Research Article www.acsami.org
Patterned Polymer Coatings Increase the Efficiency of Dew Harvesting Omar Al-Khayat,†,‡ Jun Ki Hong,† David M. Beck,† Andrew I. Minett,‡ and Chiara Neto*,† †
School of Chemistry and ‡School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia S Supporting Information *
ABSTRACT: Micropatterned polymer surfaces, possessing both topographical and chemical characteristics, were prepared on three-dimensional copper tubes and used to capture atmospheric water. The micropatterns mimic the structure on the back of a desert beetle that condenses water from the air in a very dry environment. The patterned coatings were prepared by the dewetting of thin films of poly-4-vinylpyridine (P4VP) on top of polystyrene films (PS) films, upon solvent annealing, and consist of raised hydrophilic bumps on a hydrophobic background. The size and density distribution of the hydrophilic bumps could be tuned widely by adjusting the initial thickness of the P4VP films: the diameter of the produced 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. Under low subcooling conditions (3 °C), the highest rate of water condensation was measured on the largest (80 μm diameter) hydrophilic bumps and was found to be 57% higher than that on flat hydrophobic films. These subcooling conditions are achieved spontaneously in dew formation, by passive radiative cooling of a surface exposed to the night sky. In effect, the pattern would result in a larger number of dewy nights than a flat hydrophobic surface and therefore increases water capture efficiency. Our approach is suited to fabrication on a large scale, to enable the use of the patterned coatings for water collection with no external input of energy. KEYWORDS: thin films, water harvesting, condenser, biomimetic, polymers, dew
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INTRODUCTION The World Economic Forum, in its latest annual risk report, has listed water crises as the leading global potential risk in terms of impact and eighth in terms of likelihood.1 In a recent study on monthly global water scarcity, Mekonnen and Hoekstra concluded that two-thirds of the global population (4 billion people) live under severe water scarcity for at least 1 month of the year. Furthermore, half a billion people face yearround severe water scarcity.2 Harvesting water from atmospheric humidity and fog is a promising solution3 as a source of freshwater for regions around the world without ready access to surface and groundwater resources. A beetle of the Namib Desert, Physosterna cribripes, previously misidentified as Stenocara gracipiles,4,5 has adapted its elytra to facilitate fog collection, with an array of raised hydrophilic bumps approximately 0.5−1.5 mm apart and 0.5 mm in diameter on a hydrophobic background. Water from the fog condenses on the hydrophilic bumps at dawn, when the surface is cooler than the surrounding air; the droplets grow in size until they detach and roll off the surface and into the mouth of the beetle, and the hydrophobic nature of the background helps to facilitate roll-off.4,6 Biomimetic efforts have tried to reproduce this functional pattern with the use of © XXXX American Chemical Society
different materials, pattern geometries, and material fabrication processes.7−14 Most recently, compound surfaces combining the wettability adaptations of multiple plant and animal species have been shown to be effective at collecting atmospheric water.15 We have shown that functional polymer micropatterns mimicking the desert beetle can be produced by thin polymer film dewetting, and the patterns capture significant volumes of water from humid air.16,17 Dewetting is the process by which unstable thin liquid films spontaneously break apart on a substrate, driven by unfavorable intermolecular forces at the interface.18−20 The unstable film breaks apart into holes that grow with time, eventually transforming the film into a series of isolated droplets. When the dewetting process is stopped, the resultant pattern is “frozen in”, preserving the droplet morphology. Our previous publication exploited the dewetting of a hydrophilic polymer (poly-4-vinylpyridine, P4VP) film on a polystyrene (PS) substrate to create biomimetic micropatterned surfaces, prepared by thermal annealing.16 The Received: December 19, 2016 Accepted: February 22, 2017 Published: February 22, 2017 A
