Effects of Engineered Wettability on the Efficiency of Dew Collection

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Effects of Engineered Wettability on the Efficiency of Dew Collection Konstantinos Gerasopoulos, William Lloyd Luedeman, Emre Ölçero#lu, Matthew McCarthy, and Jason J. Benkoski ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16379 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Effects of Engineered Wettability on the Efficiency of Dew Collection Konstantinos Gerasopoulos1, William L. Luedeman1, Emre Ölçeroglu2, Matthew McCarthy2, Jason J. Benkoski1* 1. Research and Exploratory Development Department, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, United States 2. Department of Mechanical Engineering and Mechanics, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States KEYWORDS Condensation, wetting, hysteresis, biphilic, superhydrophobic.

ABSTRACT: Surface wettability plays an important role in dew collection. Nucleation is faster on hydrophilic surfaces, while droplets slide more readily on hydrophobic surfaces. Plants and animals in coastal desert environments appear to overcome this tradeoff through biphilic surfaces with patterned wettability. In this study, we investigate the effects of millimeter-scale wettability patterns, mimicking those of the Stenocara beetle, on the rate of water collection from humid air. The rate of water collection per unit area is measured as a function of subcooling (ΔT = 1, 7, and

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27°C) and angle of inclination (from 10° to 90°).

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It is then compared for superbiphilic,

hydrophilic, hydrophobic, and surperhydrophobic surfaces.

For large subcooling, neither

wettability nor tilt angle has a significant effect because the rate of condensation is so great. For 1°C subcooling and large angles, hydrophilic surfaces perform best because condensation is the rate-limiting step. For low angles of inclination, superhydrophobic samples are best because droplet sliding is the rate-limiting step. Superbiphilic surfaces, in contrast to their superior fog collecting capabilities, generally collected dew at the slowest rate due to their inherent contact angle hysteresis. Theoretical considerations suggest that this finding may apply more generally to surfaces with patterned wettability.

1. INTRODUCTION 1.1. Moisture Scavenging Surfaces The collection of water from humid air governs many important industrial processes, including heat transfer,1-2 water purification,3-4 water harvesting,5-7 power generation,8-9 and dehumidification.10 For dew harvesting applications, the collection of water is especially challenging due to the scarcity of water in the environments where such systems are needed most. 11-12 Dewfall has critical importance for plants and animals in arid climates, and it is increasingly collected for human consumption. Most dew collecting devices consist of a flexible foil material placed over a gently sloping surface that guides the condensed dew towards a collector.13 For this application, the degree of subcooling cannot be freely modified without active cooling, so the rate of capture can only be maximized through careful design of surface properties.13 The ideal surface for capturing moisture from air facilitates both the condensation of water from humid air and the shedding of the condensed droplets towards a collector. According to


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Volmer’s nucleation theory, droplets should form many orders of magnitude faster on a hydrophilic surface.14, 15 Once there, the droplets do not slide until they grow enough for gravity to overcome capillary forces. The magnitude of this resistance increases with the contact angle hysteresis and liquid-solid contact area. Although hysteresis is not intrinsically different on hydrophobic and hydrophilic surfaces, the liquid-solid contact area is larger on a hydrophilic surface for a fixed droplet size. Droplets consequently must reach a larger critical mass to begin moving on a hydrophilic surface, potentially negating the faster rate of nucleation. A possible solution to this problem has been engineered by the Stenocara beetle.16-18 This beetle obtains its drinking water directly from the fog that is common to the Namib Desert. A random pattern of 0.5 mm hydrophilic bumps with 0.5-1.5 mm spacing protrude from its waxy outer shell. Water collects on the bumps, and, once the droplet volume is sufficiently large, the gravity pulls the droplets down the superhydrophobic waxy coating to the beetle’s mouth. The success of this water collection strategy suggests that a (super)biphilic surface–a surface with a controlled pattern of (super)hydrophobic and (super)hydrophilic regions–can combine the water collecting advantages of (super)hydrophilic surfaces with the water shedding advantages of (super)hydrophobic surfaces. This bio-inspired approach has been previously employed by several groups for the fabrication of fog collecting surfaces.19-22 Most employed a pattern modeled specifically after the Stenocara beetle. They used round, hydrophilic patches with a 0.5 mm diameter on either a hydrophobic or superhydrophobic background. Bai, et al., took the concept further, showing that star-shaped patches were superior to circular ones.19 The literature on dew collection shows that surfaces with patterned wettability exhibit both enhanced collection rates and heat transfer coefficients relative to unpatterned surfaces.23-25, 26


