surface patterns, and certainly not with strong topographical and chemical patterns. This leads to the need to further investigate how contact line motion develops on surfaces with topographical and chemical patterns.

Figure 1. (a) Schematic of the experimental scenario for withdrawing a vertical plate from a water reservoir at a constant velocity U. The apparent static contact angle is θ0 and the capillary rise is Z0 when the plate is at rest (the dashed line). When the plate is moved vertically, the apparent contact angle decreases and reaches a dynamic equilibrium at θd, while the capillary rise increases to Zd. (b-d) Side view images extracted from a high-speed camera recording: capillary rise of water against a P4VP substrate from a static state (b) to a dynamic equilibrium (c); capillary rise of water against a micro-patterned substrate from a static state (d) to a dynamic equilibrium (e). The red dotted lines in (c) and (e) are used to guide the eye for contact angle measurement. Both substrates were withdrawn at U = 200 mm/min. Experimental Surface preparation and characterisation Polystyrene (PS, MW = 350 kg mol-1, Sigma-Aldrich) films of 123 ± 6 nm thickness were prepared by dip-coating (KSV, NIMA, Stockholm, Sweden) from a toluene solution (2 wt %, extracted at 60 mm min-1) onto approximately 30 mm × 30 mm × 0.53 mm silicon wafers coated with a native oxide layer 4 ACS Paragon Plus Environment

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(1.8 ± 0.2 nm thick, MMRC Pty. Ltd., Australia). Poly(4-vinylpyridine) (P4VP, MW = 60 kg mol-1, Sigma-Aldrich) films of 6 nm and 85 nm thickness for nano- and micro- patterns, respectively, were subsequently dip-coated onto this substrate from an ethanol solution (0.2 or 1.5 wt %, extracted at 120 or 200 mm min-1). To induce dewetting, the bilayer films were placed in a saturated vapor environment of a mixture of ethanol (99.7%, Merck), acetone (99.8%, Merck) and water (Millipore, resistance = 18.1 MΩ cm-1), respectively a good, poor, and non-solvent for P4VP, in a customdesigned Teflon cell. The liquid weight ratio for the solvent mixture of ethanol:acetone:water is 30:30:40, as described elsewhere.12 Prior to coating, the silicon wafers were thoroughly cleaned by sonication in ethanol, followed by sonication in acetone, and blown dry with pure N2. Then the wafer surfaces were exposed to a CO2 snow jet gun (Applied Surface Technologies, NJ), and treated by air plasma (Harrick Plasma, Ithaca, NY, model PDC-002) for 5 minutes. Spectroscopic ellipsometry (J.A. Woollam Co. Inc., M2000) was used to measure the thickness of the polymer films over three spots on each sample. The final dewetted pattern was observed by optical microscopy (Nikon Eclipse LV150). Tapping mode atomic force microscopy (AFM, Bruker Multimode 8) was used to characterise the surface topography. The advancing and receding contact angles of water droplets on PS, P4VP, nano- and micro-patterned surfaces were measured by a goniometer (KSV CAM 200) using volume addition-subtraction method by adding and removing, respectively, 15 µL to a 3 µL water droplet at 0.1 µL s-1. The Young-Laplace fitting model was used.