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
controlled humidity on a Peltier plate (TE technology Inc. CP-031 with controller TC 48−20). A sensor placed 10 mm vertically above the coated copper sample measured the ambient temperature and relative humidity (RH), and the surface temperature was measured by a thermocouple located on the upper surface of the Peltier plate. Optical micrographs were analyzed to determine the area coverage of water accumulated on each surface. The volume of water (V) of each droplet was derived using eq 1 from the radius (a) of the droplet on the surface and the contact angle (θ) of water on the surface:
patterns produced consisted of micrometric hydrophilic bumps on a hydrophobic film. The height and the diameter of the bumps could be varied by varying the initial hydrophilic film thickness, but only within a limited range (7−12 μm in diameter and 0.5−2 μm in height), due to the intrinsic limitations of the thermal annealing process, mainly the fact that annealing time increases significantly with increasing film thickness and very thick films cannot be dewetted. In recent work, we have shown that the dewetting process can be significantly accelerated and the range of length scale of the patterns produced greatly enhanced, by using solvent vapor annealing, in the presence of a mixture of good and poor solvents, instead of thermal annealing.21,22 Here we harness this recent breakthrough in polymer dewetting to optimize the water-harvesting performance of the patterned polymer coatings, by varying the pattern length scale and density by several orders of magnitude. An optimized pattern for water collection needs to combine and balance three competing effects: (i) The energy barrier for nucleation of droplets is lower on wettable surfaces,23 so surfaces of hydrophilic nature are more efficient in the initial stage of water nucleation. (ii) For a liquid droplet to nucleate on a surface, the temperature of the substrate surface must be lower than that of the surrounding environment; that is, the substrate must be cooled (either passively through radiation, or actively by energy input). The heattransfer coefficient during condensation of water is much higher if the water condenses as droplets (dropwise condensation) rather than as a film (filmwise condensation),24 so for this purpose, a surface with a hydrophobic nature is advantageous; also the heat transfer, and therefore condensation rate, is more efficient on small water droplets, around 10 μm in diameter, and slows down on larger droplets (above 100 μm).25 (iii) The presence of patterns or small defects on the surface increases nucleation rate, but also may induce contact line pinning, which increases the critical volume for detachment of droplets.26,27 Delayed detachment of condensed water droplets is associated with a reduced rate of condensation.28 Any pattern designed to combine the properties required to address issues i and ii needs also to minimize contact line pinning. However, minimizing contact line pinning in experiments is still largely a trial-anderror process.29 This paper identifies the optimal dewetted pattern size and distribution and the ideal environmental conditions to maximize water capture performance on patterned surface coatings.
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V=
3 1 ⎛⎜ a ⎞⎟ π (2 − 3 cos θ + cos3 θ) 3 ⎝ sin θ ⎠
(1)
When collecting water in the custom-built humidity chamber [Figure S1, Supporting Information (SI)], the outer surface of four copper tubes was coated with the patterned polymers, prepared by solvent vapor induced dewetting, as well as plain PS as a reference. The coatings were uniformly cooled by flowing ethanol from a temperature-controlled reservoir through the tubes, facilitating the condensation of water on the coated surface. Water droplets that rolled off the tubes were collected in plastic trays situated directly below, and the weight of collected water was measured at multiple intervals following the onset of steady-state condensation. The steady-state water collection efficiency (in units of mL m−2 h−1) of each coating was directly measured. Contact angle goniometry (KSV Cam 200) was used to characterize the wettability and the roll-off angle of water on the patterned surface coatings.