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However, these surfaces also included topographical patterns. Chen created micropyramids with a nanograss texture to lift droplets off the surface for easier shedding.27 Hydrophilic pillars in a background of fluorinated nanograss caused the ejection of condensed water from the surface when adjacent droplets coalesced.28 Rahman, et al., observed similar behavior on hydrophobic pillars coated with tobacco mosaic virus-templated nanograss.30 Lastly, the Aizenberg group combined the macroscopic bump topography of the Stenocara beetle, tapered diameter of cactus needles, and lubricant-filled pores of the pitcher plant. These complex patterns enhanced the flux of water to the surface, accelerated the motion of droplets to the collector, and provided a nearly frictionless surface for droplet sliding.29 All microscopic protrusions are employed, in part, to overcome the adhesion of the condensed droplets to the surface. In order to maximize the efficiency of such surfaces for dew collection, it is critical to develop a systematic understanding of the individual contributions of both surface wettability and topography. Lee, et al., were among the first to investigate the effects of patterned wettability on dew collection in the absence of patterned topography.27 They observed that hydrophilic and superhydrophilic surfaces harvested water most efficiently on vertical surfaces cooled 10°C below the dew point. For this specific set of conditions, they collected dew at a faster rate than hydrophobic, superhydrophobic, and patterned superberbiphilic surfaces. However, dew collectors rarely operate under conditions that are so favorable to water collection. The degree of subcooling and angle of inclination are small in most dew collection installations. We therefore examine these surfaces under a wider range of conditions to determine whether conditions exist where patterned wettability provides an advantage. Particular emphasis is given to conditions of low tilt angles and low subcooling that are characteristic of dew collectors. We hypothesize that small tilt angles will exacerbate droplet


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pinning on hydrophilic surfaces with a large liquid-solid contact area, and that low subcooling will exacerbate slow droplet nucleation on hydrophobic surfaces. We further hypothesize that superbiphilic surfaces will be able to combine the rapid nucleation of the hydrophilic patches with the rapid droplet shedding of the superhydrophobic background to enhance dew collection under select conditions. To test this hypothesis, a custom environmental chamber has been constructed to precisely control the dew collection conditions over a broad parameter space. The parameters include air temperature, relative humidity, plate temperature, and tilt angle.

2. EXPERIMENTAL 2.1. Materials The samples used in this work were fabricated on Si and Cu rectangular substrates (4 cm x 4.2 cm). For the hydrophilic and hydrophobic surfaces, the Si surface was modified with SiO2 and SiO2 (optional)/Teflon AF layers, respectively. SiO2 was grown on the Si substrate by thermal oxidation while Teflon AF was applied by spin coating at a spin speed of 4000 rpm followed by baking at 175°C and 240°C for 10 minutes. Superbiphilic surfaces were produced by a combination of chemical oxidation and photolithographic patterning. Cu was either electroplated to a thickness of 25-30 µm on a silicon wafer or was used directly in Cu plate form. Cu substrates were first stripped of their native oxides in a bath of 2% hydrochloric acid for 10 minutes. Afterwards they were quickly rinsed with DI water, dried with nitrogen, and placed directly into an alkaline bath of NaClO2, NaOH, Na3P04 12H2O, and DI water (3.75:5:10:100 wt %) for 10 minutes at 96°C using the method described by Rahman et al.26 During this process, copper oxide (CuO) nanostructures are grown directly onto the copper substrates via hydrothermal oxidation.

The bath temperature is maintained using a hotplate and closely


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monitored during CuO growth. Wettability patterns were generated using photolithography, spin coating of Teflon AF (as described above), and a standard lift-off process as described by Ölçeroğlu et al.28 The wettability patterns form a square array of superhydrophilic islands of diameter 0.5 mm separated by a pitch of 2 mm. This gives an area fraction of 0.05 for the hydrophilic patches. The superhydrophobic samples were fabricated by employing a scalable coating strategy on the Si substrate. The key material selections in this approach are a polytetrafluoroethylene (PTFE) nanoparticle as the superhydrophobic material and polyisobutylene (PIB) rubber as an interface layer between the PTFE and the silicon. PIB was chosen due to its water impermeability as well as its ability to bind to PTFE. Samples were prepared by the following procedure: polyisobutylene (500 kg/mol, polydispersity index = 2.5, Sigma-Aldrich, USA) was dissolved in toluene to a final concentration of 20% w/w. The viscous solution was then spun on the Si substrate at a spin speed of 700 rpm for 2 minutes followed by baking at 100°C for 5 minutes. Nano-PTFE (Cefral Lube, V Grade, Synquest Laboratories, Japan) was dispensed in hexanes, sonicated and rigorously stirred until a uniform suspension was formed. This suspension was then spray painted on the PIB covered silicon. During spray painting, the solvent softens the PIB surface and dries rapidly, allowing good adhesion of the PTFE onto the rubber. Finally, the samples were rinsed thoroughly with DI water to remove excess PTFE. This method is scalable and compatible with different substrates. SEM images of the surface morphology for the superhydrophobic and superbiphilic samples are shown in Fig. 1a, and Fig. 1b, respectively, clearly illustrating the hierarchical and nanostructured morphology of the fabricated samples. Figure S1 shows characteristic contact angle images for the different samples used in this work. Average static contact angles of 61°,