Experimental Set-up The experiment for withdrawing a solid substrate from a glass container filled with water is schematically shown in Figure 1. The substrates studied in our experiment were silicon wafers coated with either plain P4VP films or P4VP/PS bilayer patterns, as described previously. These substrates were cleaned with Milli-Q water and high purity N2 gas before they were attached to a dip-coater (NIMA KSV). The dip-coater allowed steady vertical velocities ranging from 2 mm/min to 200 mm/min. Prior to the measurement, a substrate was immersed, by means of the dip-coater, into freshly collected Milli-Q water in a glass container that was around 80 mm in height and 60 mm in diameter. The sample was left to equilibrate for several minutes before being withdrawn vertically from the water bath at a constant velocity. Standard values were used for the density (1000 kg/m3), surface tension (72.5 mN/m), and viscosity (1.002 mPa·s) of water. The capillary length, L, was calculated to be 2.7 mm. A high-speed camera (Fastec IL3, San Diego, CA) with a magnification lens was used to capture the meniscus motion from front and side views and the dynamic contact angle and height of capillary rise were extracted using ImageJ. Figure 1 (b-d) show examples of the meniscus rise on P4VP and bilayer substrates. Results and Discussions Figure 2 shows optical and AFM micrographs of the micro- and nano-patterned surfaces, resulting from the dewetting of a P4VP/PS bilayer, which consist of a series of isolated P4VP bumps on a PS background. By varying the initial P4VP film thickness and the annealing procedure, the diameter of the P4VP bumps and their height could be varied by almost 2 orders of magnitude (1−80 µm and 40−9000 nm, respectively), and their distribution density could be varied by 5 orders of magnitude.12 In the following context, the patterns are referred to as either ‘nano-’ or ‘micro-’, based on the length scale of the diameter of the P4VP bumps. For the micro-patterned surface, the average P4VP bump 5 ACS Paragon Plus Environment

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density was 980 ± 250 per mm2, and the diameter of the P4VP bumps was 12 ± 6 µm, with a distribution shown in Figure S1 in Supporting Information, and the height 1200 ± 700 nm. The nanopatterned surface had much higher average P4VP bump density of (3.3 ± 1.6) × 105 per mm2, smaller diameters of 1 ± 0.2 µm (distribution in Figure S1), and lower height of 80 ± 20 nm. In both cases, the bilayer surfaces mimicked the Namib desert beetle exoskeleton with raised hydrophilic bumps on a hydrophobic background, but with smaller-sized bumps. The portion of surface area which was hydrophilic was 16 ± 4 % for the nano-patterns and 6.4 ± 0.9 % for the micro-patterns.

Figure 2. Optical and AFM micrographs of P4VP/PS dewetted bilayers showing isolated P4VP bumps on top of a PS film. (a) Optical micrograph of micro-patterned surface. Inset: AFM micrograph of micro-patterned surface (scale bar = 30 µm, height scale = 4 µm). (b) Optical micrograph of nanopatterned surface. Inset: AFM micrograph of nano-patterned surface (scale bar = 4 µm, height scale = 180 nm). (c) AFM cross sectional profiles of red lines in a) and b) of micro- and nano-patterned surfaces. Table 1 Advancing (ΘAdv) and receding (ΘRec) contact angles measured with a sessile water droplet on flat P4VP and PS films, and on the micro-patterned and nano-patterned bilayer surfaces.

ΘAdv (o) o

ΘRec ( )

P4VP

PS 103 ± 1

Micropatterns 99 ± 2

Nanopatterns 77 ± 3

76 ± 2 15 ± 1

90 ± 1

61 ± 3

20 ± 3

The wettability of the flat and patterned surfaces as measured through a sessile water droplet is presented in Table 1. The micro-patterned surface showed higher water contact angle values than the nano-patterned surface, because of the ~10% larger portion of PS background layer exposed. The receding contact angle of water on the micro-patterned surfaces (61°) was intermediate between those of PS (90°) and P4VP (15°) surfaces. However, the receding contact angle of water droplets on the nano-patterned surfaces was around 20°, very close to that on plain P4VP. This effect points to the strong pinning of the contact line caused by the densely packed P4VP nanoscale bumps. Similarly, the advancing contact angle of water for the micro-pattern (99°) was close to that of plain PS (103°), while that of the nano-pattern (77°) was close to that of plain P4VP (76°). The higher contact angle hysteresis on the nano-patterns, compared with the micro-patterns, and even with the plain P4VP surfaces, is likely to retard droplet roll-off, which is deleterious for atmospheric water capture. However, the sessile droplet measurements do not reveal how the velocity of the contact line may affect contact line pinning, which is highly relevant in an atmospheric water capture scenario, where 6 ACS Paragon Plus Environment