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RESULTS AND DISCUSSION Range of P4VP Bumps Produced by Film Dewetting. In our previous paper and in most dewetting work, polymer films are thermally annealed above the polymer’s glass transition temperature (Tg) to induce flow of the top unstable film. However, thermal annealing is limited to relatively thin films (around 100 nm), as it exploits unfavorable intermolecular interactions at the interface.18−20 Also, as both the top P4VP film and the bottom PS film are liquid at the annealing temperature, layer inversion occurs, driven by the lower surface tension of the bottom PS film.30 This is clearly illustrated in Figure S2 (SI), where the P4VP film does not completely dewet into isolated bumps on the PS substrate, as the P4VP is partially inverted with PS, but retains interconnected cylinders. In this paper, patterned surfaces were prepared by dewetting P4VP films of varying thickness on PS films on silicon and copper substrates, by exposure to a 70:30 (w/w) acetone− water mixture, which induced very fast dewetting of P4VP, as expected on the basis of our recent work.22 Figure 1 illustrates the typical phases of P4VP films dewetting on PS: random nucleation of holes in the film, growth and coalescence with neighboring holes with the formation of liquid cylinders along the line where the holes touch, and final breakup of the cylinders into isolated P4VP droplets. The time required for complete dewetting (300 s) was much shorter than that required for thermal annealing (>20 min, Figure S2, SI), as the presence of the nonsolvent water and the poor solvent acetone reduced polymer viscosity, lubricated interfacial flow, and modified the interfacial energy, as detailed in our recent work.22 Solvent annealing also allowed dewetting of very thick films, as thick at 820 nm, which do not dewet upon thermal annealing. For a detailed understanding of the solvent annealing mechanism, we refer the reader to our previous publications.21,22 P4VP films of three thicknesses were dewetted and resulted in different pattern sizes, as shown in Table 1. The diameter and density of the hydrophilic P4VP bumps are relatively monodisperse, with a standard deviation increasing to around
EXPERIMENTAL SECTION
Polystyrene films (PS, Mw = 350 kg mol−1, 110 nm thick) were prepared by dip-coating from solution (2 wt % in toluene, extracted at 120 mm min−1) onto flat copper sheets and on the outer surface of copper tubes 10 mm in diameter (KSV Nima dip coater). Poly(4vinylpyridine) films (P4VP, Mw = 60 kg mol−1) of varying thicknesses (5−900 nm) were subsequently dip-coated onto this substrate from solution (0.5−7.5 wt % in ethanol). To induce dewetting, the bilayer films were placed in a saturated vapor environment of a mixture of acetone and water in a 9:1 vapor mole ratio, respectively a poor and a nonsolvent, as this had been established to be the ideal dewetting conditions for the bilayer.22 Spectroscopic ellipsometry (Woollam M2000) was used to establish film thickness onto flat films, with measurements over three points on each sample. Tapping-mode atomic force microscopy (Asylum MFP3D, AFM) was used to image the isolated P4VP bumps. The water capture efficiency of the coatings was evaluated in two ways: (i) by optical microscopy and (ii) by collecting water in a custom-built humidity chamber. With optical microscopy, the rate of condensation was observed on cooled, coated copper sheets in a B
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Surface Wettability and Roll-off Behavior. As the rate of water condensation is related to wettability, the contact angle of a 5 μL sessile water droplet on the patterned substrates is presented in Table 2. The contact angle of water on the three patterns was similar, increasing from 81° for the nanopattern to 85° for the micropattern and 88° for the macropattern. The effect can be explained by the Cassie−Baxter relationship for composite surfaces:34 cos θeCB = f1 cos θ1 + f2 cos θ2
(2)
where the contact angle of a water droplet on a composite surface (θCB) is equal to the weighted average of the contact angle of water on each region (i) of surface fraction (f i) within the droplet footprint or, as more recently suggested, localized near the liquid−substrate contact line.35 The projected surface area coverage of the P4VP bumps in the nano- and micropatterned surfaces is approximately 9%, but much lower, around 2.6%, in the macropatterned surface. Entering the values of contact angle of water on plain P4VP and PS from Table 2 into eq 2 yields a theoretical contact angle value of 88° for the nano- and micropatterns and 91° for the macropattern. These values slightly overestimate the contact angle obtained from experimental measurements. As the employed water droplets are much larger in size than single P4VP droplets, the contact line lies along multiple hydrophilic domains, which increases contact line pinning and could partially explain the discrepancy between the measured and calculated values. Furthermore, the calculation does not take into account the larger P4VP surface area related to the curvature of the bumps. An estimate of contact line pinning on the surfaces was obtained by two methods:36 first, the contact angle hysteresis, i.e., the difference between the advancing and receding contact angles, was measured by the volume addition−subtraction method on a horizontal surface. Second, the minimum critical droplet volume Vcrit to slide down a surface inclined at 45° was measured (Table 2). The results for the two types of measurement are in good agreement, except for the results on the macropattern. On the basis of the contact angle hysteresis on the macropattern (36°), this surface was expected to have the highest critical volume, Vcrit. In actual fact, the critical volume for droplet sliding on the macropattern, Vcrit =12.5 μL, was lower than that on the other patterns (15 μL) but greater than that on the flat PS surface (7.5 μL). The high value of contact angle hysteresis on the macropatterned surface was attributed to the large size and sparse distribution of the P4VP bumps on the PS background. A receding droplet experiences significant distortion of the contact line on the hydrophilic bumps.26 The significant distance between each bump resulted in “stick−slip” motion observable by eye, which was largely absent on the patterned surfaces with numerous
Figure 1. Optical micrographs illustrating the main stages of dewetting of a poly-4-vinylpyridine film (P4VP, 80 nm thick) on a polystyrene film (PS, 110 nm thick) on a silicon substrate by solvent annealing in 70:30 (w/w) acetone−water mixture. (a) Hole nucleation, followed by (b) hole growth and (c) coalescence between neighboring holes, culminating in the formation of (d) isolated droplets on the substrate. Scale bars = 50 μm.