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121° and 157° were measured for the hydrophilic, hydrophobic, and superhydrophobic samples (both PIB and CuO), respectively. On superhydrophilic CuO, water forms a film (static contact angle ~ 0° which cannot be reliably measured with the goniometer). Advancing and receding contact angles were measured at the conclusion of the study in order to compare the contact angle hysteresis and determine whether testing affected the wettability of the surfaces. The hydrophilic sample had an advancing contact angle of 77° and a receding contact angle of 55°. The hydrophobic sample had an advancing contact angle of 121° and a receding contact angle of 111°. The superhydrophobic sample had an advancing contact angle of 160° and a receding contact angle of 154°.

Figure 1: SEM images showing the morphology and microstructure of a) superhydrophobic and b) superbiphilic surfaces

2.2. Environmental Chamber To measure the efficiency of water collection, a test setup capable of measuring the rate of water collected per area as a function of ambient temperature, relative humidity, sub-ambient


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cooling, and surface angle (Fig. 2) was constructed. Fig. 2a conceptually describes the water collection experiment. Briefly, the 4 x 4.2 cm samples are mounted on a Peltier plate that cools the surface down to the desired temperature. A goniometer controls the angle of the sample between 0 and 90°. To measure the rate of water collection, the setup measures the mass of water with a load cell in real time. Throughout this study, the conditions inside the chamber are maintained at a temperature of 37°C and 98% relative humidity. The relatively high temperature and relative humidity were chosen to minimize variations due to changes in ambient conditions. Though less relevant to outdoor dew collection, the conditions could be relevant for scavenging water vapor from sweat or breath. The test set up consisted of an environmental chamber (ETS, Model 5503) equipped with a heater assembly with circulating fan (ETS, Model 5474), an ultrasonic humidification system (ETS, Model 5462) a temperature compensated humidity/temperature sensor (ETS, Model 556) and a microprocessor humidity/temperature controller (ETS, Model 5200-240-230). A goniometer stage (Sherling products, P/N 3750) with 90 degree range of motion was placed on a height-adjustable stage. Sample temperature control was provided by a thermoelectric (TE) module (TE-2-(127-127)-1.15, TE Technology, Inc) that was attached on a custom-made copper heat sink (mounted on the stage with screws) through thermal interface foil (Custom Thermoelectric) and high thermal conductivity paste (Omegatherm 201, Omega Engineering). The copper heat sink allowed circulation of cold water through a chiller (ThermoNESLAB, RTE7) outside the chamber. To prevent condensation from the stage, the TE module, tubing, and other aforementioned parts were covered with thermal insulating tape and foam. Sample mounting was done through a thin Cu plate attached to the TE module through the thermal paste. The Cu plate was also covered with thermal insulating foam around its exposed area. This


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attachment approach was preferred over directly attaching the samples to the TE module since the thermal insulating foam around the TE could otherwise absorb shedding water. A K-type thermocouple was mounted on the cold side of the TE module using thermally conductive epoxy and connected to a digital thermometer (RDXL4SD, Omega Engineering). In separate experiments, the sample temperature was found to only vary by no more than 1°C compared to the temperature on the cold side of the TE plate. As a result, this temperature is used as a reference in the subsequent sections. Finally, electrical connections to a DC power supply (Keithley 2260B) were made using water resistant cables. Water collection was monitored in real time using two different methods. Quantitative measurements were obtained using a load cell (LSB200, Futek). The load cell was connected to a computer using a USB interface (USB200, Futek) and operated through commercial software (Sensit, Futek). A graduated cylinder with a funnel was then placed on the load cell. At the same time, the water collection process was optically monitored using a digital camera (DFK 23UX236 operated by ICapture software, The Imaging Source). Throughout the duration of the experiments, a heat gun was used to maintain a clear optical window inside the chamber. Fig. 2b is a close-up view of the interior of the chamber where the key components are highlighted.


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Figure 2. a) Schematic of test setup that measures the rate of water collection as a function of sample inclination, b) optical image of the actual setup inside the humidity chamber.

2.3. Water Collection Measurements As discussed above, nucleation and growth depend on the degree of subcooling below the dew point. To investigate this effect, water collection from the four different surfaces was compared at two different sample surface temperatures while other parameters were held constant. The surface temperatures chosen for this set of experiments were 30°C (7°C subcooling) and 10°C (27°C subcooling). Observe that the dew point at 37°C and 98% relative humidity is 36.6°C, so the subcooling below air temperature is effectively equal to the subcooling below the dew point. The small offset from 100% relative humidity was maintained primarily to minimize stray condensation on the graduated cylinder or other surfaces that would introduce error into the measurement. Droplet shedding occurs when gravity overcomes capillary forces. This effect was studied at varying angles of inclination. Specifically, angles of 10°, 30°, 60°, 90° were used in this work.


