Bioinspired Dual-Tier Coalescence for Water-Collection Efficiency

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Biological and Environmental Phenomena at the Interface

Bioinspired Dual-Tier Coalescence for Water Collection Efficiency Enhancement Barbara Ting Wei Ang, Choon Hwai Yap, Wee Siang Vincent Lee, and Jun Min Xue Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02474 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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Bioinspired Dual-Tier Coalescence for Water Collection Efficiency Enhancement Barbara T. W. Anga, Choon Hwai Yapb, Wee Siang Vincent Leea*, Junmin Xuea* a

Department of Materials Science and Engineering, National University of Singapore, Singapore

117573. b

*

Department of Biomedical Engineering, National University of Singapore, Singapore 117583. To whom correspondence should be addressed. E-mail: [email protected] (Junmin Xue),

[email protected] (Wee Siang Vincent Lee). Tel./fax +65 65164655.

Abstract Directly harvesting water from the atmosphere could aid in negating the issue of fresh water scarcity, garnering increased research interest in recent years. Typically, atmospheric water collection occurs via three main steps: accumulation, transportation and collection. While multiple studies have been published on bioinspired structures with enhanced directional fluid transportation, there is a significant lack of designs for enhancing water droplet coalescence. Long mean times before coalescence result in the re-evaporation of micro droplets, severely impeding the efficiency of atmospheric water collection. Herein, a water accumulator derived from a synergistic combination of inspiration from cacti spines and Tillandsia trichomes has been designed to encourage rapid coalescence. The drip-off volume measured in a fog chamber was found to be 220% that of a flat surface within 15 minutes, suggesting that improving coalescence efficiency will be important in the future development of water collection devices.

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Introduction The air contains a large amount of moisture that can be harvested in times of emergency when freshwater access is limited. Despite the ubiquity of this water source, it remains untapped due to the low energy efficiency of commercial water collectors. In order to exploit this source of water, there has been increased research interest in developing zero energy bioinspired water collectors. Flora and fauna native to arid environments have adapted over years of evolution, developing physical features that allow them to trap as much moisture as possible during rare events of high humidity. For example, the Namib desert beetle 1–3 has hydrophilic bumps surrounded by hydrophobic grooves, allowing the beetle to collect fog droplets on its back before transporting condensed droplets via a wettability gradient to the beetle’s mouth. Syntrichia Caninervis desert moss 4 have multi scale structures that serve specialized functions allowing for efficient water collection. Cacti spines 8–10

5–7

are capable of unidirectional water transportation via a Laplace gradient

. From these mechanisms found in nature, water collection can be summarized into

three main steps: accumulation, transportation and collection. All three components have significant effect on the efficiency of water collection. It is insufficient to solely investigate and enhance only one of the three components. Knowing that accumulation and re-evaporation are two mutually competitive mechanisms, it is imperative to enhance the coalescence of water droplets to reduce the onset time before droplets are large enough to be harvested. For the current bioinspired water collectors, random accumulation presents an increased likelihood of re-evaporation due to the wide separations between adjacent droplets, lowering the droplet growth rate. Hence, controlling the distance between the nucleated droplets by bringing accumulation loci


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close to one another will aid in reducing the re-evaporation rate by increasing the likelihood of coalescence. This phenomenon can be achieved via three strategies; (1) specialized structure for water to preferentially condense on, (2) prolonged air flow at specific areas, and (3) a central catchment area for localized accumulation of water. Herein, a water collector design inspired by the synergistic combination of cacti spines and Tillandsia trichomes is proposed. The design features sharp conical structures that act as a site for accumulation at the tips due to their small surface area. Arranging the sharp tips in a circular configuration minimizes the distance between accumulation loci, increasing the coalescence rates. Thus, this sharp circular brim (SCB) performs the function of focusing accumulation, while simultaneously encouraging microdroplets to coalesce with their neighbours. In addition to SCB, a bowl-like well is embedded within to serve as a rally site for the droplets that coalesced on the SCB. As the well starts to get filled, the coalescence between neighbouring bowls can be manipulated by controlling the bowl separation distances. Furthermore, the fluid dynamics simulation results show that such an indented bowl-like structure creates prolonged cyclic air flow at the brim of the well, in turn facilitating further accumulation of water droplets on the SCB. Hence, in this work, an efficient water accumulator with engineered water accumulation loci was designed and fabricated using polymethyldisiloxane (PDMS). By focusing accumulation on specific structures, the distance between neighbouring condensing droplets has been shortened, reducing the mean time before coalescence occurs, and preferential accumulation of water on the peaks of the SCB uniformly over the area of impingement was observed. Water accumulation was found to be improved by a factor of 120% for a 3 cm by 3 cm sample within 15 minutes. Consequently, by designing controlled water