10%. The three patterns are shown in Figure 2, and for ease of presentation, they are grouped into three categories based on their P4VP bump height: (a) micropatterns, for surfaces with a bump height around 1 μm and bump diameter around 10 μm; (b) macropatterns, for surfaces with a bump height around 13 μm and bump diameter around 80 μm; and (c) nanopatterns, for surfaces with a bump height around 40 nm and bump diameter around 1 μm. A 2 orders of magnitude increase in thickness induced a 2 orders of magnitude increase in the diameter and height of the dewetted bumps and a 5 order of magnitude decrease in the distribution density (Table 1). The macropatterns consisted of 5 bumps mm−2 while the nanopatterns, as characterized by AFM as the bumps were too small to be observed optically, had a density of 136 000 bumps mm−2 [Figures 2c and S3 (SI)]. The contact angle of the P4VP droplets on the PS bottom film was measured using tapping-mode AFM. The micro- and macropatterned surfaces both presented an average P4VP droplet contact angle of around 25°, as expected for bilayers annealed in this saturated vapor mixture.22 The contact angle measurement in the nanopatterns was difficult due to an edge around the base of the bump, at a height of approximately 20 nm (Figure S3, SI), likely due to the onset of partial layer inversion in these ultrathin P4VP/PS bilayers,31−33 the effect of which is negligible in thicker films. Therefore, the measured value of 10° might be affected by a large error.
Table 1. Average Density Distribution, Diameter, Height, Contact Angle, and Surface Area Coverage of P4VP Bumps on a PS Film, after Annealing in a 70:30 (w/w) Acetone−Water Mixturea, pattern
P4VP film thickness (nm)
distribution (mm−2)
average diameter (μm)
average heightb (nm)
contact angle of P4VP droplet on PS (deg)
P4VP surface area coverage (%)
nano micro macro
7.3 ± 0.2 86.2 ± 0.2 821.4 ± 8.6
136 000 ± 11 000 793 ± 102 5±1
0.9 ± 0.1 12 ± 1 82 ± 9
44 ± 10 1300 ± 100 9000 ± 2000
10 ± 1c 24 ± 1 26 ± 3
8.7 ± 1.5 8.8 ± 1.1 2.6 ± 0.7
PS film thickness was typically constant at 110 nm. bThe P4VP bumps were assumed to be spherical caps; therefore, the average P4VP bump height (hbump) was calculated from the contact angle of the P4VP bumps on PS (θP4VP/PS) and the average radius of the bumps (rbump) by the relationship h = rbump tan(θP4VP/PS). cValue affected by layer inversion; see the text and Figure S3 (SI). a
C
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Time lapse series of optical micrographs of a P4VP film dewetting from a PS-coated copper substrate. (a) Micropatterned surface coating (scale bar = 50 μm). Inset: AFM micrograph (100 × 100 μm2) of P4VP bumps on a PS background. Height scale = 2.5 μm. (b) Macropatterned surface coating (scale bar = 200 μm) and (c) AFM micrograph of the nanopatterned surface coating (scale bar = 5 μm) with a cross sectional profile of the P4VP bumps on the PS substrate. Time stamps indicate the time from the start of vapor annealing of the films.