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Water collection was monitored for at least 1 hour, and all experimental conditions were studied by taking at least 3 different measurements for each sample type.

3. RESULTS AND DISCUSSION 3.1. Low Subcooling Figure 3 shows the collected water mass as a function of time at different inclination angles. The sample surface temperature was fixed at 30°C, corresponding to a 7°C subcooling. Several interesting observations can be made. First, the effect of the inclination angle is significant at low angles. At 10°, one of the hydrophilic samples does not collect any water during the first 60 minutes. Fig. S2 in the Supporting Information follows the experiment for 75 minutes, and it shows a large jump in the amount of collected water (~0.4 g) at 62 minutes. Thus the inability to shed water dominates the water collecting behavior under these conditions. The hydrophobic sample performed reasonably well in terms of total water mass collected; however the collection onset is still delayed significantly (~ 38 minutes for this sample). The real-time data indicate that water collection still mostly occurs in discrete shedding events. Superhydrophobic surfaces exhibited the best behavior, with a collection onset below 10 minutes and a mostly linear steady state collection rate. Discrete events observed in these samples are mostly attributed to water droplets hanging at the edge of the sample until they are shed into the collection beaker. Interestingly, superbiphilic surfaces exhibited a fairly similar behavior to hydrophilic samples, showing minimal water collection until a large amount is shed near the 60-minute mark. The delay in the onset of water collection has the most pronounced impact at the beginning of the experiment. Note how the water collection from the four samples begins to converge by 60 minutes. However, the total amount of water collected never fully converges. Differences


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remain due to the fact that the samples prone to droplet pinning always return to a state where water pools on the surface rather than being shed to the collector. The pooled water has the additional effect creating additional thermal resistance; the surface of the thick water droplets is necessarily higher than the underlying substrate. The reduction in subcooling consequently reduces the rate of droplet growth. Thus, longer experiments generally continue the steady state behavior reached by the end of 60 minutes.

Figure 3: Collected water mass vs. time at 7°C subcooling for different angles of inclination: a) 10°, b) 30°, c) 60°, and d) 90°.


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Figure 4: Images of the water collection process at distinct times (t=5, 15, 30, 60 minutes) for all four sample types at a 10° angle and 7°C subcooling.

Qualitative inspection of the water collection process further supports the quantitative results. Figure 4 shows snapshot images at distinct times during the condensation experiments (t=5, 15, 30, 60 minutes) for all four sample types at 10°. Early on (t=5 minutes), nucleation begins on the surfaces in the form of small droplets. Only on the superhydrophobic surface do droplets begin sliding during the first 15 minutes of the experiment. Every other surface exhibits a droplet coalescence behavior to progressively larger sizes. Hydrophilic samples nucleate water readily, but it collects in pools along the lower edge due to the low angle of inclination. Hydrophobic samples allow sliding without pooling, but at diameters significantly larger than the superhydrophobic counterparts. Superbiphilic samples exhibit rapid nucleation at the


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hydrophilic patterned islands; however these patterns act as localized collection sites, preventing sliding until droplets grow to very large diameters at this low inclination angle. Low sliding resistance is what gave superhydrophobic PTFE the performance edge. Droplets began sliding almost as soon as they nucleated. Moreover, they also slid to the collector at a smaller diameter. This last point is critical, as the thickness of the adsorbed water layer increases the thermal resistance proportionally. The thicker the droplet, the smaller the temperature difference is at the vapor-liquid interface. Thus, the degree of subcooling is lower than 7°C on the surface of the water droplet. For very large droplets, the surface temperature may increase beyond the dew point. Removing the driving force for condensation means a much slower rate of water collection relative to the bare surface. Shedding the droplets when they are still small is therefore key to maximizing the rate of water collection.

Figure 5: a) Average water collection rate and b) onset of water collection at different inclination angles for samples tested at 7°C subcooling. The error bars represent the standard deviation.

Similar visual inspection at higher inclination angles validates that the onset of sliding and water collection is accelerated as the angle increases (Fig. S3, Supporting Information, images at