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droplets may roll-off a vertical surface and bring with them sheets of water. To obtain this insight, withdrawal experiments from a water bath were necessary. When the patterned surfaces were withdrawn from a clean water bath, the contact line height increased from the initial value Z0 to a new equilibrium value Zd. Figure 3(a) shows typical examples of the height of the water contact line on the P4VP surface and the patterned surfaces due to capillary rise at a withdrawing velocity of 200 mm/min. As predicted by Eq. 1, the contact line on the most hydrophilic P4VP surface rose higher over time than the other two surfaces before reaching the relative equilibrium in the dynamic state. Further to that, since the micro-patterned surface was most hydrophobic, it exhibited the smallest contact line rise. In the cases presented here, the dynamic contact line heights were around 5 mm on the P4VP surface, 3 - 3.5 mm on the nano-patterned surface, and 1.5 mm on the micro-patterned surface. Figure 3(b) shows the equilibrated height in the dynamic state Zd and its normalised form by the capillary length, L, as a function of withdrawing velocity/capillary number Ca. As the velocity increased from 2 mm/min to 200 mm/min, the height in the dynamic equilibrium increased approximately from 2.3 mm to 3 mm for the nano-patterned surfaces and from 1.3 mm to 1.6 mm for the micro-patterned surfaces. For the hydrophilic P4VP substrate, a thin film of liquid water was entrained at 200 mm/min (Figure S2, Supporting Info). Yet this thin film ruptured before an equilibrated meniscus could be formed, possibly due to the low viscosity of water. Its contact line height was hence measured based on an apparent meniscus, not including the thin film.

Figure 3. (a) Development of the contact line height on the nano-patterned and micro-patterned surfaces from a stationary state Zo as a function of time at the velocity of 200 mm/min. (b) Dynamic contact line height Zd normalised by capillary length L as a function of capillary number, Ca.

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The normalised height, Z/L, according to Eq. 1, is proportional to 2(1 − sin ) in a static equilibrium. For non-wetting liquids (θ > 0°), Z/L will be lower than √2 = 1.41. In the dynamic states, the motion of the receding contact line will lead to liquid deposition at high capillary numbers.19, 23 As mentioned above, film entrainment indeed occurred during the development of the dynamic contact line on the P4VP surface. Neither patterned surface exceeded the equilibrium limit arising from Eq. 1 within our experimental range, as shown by the dashed line in Figure 3(b). The normalised height for the nano-patterned surface approached the static limit faster than the micropatterned surface as the capillary number increased. Based on the partially wettable behaviour of the patterned surfaces, it is predicted that there will be a threshold capillary number leading to the entrainment of a thin film of liquid water on both patterned surfaces.22 At withdrawing rates higher than investigated here, the nano-patterned surfaces will likely entrain a liquid film sooner than the micro-patterned surfaces, and this effect again identifies the nano-patterns as non-ideal surfaces for atmospheric water capture, as their wettability encourages film-wise condensation. While the height of the meniscus increased as the surfaces were being withdrawn, the contact angle values decreased, as expected, starting from the static values and reaching a new equilibrium receding value over time. Figure 4(a) shows some representative apparent contact angles measured on flat P4VP, nano-patterned and micro-patterned surfaces at the velocity of 200 mm/min from the stationary state as a function of time. In the stationary state, the contact angle value of the meniscus may be subject to the specific spot chosen. When the motion began, the apparent contact angle decreased to reach a new equilibrium state: θd = 56° ± 2° for the micro-patterned surface, 40° ± 4° for the nanopatterned surface, and 24° ± 2° for the flat P4VP surface (Figure 4(a)). These values of receding contact angles are close to those measured with sessile water droplets (Table 1), and follow the same trend of higher pinning on the nano-pattern than on the micro-pattern. The measurements with sessile droplet are also dynamic of course, with a rate of the contact line movement around 0.30 ± 0.01 mm s1 on all surfaces, as established by examining the video of the droplet profile collected while dispensing or withdrawing water from a needle at a rate of 0.2 µL s-1.