By comparison, the contact angle hysteresis on a micropattern prepared by thermal annealing was found to be higher than for all the solvent-annealed patterns (40°, for the pattern shown in Figure S2, SI), and the critical volume for sliding was higher (30 μL). This might be partially explained by the presence on the surface of the thermally annealed sample of P4VP cylinders, which have not broken up into isolated droplets due to layer inversion. This demonstrates a known observation, that in improving water-shedding properties it is more effective to design a discontinuous three-phase contact line than to increase the contact angle.37 Condensation on Surface Coatings. Water condensation was observed in situ using optical microscopy in a customdesigned controlled-humidity chamber, on both patterned and plain (PS and P4VP) coated copper sheets. Figure 3 shows a representative series of optical micrographs of water condensing on the patterned and plain surface coatings over time, measured at the dew point, as obtained from the simplified equation by Lawrence:38 air temperature Tamb = 20.3 ± 0.5 °C; subcooling ΔT = 3.0 ± 0.5 °C, where ΔT is the difference in temperature between the ambient atmosphere and the surface; and relative humidity 85.3 ± 3.6%. Water vapor condensed on PS-coated copper, the reference hydrophobic system, in characteristic uniformly circular droplets, with a semispherical
Table 2. Sessile, Advancing, and Receding Contact Angle of Water on P4VP/PS Patterned Coatings and Plain Flat PS and P4VP Surfaces Deposited on Copper Substrates surface nanopattern micropattern macropattern flat PS flat P4VP
sessile contact angle of water (deg)a 81 85 88 92 45
± ± ± ± ±
1 2 3 1 1
advancing contact angle (deg) [hysteresis (deg)]b 90 91 95 102 71
± ± ± ± ±
1 1 2 3 3
(28) (26) (36) (18) (61)
Vcrit for droplet sliding at 45° (μL) [hysteresis (deg)] 15.0 (25) 15.0 (25) 12.5 (27) 7.5 (19) −c
a
Sessile drop contact angle of water on the surfaces was measured using a 5 μL water droplet. bAdvancing and receding contact angles obtained by volume addition-subtraction method by sequentially adding and removing 10 μL to a 5 at 0.5 μL s−1. cDroplet roll-off was not observed, up to the largest droplet size measured (35 μL).
hydrophilic bumps. On an inclined plane, the momentum of the droplets was sufficient to overcome the adhesive forces on the sparsely distributed hydrophilic bumps on the macropatterned surface. On the other hand, the large density of bumps on the nano- and micropatterned surfaces resulted in numerous pinning points, and a larger droplet mass was required to initiate sliding. D
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. Time lapse optical micrographs of water condensation at the dew point (ΔT = 3.0 °C and 85.3% RH) on (a) PS film, (b) P4VP film, (c) nanopatterned coating, (d) micropatterned coating, and (e) macropatterned surface coating on flat copper substrates. Scale bars = 50 μm. A selection of P4VP bumps in panels d and e is highlighted in blue. Droplets encircled in red lines coalesce together over the period of the experiment; the red arrows indicate the direction of motion of the center of mass of these droplets during coalescence. The time stamp indicates the time from the start of cooling.
large “daughter” droplets that result from coalescence events was located approximately at the center of mass of the original “parent” droplets. According to accepted models, the condensation rate plateaus at a surface area coverage of 55% due to competition between two opposing effects: droplet growth decreases the free surface area and slows down condensation, while droplet coalescence increases the free surface area available for the nucleation of new families of droplets.23 The rest of the figure describes condensation onto P4VP/PS patterns. As shown also in Figure S6 (SI), water condensation on micropatterns preferentially occurs on the hydrophilic bumps, with coalescence occurring when adjacent droplets are large enough to span the hydrophobic region between them. In Figure 3c the nanopatterns cannot be distinguished as the hydrophilic bumps are too small to detect optically. In Figure 3d,e, some of the P4VP bumps are labeled in blue to distinguish
appearance characteristic of the high water contact angle (92°, Figure 3a). Similar condensation patterns are observed on PS films on silicon substrates (Figure S4, SI). In comparison, water droplets nucleating and growing on a hydrophilic P4VP surface were irregular in shape as the threephase contact line spread and remained pinned on the higher energy surface, eventually forming films of water on the surface (Figure 3b). Similar condensation patterns are observed on P4VP films on silicon substrates (Figure S5, SI). Film-wise condensation is detrimental for the efficiency of water capture, as the water films thermally insulate the surface from the humid air and slow down further condensation.39,40 On both the flat P4VP and PS surfaces, water droplets grew in size and coalesced with adjacent droplets over the course of the experiment. For clarity, the droplets that coalesced together in the fourth panels of Figure 3a,b are highlighted by a red line (in their state prior to coalescence). The center of mass of the E
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. (a) Percentage surface area coverage of condensed water over time as obtained by optical microscopy and (b) volume of water condensed over time as obtained by optical microscopy, normalized against the coated surface area. Sample studies were PS (blue), nanopatterned (orange), micropatterned (red), and P4VP (green) coated copper sheets at Tamb = 18.4 ± 2.0 °C and 85.3% RH for surface subcooled by ΔT = 5.0 ± 0.5 °C (solid line), ΔT = 3.0 ± 0.5 °C (dotted−dashed lines), and ΔT = 2.0 ± 0.5 °C (dotted lines). (c) Schematic illustrations of water droplet nucleation, growth, and coalescence on the plain PS, P4VP, nanopattern, and macropattern surfaces. The red layer is PS, the green layer is P4VP, and the shades of blue illustrate the growth and coalescence of water droplets (the blue color gets lighter as condensation progresses).