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accumulation loci, water accumulation efficiency can be improved, which may provide further insight for future development of water collection devices.

Materials and Methods Materials Sylgard 184 Silicone elastomer (Dow Corning) was bought from element14 Pte Ltd and was used without further modification. A Novita NA600 humidifier was employed to maintain 100% humidity during water collection testing. Millipore purified water was used in water collecting testing, with pink food dye mixed in the deionized water for visibility in some photographs and videos. Preparation of PDMS Sylgard 184 was mixed with its curing agent in 10:1 ratio, degassed for 15 minutes in a vacuum chamber before immersing a conical rod array in the pre-cured solution. Both the conical rod array and PDMS are put into a drying oven at 80 °C for 1 hour. The cured PDMS and conical rod array are carefully separated to obtain the final structure. 12.5g of pre-cured PDMS mixture was put into four separate identical circular-based moulds to ensure that all samples would have the same size and thickness after casting. Four samples were prepared using this method: a flat piece made without immersing a conical rod hexagonal array and three other SCB structures created with varied tip radii sizes of 1 mm, 1.5 mm, and 4 mm. The


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samples were then cut into identical 3 cm by 3 cm pieces and tested for their water collection abilities. Water collection setup Water collection was performed at 100% humidity in an enclosed chamber to eliminate external influences on the experiment. The humidifier nozzle was maintained at a distance of 8 cm away from the surface of each sample during testing (Figure S5). The amount of water collected was weighed at 15 min intervals for a total of 1 hour. All four samples were tested individually, using the flat PDMS sample as the control experiment. The nucleation of the water droplets was filmed using a Teslong MS1000 USB microscope video camera, and the surface coverage by the nucleated water droplets were measured and calculated using Fiji 11. Material Characterizations Scanning Electron Microscope (SEM) images were taken using a Zeiss Supra 40 SEM. Wetting angle was taken using AST Products Inc. VCA Optima contact angle instrument. Computational simulation The fluid dynamics of the surface structures were investigated using SolidWorks Flow Simulation. The ambient conditions were set as 101325.00 Pa, 293.20 K, 100.00% relative humidity, wind velocity was set as 0.100 m/s, the initial solid temperature was 293.20 K and the flow type was set as turbulent flow.


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Results and Discussion

Figure 1. (a) Water collector design inspired by a combination of the physical structures of cactus spines and tillandsia trichomes (b) A schematic representation of the fabrication process using polydimethylsiloxane (PDMS)

Cacti spines are known to be able to collect water in arid environments and have been applied in various bioinspired water collection devices

6,7,12

. Their conical structure

provides a Laplace gradient, allowing water droplets to be unidirectionally transported towards the base of the spine

5,6

. On the other hand, plants of the Tillandsia genus have

the ability to harvest moisture from air via the unique structure of their trichomes that cover their leaves

13,14

. A single leaf of a Tillandsia Ionantha was studied under a

Scanning Electron Microscope (SEM) as shown in Figure 1 (a), illustrating the trichomes growing on the surface of the leaves. The trichomes are observed to grow around the circumference of the stomata of the Tillandsia leaves, fanning radially in an outward direction, increasing the surface area with which water can be collected, concurrently


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ensuring that collected water moves towards the stomata (Figure S1). The two biostructures were amalgamated to form the SCB structure featuring micro-reservoirs (MRs) surrounded by a sharp ridged circle. The sharp ridges (made up of an infinite array of sharp conical spikes arranged in a circle) provide a site for water to condense upon and the cavity in the middle acts as a reservoir to accumulate water. PDMS is a material that is fundamentally hydrophobic, therefore it encourages dropwise condensation; and dropwise condensation is known to be more effective in heat removal from a surface, facilitating condensation