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30°). Data from several repeated experiments at these conditions were extracted for both the average water collection rate as well as the onset of water collection and they are plotted in Figure 5. In terms of the average collection rate, superhydrophobic surfaces notably outperform all other sample types at a 10° inclination angle, showing 42% increase (9.34 g/m2 min) compared to the second closest competitor (hydrophobic – 6.59 g/m2 min) with a p-value of 0.0005. As the angle of inclination increases the average collection rate increases to roughly 12 g/m2 min for the superhydrophobic samples. At the same time, all other surfaces begin catching up, as the variations in the average collection rate are not statistically significant. Another metric of importance is the collection onset, which is defined as the time at which the steady state of water collection begins. Superhydrophobic surfaces exhibit a remarkably narrow and stable water collection onset window, ranging from 9-12 minutes on average. For hydrophobic surfaces the collection onset is 36 minutes at 10°, and then stabilizes around 20 minutes at all other angles. Hydrophilic and superbiphilic surfaces exhibit a very large average collection onset at low angles (56 minutes), which progressively becomes smaller as the angle of inclination increases. Interestingly, for the hydrophilic samples at 60° and 90° as well as the superhydrophobic samples at all angles, the average collection onset shows a somewhat counterintuitive, but non-statistically significant variability. This is attributed to some experimental and sample-to-sample variability, as it was observed that, despite earlier sliding as the angle increases, water droplets tend to hang on to the edge of the sample until they become large enough to shed and collect into the beaker, affecting the collection onset. This is further validated by imaging data shown in the Supporting Information (Fig. S4). Overall, superhydrophobic surfaces outperform all other samples in terms of both metrics for water collection efficiency from humid air. Not only do they collect sufficient water at all angles of


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inclination, but do so with a smaller collection onset, implying that they are appropriate for both long and short term water harvesting. The benefits of superhydrophobic surfaces are significantly enhanced at low angles of inclination, which are more likely to occur in naturally occurring emissive surfaces where condensation for dew collection can be readily promoted.

3.2. High Subcooling The experiments were repeated at high subcooling (sample temperature 10°C, ΔT=27°C) in order to investigate if the benefits of the superhydrophobic surfaces are maintained when the driving force for nucleation is not a limiting factor.


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Figure 6: Collected water mass vs. time at 27°C subcooling for different angles of inclination: a) 10°, b) 30°, c) 60°, and d) 90°. Each plot shows one representative sample for clarity, but 3-5 different samples were run for each surface type.

Collected water mass as a function of time for the four different sample types is plotted in Figure 6 for all angles of inclination. Compared to the results at low subcooling (Fig. 3), some interesting observations can be made. First, the average collected water mass during the first hour of the condensation experiments is 3-4 times larger compared to a ΔT of 7°C, indicating the strong effect of subcooling in the condensation and collection process. Second, a similar trend of reduced variability between sample types is observed as the angle of inclination increases with


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an even smaller variation compared to low subcooling. Third, at a 10° angle, a similar quantity of water is collected for all sample types.

Figure 7: Images of the water collection process at distinct times (t=5, 15, 30, 60 minutes) for all four sample types at 10° angle and 27°C subcooling. The error bars represent the standard deviation.

This result is further illustrated in Fig. 7, which shows images of the water collection process at distinct times (t=5, 15, 30 minutes). All samples exhibit similar nucleation and shedding behavior, albeit with an accelerated timeframe compared to 7°C subcooling. By the 30-minute mark, measurable water collection is observed for all sample types. The process begins very rapidly on the superhydrophobic samples, where sliding lanes for sufficiently formed droplet aggregates are observed within 5 minutes. The pooling and small-to-large droplet coalescence are still observed for the hydrophilic and hydrophobic/superbiphilic surfaces, respectively;


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however, the droplet surface temperature is significantly lower than the dew point, promoting faster nucleation and growth to the critical diameter size.

Figure 8: a) Average water collection rate and b) onset of water collection at different inclination angles for samples tested at 27°C subcooling. The error bars represent the standard deviation.

Figure 8 shows statistical data for the average water collection rate and water collection onset at 27°C subcooling. As implied from the real-time data in Fig. 6, smaller variations between the means and standard deviations are observed for this experimental condition compared to 7°C subcooling in terms of the average water collection rate. Interestingly at 10°, while the mean for the hydrophilic samples is lower compared to both the hydrophobic and superhydrophobic (29.8 g/m2 min vs. 35.7 g/m2 min and 33.4 g/m2 min, respectively), the standard deviation is sufficiently high to make the result statistically insignificant at p Vcrit, Equations 1 and 2 can be combined to yield the net force drawing the droplet toward the collector.

F = mgsin φ − γ D ( cosθ rec − cosθ adv )

(4)

Noting that energy is simply force times distance (x), Equation 10 yields the potential energy as a function of position.

E = mgx sin φ + γ Dx ( cosθ rec − cosθ adv )

(5)


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Velocity, too, can be calculated by noting that the sum of the potential and kinetic energy is a constant. This analysis ignores contributions due to air resistance and viscous dissipation, but it is not a bad approximation for small droplets on superhydrophobic surfaces, which experience little viscous dissipation.