time. Furthermore, the preferential coalescence of droplets on the P4VP bumps is expected to enhance the rate at which water droplets approach Vcrit to slide off. A quantitative analysis of the condensation process was obtained from optical micrographs on different polymer-coated flat surfaces (Figure 4, at 85.3 ± 3.6% RH, ΔT = 2−5 °C). The values of ΔT were chosen to highlight the effect of surface wettability on the condensation rate. It was hypothesized that the effect of surface wettability on condensation rate would be more important at low ΔT at a constant ambient humidity level, as at low subcooling the energy barrier for nucleation is higher.23 The results are reported as percentage area coverage (Figure 4a) and the calculated volume (Figure 4b) of water condensed on each surface. The condensation of water vapor on the macropatterned surface was not analyzed, as the large size of the bumps prevented accurate analysis of the water droplet size forming on the bump surface at the used magnification. The surface area covered by water was directly related to the surface wettability (Figure 4a,c). At all subcooling temperatures, the flat P4VP films (green lines) had a higher water surface coverage than the flat PS film coatings (blue), due to the high nucleation rate and rapid spreading of water droplets. When the surfaces were subcooled by 2 and 3 °C, the condensation rates on the nano- and micropatterned surfaces, the orange and red lines in Figure 4a, respectively, were in between those of the flat
them from the water droplets, and the position of the bumps remains constant throughout the series. The nano- and micropatterns are more hydrophilic than the macropatterns, so a higher nucleation density of water droplets can be observed, and the size of the growing water droplets relative to the dimensions of the hydrophilic P4VP bumps is large. This leads to water droplets becoming “pinned” to the bumps, resulting in the contact line becoming deformed (Figure 3c,d). The condensation of water droplets on the macropatterned surface (Figure 3e) combines features of condensation on the P4VP coating and on the PS coating due to the relatively small (2.6%) hydrophilic surface area coverage of the P4VP bumps. At 2 min from the start of cooling, small, uniformly circular water droplets nucleate on the hydrophobic PS, and irregularly shaped droplets condense on the hydrophilic P4VP bumps. The P4VP bumps are large enough to support the nucleation of multiple water droplets, which are visible in the micrographs until 8 min into the experiment. In contrast to droplet coalescence on the plain PS and P4VP surfaces, water droplets that coalesce on the patterned surfaces have a preferred direction of motion toward the water droplets on the hydrophilic P4VP bumps, indicated by red arrows on the micrograph series. This directional coalescence of droplets enhances the condensation rate by providing free surface area for further droplet nucleation,41−43 in contrast to the flat PS surface, where the nucleation of new droplets slows down over F
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces P4VP and PS for experiments. Comparing the two patterned surfaces, the nanopattern exhibited a faster condensation rate, as expected due to the lower contact angle (Table 2). At ΔT = 5 °C (and above), the surface area coverage with water was similar for the P4VP coating and the two patterned surfaces, and in fact, the initial condensation rates, up to 50 s into the experiment, for all four surface coatings were very similar. This confirmed our expectation that, as ΔT was increased, all surfaces nucleated water droplets at similar rates, irrespective of surface wettability. The volume of water condensed on each surface, plotted in Figure 4b, was calculated using eq 1, from the contact angle values (Table 2) and droplet radii from image analysis. As expected, the rate of increase in water volume on all of the surfaces increased as the level of subcooling was increased from 2 to 5 °C. Due to the low contact angle of water on the P4VPcoated copper, the volume of water condensed on P4VP is lower at all subcooling temperatures than on the patterned and PS surface coatings. At ΔT = 2 °C, the rate of increase in water volume on the PS coating (blue, dotted line) was higher than on the micropattern (red, dotted line) and the nanopattern (yellow, dotted line). This trend is attributed to two effects that are schematically illustrated in Figure 4c: (1) the high contact angle of water on the PS surfaces results in a higher volume for a given surface area coverage by a water droplet, and (2) the