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Figure 4 (a) Development of the contact angle at the contact line on P4VP, nano-patterned and micropatterned surfaces from a stationary state as a function of time at withdrawing velocity of 200 mm/min. (b) Dynamic receding contact angles on the three surfaces as a function of withdrawing velocity. (c) The cube of the dynamic receding contact angle θd as a function of the capillary number. The straight lines are linear fits of the relatively high Ca regime (approximately from 1.15×10-5 to 4.61×10-5) with extrapolation to the low Ca regime. The maximum changes in contact angle values over the first 5 seconds of withdrawal motion in Figure 4(a) are listed in Table 2 for the different surface types and velocities. The decrease in apparent contact angles (CA) became larger with increasing velocity for all three types of surfaces, with the plain hydrophilic P4VP surface showing the highest decrease, then the nano-pattern, followed by the micro-pattern. Specifically, when the velocity increased from 2 mm/min to 200 mm/min, the decrease in apparent contact angle grew from around 17° to 30° for the P4VP surface, from 22° to 28° for the micro-patterned surface, and from 33° to 41° for the nano-patterned surface. Uncertainties may exist here due to the entraining and rupturing of thin liquid water film on the P4VP surface.

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Table 2 Decrease in contact angle values of the surfaces withdrawn from the water bath at different velocities. Velocity (mm/min) 2 10 20 50 100 200

P4VP 17.0±10.3 22.0±12.3 23.7±9.7 27.8±8.7 30.1±8.4 30.4±8.3

Decrease in CA, θ0 - θd. (°) Micro-Pattern Nano-Pattern 21.8±3.7 33.1±5.0 22.0±3.9 36.1±4.6 24.3±4.1 36.7±4.5 25.3±4.7 39.4±5.2 28.3±4.1 40.2±5.0 27.9±3.6 40.6±4.9

Figure 4(b) shows the dynamic receding contact angle values as a function of velocity for the three surfaces. In the relatively low velocity regime (up to 50 mm/min), the dynamic receding contact angles all decreased gradually with velocity. For higher velocities, the receding contact angle values reached a steady state and nearly plateaued. The contact line friction is dominant over the viscous dissipation in such receding scenarios.40 Figure 4(c) shows the experimental results plotted as the cube of the receding contact angles versus the capillary number. Two velocity regimes were identified: the receding contact angle values decreased strongly at low velocities (for Ca < 1×10-5), and then more gradually at higher velocities, up to a value of Ca of around 5×10-5. The latter, slower decrease part could be fitted well with the Cox-Voinov relation as described in the form of Eq. 2, leading to linear fits. Consistent to the previous results obtained by Henrich et al.,44 a deviation from the model was observed at low capillary numbers (velocities ≤ 10 mm/min) for all surfaces, which could arise from surface roughness, and the presence of the pattern. The fluctuation in contact angles shown in Figure 4 depends on the difference in surface morphology and P4VP surface fraction between the three types of samples. The deviation from the Cox-Voinov model was most pronounced for the micro-patterned surfaces, with large deviations of the cube of the dynamic receding contact angle from the hydrodynamic theory fit, even at high velocities. It is likely that the large size of the P4VP bumps on top of the PS background in the micro-patterned surfaces, which create large jumps in surface wettability and large pinning points, is responsible for this effect. The bare P4VP surface presented some deviation from the model at low velocities, which is not surprising given the finite roughness of even the flat P4VP; but when the velocity increased, the cube of the dynamic contact angles for P4VP became more stable, with progressively lower scatter. The scatter in the cube of the dynamic contact angles was less significant for low contact angles as compared with large contact angles. The nano-patterned surface was in an intermediate state between the micro-patterned and flat P4VP surfaces, as previously demonstrated by the contact angle measurement with water droplets. In accordance with the hydrodynamic theory, the cube of the receding contact angles on the nano-patterns nearly reached a plateau at high capillary numbers; the deviation from the model was smaller for the nano-patterns than for the micro-patterns, yet bigger than for the flat P4VP surface. Conclusions In these measurements, it was shown that the dynamic receding contact angle of water on patterned P4VP/PS surfaces depends on the size and distribution of the hydrophilic P4VP bumps in the range of velocities 2 - 200 mm/min. The surface with micro-sized P4VP bumps showed higher contact angle values, because of the slightly larger exposure of the PS background layer, than the nano-patterned surface. This resulted in lower height values of the contact line at the micro-patterned surface than at the nano-patterned surface when they were being withdrawn vertically at constant velocities from a water bath. Relative to the micro-patterned surface, the nano-patterned surface had faster increase in 10 ACS Paragon Plus Environment