E + K = E0

(6)

1 mgx sin φ + γ Dx ( cosθ rec − cosθ adv ) + mv 2 = mgh0 2

(7)

v = 2 mg ( h0 − x sin φ ) − γ Dx ( cosθ rec − cosθ adv ) / m

(8)

The force, potential energy, and velocity are plotted in Fig. 10 for a 3 mm droplet (V = Vcrit) sliding down a 30° incline over 0.5 mm patches with a 2 mm pitch. Equations 4, 5, and 8 are evaluated for θadv = 5° and θrec = 170°. (Evaluations of the three equations for additional conditions are provided as supporting information, Figs. S5-S6.) The force plot shows that the net force just reaches zero as the droplet detaches from a hydrophilic patch. The patches then introduce a periodic sawtooth perturbation to the potential energy, where the droplets are initially pulled onto the hydrophilic patch. The droplets then experience severe hysteresis while crossing the edge of the patch, where ܿ‫ߠݏ݋‬௥௘௖ − ܿ‫ߠݏ݋‬௔ௗ௩ pushes the capillary force to its maximum value of 2γD.


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b)

c)

Figure 10. Calculated a) net force, b) potential energy, and c) velocity as a function for a 3 mm droplet sliding down a 30° incline over 0.5 mm patches with a 2 mm pitch. Since we ignore dissipative losses in this analysis, the patches act essentially like speed bumps for the velocity. The acceleration of the droplet is critical to generate sufficient momentum to prevent pinning on defects or adjacent patches. Viscous dissipation could limit the terminal velocity to a point where these “speed bumps” become a significant factor in reducing the rate of water collection. Figure 7 shows experimentally that the hysteresis is large enough to prevent sliding even after the droplets grow by coalescing with their neighbors. Note how such droplets span multiple patches. Similarly, equation 4 can be also be used as the basis to understand the effect of contact angle hysteresis in the droplet shedding behavior of the unpatterned surfaces. In fact, this contact angle hysteresis was measured experimentally on hydrophilic, hydrophobic, and superhydrophobic samples after testing, exhibiting average values


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of 24°, 10°, and 6.2°, respectively (data and images also shown in the Supporting Information, Fig. S7). As anticipated, superhydrophobic surfaces have the lower contact angle hysteresis even after extensive testing. Taken together, the experimental data and theoretical considerations suggest that superbiphilic surfaces with diameter and pitch close to that of the scarab beetle are possibly the worst design for water collection from humid air. This finding might come as a surprise considering the success of the scarab beetle to collect water from air. Others have pointed out a critical difference in the conditions.27 The scarab beetle collects water from fog. The water has already condensed in air, and is carried across the desert by a steady wind. The beetle must capture the droplets from air before they evaporate. The hydrophilic patches, therefore, function primarily as traps for drifting water droplets. They exploit the contact angle hysteresis of a biphilic surface to overcome drag. Then, as additional droplets coalesce with the initial droplet, gravity eventually overcomes both capillary forces and drag to pull the droplets towards the beetle’s mouth. Observe that previous success mimicking the Stenocara beetle were demonstrated on fog collection rather than dew.19-22 As noted in the introduction, several studies have shown that it is also possible to improve dew collection through micropatterned wettability. However, these improvements are relatively small and depend strongly on the experimental conditions. These studies did not examine a wide range of inclination angles or subcooling, so it is not clear whether the performance advantage would be maintained under the operating conditions that are most relevant to dew collection. Comparisons are also hindered because other controls, such as smooth silica, are not provided.2930


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Further complicating interpretation of the data is the influence of surface topography. The Aizenberg group recently showed that the combination of wettability patterns and carefully controlled topography can greatly enhance the rate of water collection on a cold surface.25 They showed specifically that convex surface features increase the local vapor diffusion flux, and that an apex geometry with a widening slope creates a driving force for fast directional transport of droplets towards the collector. The results of the current study therefore do not preclude that patterned wettability can enhance dew collection when coupled with other design strategies. Overall, it is the authors’ opinions that approaches that combine patterned wettability with surface topography and potentially involve alternate droplet shedding mechanism can be optimized to improve water collection rates by taking into account the specific operational conditions and size scale of the application of interest. For conditions that are most relevant to dew collection–moderate subcooling and small angles of inclination–the rate-limiting step appears to be the initiation of sliding. Nucleation occurred so instantaneously that rate differences spanning 100 orders of magnitude could not be readily distinguished in the water collection data. Neither could a propensity for enhancing droplet growth rates be detected. Accordingly, a surface with high contact angle (smaller liquid-solid contact area) and low hysteresis provides the greatest rate of water collection under typical dew collection conditions. Being defined by these two properties, superhydrophobic surfaces appear to be best suited for most dew collection installations.