hydrophilic bumps on the patterned surfaces resulted in higher nucleation rates and smaller droplets than on the PS surface. As the subcooling was increased, the difference in the volume of water condensed on the patterned surfaces and PS diminished. At ΔT = 3 °C (dashed−dotted lines), the two patterned surfaces had very similar condensation rates, which were lower than the rate of condensation on PS, and at ΔT = 5 °C, the rate of increase in the volume of condensed water vapor on the two patterned surfaces and PS was similar and within the experimental error. These water condensation measurements provided qualitative insight into the early stages of water harvesting, namely, droplet nucleation and early growth and coalescence (Figure 4c). However, within the time frames of these experiments, water droplets did not approach the critical volumes necessary to slide off the surfaces (Table 2). A crucial part of the collection efficiency is related to the ability of water droplets to roll-off the surface after condensation, providing new sites for droplet nucleation on the surface. Both these phases of water capture were included in the experiments conducted on threedimensional cooled copper tubes. Water-Harvesting Efficiency of Patterned Coatings. By combining the condensation rate measurements from Figure 4 and the observations of the nucleation and growth of water droplets on the macropatterned surface (Figure 3e), it was predicted that the sparse distribution (5 mm−2) of the large hydrophilic bumps, coupled with the movement of water droplets toward the bumps during coalescence, would result in a high water collection efficiency at low subcooling temperatures on this surface. This hypothesis was tested by comparing the steady-state water collection rate of the three patterned coatings and the plain PS coating over a 6-h period. The weight of water collected over time was measured in a custom-built humidity chamber containing four coated copper tubes, at ΔT = 3, 5, and 10 °C and 95% RH (Figure 5). At ΔT = 3 °C (Figure 5, blue bars), the patterned surfaces were more efficient at water collection than the flat hydrophobic PS surface, with the macropattern harvesting the largest
Figure 5. Water collection efficiency of the PS, nanopatterned, micropatterned, and macropatterned surface coatings on copper tubes at 95 ± 3.6% RH; Tamb = 22.1 ± 1.0 °C; and ΔT = 3 °C (blue), ΔT = 5 °C (red), and ΔT = 10 °C (green) for a collection time of approximately 6 h.
amount of water (14.5 ± 1.1 mL m−2 h−1). The nano- and micropatterns performed similarly, collecting 11.3 ± 1.1 and 12.6 ± 1.1 mL m−2 h−1, respectively, and the PS coating produced a water collection rate of 9.2 ± 1.1 mL m−2 h−1. At this low level of subcooling, which is easily reached by radiative cooling of surfaces upon exposure to the night sky,44 the macropatterns were 57% more efficient than the plain hydrophobic coatings at collecting atmospheric water. This result indicates that macropatterned surfaces would achieve a higher number of dew events than plain hydrophobic surfaces and therefore illustrates that the presence of a pattern is beneficial when the surface is passively, and not actively, cooled. This result is linked, on the one hand, to the lower barrier for water nucleation on hydrophilic bumps than on the hydrophobic films and, on the other, to the lower critical volume for droplet sliding measured on the macropatterns compared to the other patterns (Table 2). When the subcooling was increased to ΔT = 5 °C (red bars), the PS coating was the most efficient at water harvesting, collecting 38.1 ± 2.7 mL m−2 h−1 compared with a collection rate of 32.5 ± 2.7 mL m−2 h−1 by the macropatterns. At a subcooling of 10 °C (green bars), the water collection rate of all four surface coatings approximately doubled over the collection rate at ΔT = 5 °C, and the difference in water harvesting between the four coatings over the course of the experiment was within the error margin of the experimental apparatus. This observation can be explained by considering the thermodynamics of condensation: the higher the level of subcooling (higher supersaturation), the lower the barrier for droplet nucleation.23 At 5 and 10 °C subcooling, where nucleation is facilitated compared to 3 °C subcooling, the probability for the hydrophobic PS to induce nucleation is almost as high as that of the hydrophilic P4VP. Indeed, at higher supersaturation, the presence of hydrophilic bumps is a drawback compared to an entirely hydrophobic surfaces, as the roll-off of droplets is delayed and therefore water collection is less efficient.