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the normalised height Z/L with the capillary number. This comparison is useful for the evaluation of the performance of these surfaces as platforms for atmospheric water capture: the nano-patterned surface is more likely to trap a thin water film and lead to film-wise condensation, than the micropatterned surface. The dynamic contact angles of the micro-patterned and nano-patterned surfaces were compared with a flat P4VP surface. For all surfaces studied in this work, the receding contact angles decreased with velocity. The change of the receding contact angles could be fitted well by the Cox-Voinov hydrodynamic theory at high velocities, corresponding to Ca values higher than 1·10-5, and this is remarkable as the patterned surfaces studied in this work presented large variation in topography and wettability, and large hysteresis. Below this value of Ca, the contact angle decreased more rapidly than predicted by the model, consistent with observations made with uniform surface structures. Confirming our recent work on water capture, these results demonstrate that the size and distribution of the hydrophilic P4VP bumps in the patterned surfaces are critical to the contact line dynamics, which in turn is important to the use of the surfaces in water collection. The large number of hydrophilic bumps on nano-patterned bilayer surface provides many sites for condensation, which is advantageous under low humidity or low surface sub-cooling conditions. However, the large contact angle hysteresis associated with the nano-patterns is not a desirable feature for overcoming adhesion of water droplets. On the other hand, this paper confirms our previous findings that increasing the size of the hydrophilic bumps and reducing their number on the surface, with a corresponding increased exposure of hydrophobic background layer, is a more efficient way to achieve an optimal drop-wise state for water collection that will ensure reasonable water condensation and detachment of water droplets at smaller volumes. Supporting Information Size distribution of P4VP bumps on the micro-patterned surfaces; micrographs of a water film entrained on the P4VP substrate upon withdrawing from the water bath. Acknowledgments Dr Gao was the recipient of the Endeavour Research Fellowship of the Australian Government. References: 1. Parker, A. R.; Lawrence, C. R., Water capture by a desert beetle. Nature 2001, 414 (6859), 33-34. 2. Rose, J. W., Condensation Heat Transfer Fundamentals. Chemical Engineering Research and Design 76 (2), 143-152. 3. Varanasi, K. K.; Hsu, M.; Bhate, N.; Yang, W.; Deng, T., Spatial control in the heterogeneous nucleation of water. Applied Physics Letters 2009, 95 (9), 094101. 4. Thickett, S. C.; Neto, C.; Harris, A. T., Biomimetic Surface Coatings for Atmospheric Water Capture Prepared by Dewetting of Polymer Films. Adv. Mater. 2011, 23 (32), 3718-3722. 5. Telford, A. M.; Thickett, S. C.; Neto, C., Functional patterned coatings by thin polymer film dewetting. J. Colloid Interface Sci. 2017, 507, 453-469. 6. Neto, C., A novel approach to the micropatterning of proteins using dewetting of polymer bilayers. Physical Chemistry Chemical Physics 2007, 9 (1), 149-155. 7. Vrij, A., Possible mechanism for the spontaneous rupture of thin, free liquid films. Discussions of the Faraday Society 1966, 42 (0), 23-33. 8. Seemann, R.; Herminghaus, S.; Jacobs, K., Dewetting Patterns and Molecular Forces: A Reconciliation. Phys. Rev. Lett. 2001, 86 (24), 5534-5537. 11 ACS Paragon Plus Environment

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