4. CONCLUSIONS At moderate levels of subcooling, the rate of water collection due to water condensation appears to be dominated by droplet shedding. Superhydrophobic surfaces, having the lowest


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contact angle hysteresis, collect water most rapidly overall, with their comparative advantage being greatest at moderate subcooling and low angles of inclination. The improvement is driven by the delay in the onset of water collection, which is smallest in superhydrophobic surfaces. Since the resistance to sliding is lower, droplets could begin sliding at smaller sizes and therefore earlier in the experiment. The advantage for superhydrophobic surfaces diminishes at higher angles of inclination. The increased gravitational component parallel to the surface means that gravity overcomes droplet pinning at smaller sizes for all surface types. Surfaces with larger contact angle hysteresis therefore catch up to the superhydrophobic surface when the angle becomes steeper. Hydrophilic surfaces were even shown to surpass superhydrophobic surfaces at steep angles when the rate of subcooling was low enough, because the rate of condensation became the rate limiting step under those conditions. For high levels of subcooling, water condensed so rapidly that neither the angle of inclination nor wettability had a significant effect on the rate of water capture. Such conditions are less relevant for dew collection, but apply to industrial processes such as distillation. Although they were designed to combine the best aspects of hydrophilic and hydrophobic surfaces, the superbiphilic surfaces are the least efficient owing to the importance of contact angle hysteresis. The border between the hydrophilic patch and hydrophobic background inherently causes large differences between the advancing and receding contact angles. Any advantage in nucleation rate conferred by the hydrophilic patch is more than overcome by the penalty of droplet pinning. Droplets pool on the superbiphilic surface, often spanning multiple patches. The impact on water collection worsens as the degree of subcooling decreases, because the droplets take a long time to grow large enough for gravity to overcome droplet pinning.


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5. ASSOCIATED CONTENT Supporting Information. Supporting Information (Figures S1-S7) is provided. 6. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Office of Naval Research Award N0014-15-2107 and National Science Foundation Award 1511453. Notes Any additional relevant notes should be placed here. 7. ACKNOWLEDGMENT This work was supported by the Office of Naval Research under Award N00014-15-1-2107. Student support for this work was partially provided by the National Science Foundation under Award 1511453. The authors would like to thank the staff of the microfabrication and materials characterization facilities at APL for assisting with sample preparation and characterization. ABBREVIATIONS


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BRIEFS (Word Style “BH_Briefs”). If you are submitting your paper to a journal that requires a brief, provide a one-sentence synopsis for inclusion in the Table of Contents. SYNOPSIS (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis, see the journal’s Instructions for Authors for details.

8. REFERENCES (1) Sha, M. M., A General Correlation for Heat Transfer During Film Condensation inside Pipes. International Journal of Heat and Mass Transfer 1979, 22, 547-556. (2) Stephan, K., Heat Transfer in Condensation and Boiling. Springer-Verlag: Berlin, 1992. (3) Al-Hayek, I.; Badran, O. O., The Effect of Using Different Designs of Solar Stills on Water Distillation. Desalination 2004, 169 (2), 121-127. (4) Aybar, H. S.; Egelioglu, F.; Atikol, U., An Experimental Study on an Inclined Solar Water Distillation System. Desalination 2005, 180 (1-3), 285-289. (5) Tian, X. L.; Chen, Y.; Zheng, Y. M.; Bai, H.; Jiang, L., Controlling Water Capture of Bioinspired Fibers with Hump Structures. Advanced Materials 2011, 23 (46), 5486-+. (6) Zhang, S.; Huang, J.; Chen, Z.; Lai Y., Bioinspired Special Wettability Surfaces: From Fundamental Research to Water Harvesting Applications. Small 2016, 1602992. (7) Yang, X. L.; Liu, X.; Lu, Y.; Song, J. L.; Huang, S.; Zhou, S. N.; Jin, Z. J.; Xu, W. J., Controllable Water Adhesion and Anisotropic Sliding on Patterned Superhydrophobic Surface for Droplet Manipulation. Journal of Physical Chemistry C 2016, 120 (13), 7233-7240. (8) Feeley, T. J.; Skone, T. J.; Stlegel, G. J.; McNemar, A.; Nemeth, M.; Schimmoller, B.; Murph, J. T.; Manfredo, L., Water: A Critical Resource in the Thermoelectric Power Industry. Energy 2008, 33 (1), 1-11. (9) Prieto, M. M.; Montanes, E.; Menendez, O., Power Plant Condenser Performance Forecasting Using a Non-Fully Connected Artificial Neural Network. Energy 2001, 26 (1), 65-79. (10) Campen, J. B.; Bot, G. P. A., Dehumidification in Greenhouses by Condensation on Finned Pipes. Biosystems Engineering 2002, 82 (2), 177-185. (11) G, S., Dew Harvest: To Supplement Drinking Water Sources in Arid Coastal Belt of Kutch. Foundation Books: 2006. (12) Nikolayev, V. S.; Beysens, D.; Gioda, A.; Milimouk, I.; Katiushin, E.; Morel, J. P., Water Recovery from Dew. Journal of Hydrology 1996, 182 (1-4), 19-35. (13) Khalil, B.; Adamowski, J.; Shabbir, A.; Jang, C.; Rojas, M.; Reilly, K.; Ozga-Zielinski, B., A Review: Dew Water Collection from Radiative Passive Collectors to Recent Developments of Active Collectors. Sustainable Water Resources Management 2016, 2 (1), 71-86. (14) Sigsbee, R. A., Nucleation. Marcel Dekker: New York, 1969.