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CONCLUSIONS Three patterned polymer surface coatings were fabricated onto three-dimensional copper tubes by the dewetting of P4VP films on PS films, and their water capture ability was tested. By using solvent annealing and varying the initial thickness of the P4VP G
DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces film, the diameter and height of the P4VP hydrophilic bumps could be varied by 2 orders of magnitude and the distribution density by 5 orders of magnitude. Solvent annealing leads to dewetted patterns with lower contact line pinning than thermal annealing, as isolated P4VP bumps can be achieved, which facilitate droplet roll-off, rather than interconnected P4VP bumps. The effect of surface patterning and pattern geometry on water condensation and collection was determined using contact goniometry, in situ microscopy, and water collection experiments. The patterned surfaces were observed (1) to increase the nucleation rate of water droplets and surface area covered by water as the distribution density of P4VP bumps increased, over a plain PS surface; (2) to increase the contact angle hysteresis and critical volume for droplet detachment over those of a plain PS surface; (3) to promote coalescence of the water droplets nucleated on the PS toward those nucleated on the P4VP bumps, accelerating the growth rate of droplets on the bumps and increasing the surface area available for nucleation of new water droplets; (4) to significantly enhance water collection at low surface subcooling, with the macropatterned surface coating (with 82 μm wide hydrophilic domains) harvesting 57% more water, by volume, than the plain PS coating at ΔT = 3 °C and 95% RH; and (5) to have a negligible effect on water collection rate over the plain PS coating as the temperature differential was increased. At ΔT = 5 and 10 °C the water collection rate of all the surfaces increased significantly, but there was no significant difference in the water collection performance between the three patterned coatings and flat PS. The results of this study are in agreement with the experiments in the field of condensation heat exchange, which find that dropwise condensation, experienced on a hydrophobic substrate, leads to higher condensation and heat transfer rates due to the smaller value of Vcrit for droplet sliding off the surface than experienced during filmwise condensation on hydrophilic substrates.39 Although water condensation is slower at low values of ΔT and relative humidity on all surfaces, the condensation rate is lower on purely hydrophobic surfaces due to the lower droplet nucleation rate, as expected.23,45 Under these conditions, a micropatterned surface coating (hydrophilic bumps diameter ∼80 μm, height ∼9 μm, bump separation on the millimeter scale) has the highest water collection efficiency. It was shown that this pattern generation approach is intrinsically up-scalable, and it could be applied to threedimensional objects several tens of centimeters long. The dewetted patterned surfaces could be tailored to selectively adsorb and collect a variety of solvents from the vapor phase at atmospheric temperature and pressure and with low energy demands for surface cooling. There remain extensive opportunities to optimize the wettability contrast and pattern geometry and to understand the fundamental role of surface topography and chemistry on the water condensation and collection process; however, the concept of water collection onto patterned surfaces, prepared by a simple, low-cost, and reliable solvent annealing method, has been proven successful.
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Layout of the water collection experimental setup, micrograph series of dewetting P4VP/PS bilayer film by thermal annealing, AFM micrograph of the nanopattern surface, optical micrographs of water condensation on plain PS, optical micrographs of water condensation on plain P4VP, and optical micrographs of water condensation on dewetted P4VP/PS bilayers (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chiara Neto: 0000-0001-6058-0885 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.N. acknowledges the Australian Research Council for funding. REFERENCES
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DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b16248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