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(15) Varanasi, K. K.; Hsu, M.; Bhate, N.; Yang, W. S.; Deng, T., Spatial Control in the Heterogeneous Nucleation of Water. Applied Physics Letters 2009, 95 (9). (16) Hamilton, W. J.; Seely, M. K., Fog Basking by Namib Desert Beetle, OnymacrisUnguicularis. Nature 1976, 262 (5566), 284-285. (17) Norgaard, T.; Dacke, M., Fog-Basking Behaviour and Water Collection Efficiency in Namib Desert Darkling Beetles. Frontiers in Zoology 2010, 7. (18) Parker, A. R.; Lawrence, C. R., Water Capture by a Desert Beetle. Nature 2001, 414 (6859), 33-34. (19) Bai, H.; Wang, L.; Ju, J.; Sun, R. Z.; Zheng, Y. M.; Jiang, L., Efficient Water Collection on Integrative Bioinspired Surfaces with Star-Shaped Wettability Patterns. Advanced Materials 2014, 26 (29), 5025-5030. (20) Garrod, R. P.; Harris, L. G.; Schofield, W. C. E.; McGettrick, J.; Ward, L. J.; Teare, D. O. H.; Badyal, J. P. S., Mimicking a Stenocara Beetle's Back for Microcondensation Using Plasmachemical Patterned Superhydrophobic-Superhydrophilic Surfaces. Langmuir 2007, 23 (2), 689-693. (21) Wang, Y. C.; Zhang, L. B.; Wu, J. B.; Hedhili, M. N.; Wang, P., A Facile Strategy for the Fabrication of a Bioinspired Hydrophilic-Superhydrophobic Patterned Surface for Highly Efficient Fog-Harvesting. Journal of Materials Chemistry A 2015, 3 (37), 1896318969. (22) Zhang, L. B.; Wu, J. B.; Hedhili, M. N.; Yang, X. L.; Wang, P., Inkjet Printing for Direct Micropatterning of a Superhydrophobic Surface: Toward Biomimetic Fog Harvesting Surfaces. Journal of Materials Chemistry A 2015, 3 (6), 2844-2852. (23) Chen, X. M.; Wu, J.; Ma, R. Y.; Hua, M.; Koratkar, N.; Yao, S. H.; Wang, Z. K., Nanograssed Micropyramidal Architectures for Continuous Dropwise Condensation. Advanced Functional Materials 2011, 21 (24), 4617-4623. (24) Hou, Y. M.; Yu, M.; Chen, X. M.; Wang, Z. K.; Yao, S. H., Recurrent Filmwise and Dropwise Condensation on a Beetle Mimetic Surface. Acs Nano 2015, 9 (1), 71-81. (25) 25. Park, K. C.; Kim, P.; Grinthal, A.; He, N.; Fox, D.; Weaver, J. C.; Aizenberg, J., Condensation on Slippery Asymmetric Bumps. Nature 2016, 531 (7592), 78-82. (26) Rahman, M. M.; Olceroglu, E.; McCarthy, M., Role of Wickability on the Critical Heat Flux of Structured Superhydrophilic Surfaces. Langmuir 2014, 30 (37), 11225-11234. (27) Lee, A.; Moon, M. W.; Lim, H.; Kim, W. D.; Kim, H. Y., Water Harvest Via Dewing. Langmuir 2012, 28 (27), 10183-10191. (28) Olceroglu, E.; McCarthy, M., Self-Organization of Microscale Condensate for Delayed Flooding of Nanostructured Superhydrophobic Surfaces. Acs Applied Materials & Interfaces 2016, 8 (8), 5729-5736. (29) Ghosh, A.; Beaini, S.; Zhang, B. J.; Ganguly, R.; Megaridis, C. M., Enhancing Dropwise Condensation through Bioinspired Wettability Patterning. Langmuir 2014, 30 (43), 13103-13115. (30) Thickett, S. C.; Neto, C.; Harris, A. T., Biomimetic Surface Coatings for Atmospheric Water Capture Prepared by Dewetting of Polymer Films. Advanced Materials 2011, 23 (32), 3718-+.


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For Table of Contents Only Under typical dew collection conditions, the initiation of sliding is the rate-limiting step. Contact angle hysteresis negates the benefits of enhancing the nucleation rate of condensation by patterning hydrophilic patches on a hydrophobic surface.


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Effects of Engineered Wettability on the Efficiency of Dew Collection

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