15–18

. A preliminary study to modify the surface energy of a

PDMS surface was conducted by coating it with polytetrafluoroethylene (PTFE) and poly(vinyl alcohol) (PVA). However, the results showed no significant improvement in water collecting ability as shown in Figure S3. To fabricate a SCB structure using PDMS, it is necessary to study the interaction between the mould and PDMS. For any liquid at the liquid-air interface in contact with any solid surface, three forces will be acting on the fluid, namely; (i) adhesion of fluid with solid surface, (ii) surface tension of the liquid-air interface, (iii) gravity. The interaction of these forces will result in capillary rise which causes the liquid climb up the walls of the solid surface around the entire circumference in the shape of a catenary curve

19,20

. Taking this capillary rise into consideration,

immersing a uniform pillar array into pre-cured PDMS will result in uniformly spaced indented structures after curing. By leveraging on this phenomenon of capillary rise, whole arrays of SCBs surrounded by sharp peaks can be obtained simultaneously. Thin conical rod arrays are immersed into the pre-cured PDMS, to obtain fins that extend radially outward. After curing, the PDMS solidifies, taking the same shape as the pre-


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cured solution. The needle array can be removed, with the PDMS forming half angle peaks around MRs, as shown schematically in Figure 1 (b).

Figure 2. (a) A large array of SCB fabricated using PDMS (b) Close-up view of a single SCB structure, including an inset with a schematic cross-section of the structure (c) SEM micrograph of the cross-section of one SCB peak (d) First tier coalescence occurring on the peak of the SCB, where the scale bar represents 1 mm. This series of photographs was taken with the camera almost 180 ° to the sample within the experimental chamber. (e) A chronological series of images showing the preferential nucleation of condensing water droplets, where the scale bar represents 10 mm. This series of photographs was taken with the camera at a 45 ° from the sample outside the collection chamber.

Figure 2 shows a hexagonal array of SCB fabricated using PDMS, including a close-up view of a single SCB and the cross-sections of single SCBs were studied under a SEM. The radii of the structures were measured using the optical microscope images, where an


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average value of 1 mm radius was obtained from six measurements of different SCB. The 15 ° half-angle of the sharp peaks were measured thrice from SEM micrographs of three separate cross sections of SCB. These measurements demonstrated that the PDMS was able to perfectly replicate the rounded tips of the conical arrays that were immersed in the pre-cured solution, with significant capillary rise around the structures, reproducing the desired SCB structure. The water nucleation on a single SCB structure was recorded in Supplementary Video 1 to study the nucleation mechanism. From Figure 2 (d), the nucleation of two small droplets can be seen on the top left edge of the SCB (t = 10 s). The droplets grow in size (t = 20 s) before they coalesce into a larger droplet (t = 30 s). This demonstrates the microscopic growth of the droplets (tier 1 growth) before they roll into the SCB for further accumulation. The droplets are both suitably small in volume and have sufficient adhesion force to resist the force of gravity to coalesce in the upwards direction (Supplementary Video 1). This observation can be attributed to the effects of Laplace gradient

12

. Preferential nucleation of condensing droplets is illustrated in the

series of chronological images in Figure 2 (e), showing that the SCB are able to encourage water droplets to form on their peaks. It can be seen that the droplets grow in size at fixed points around the peaks of the SCB, showing that the CB mechanism holds true for the SCB structures. This also demonstrates the growth of condensing droplets on an array of SCB at the macroscopic scale (tier 2 growth). Further information can be found in Figure S8 and S8 in the Supporting Information.


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Figure 3. (a) Flow simulation of a single MR structure where the velocity of the incoming air is retarded by the cavity (b) 3D flow simulation surface plot of the vorticity of air around the MR structure (c) Flow simulation slice plot of the vorticity of the air around the MR structure (d) The surface area of water nucleated on the surface of 3 cm by 3 cm samples with MR surface structures of varying radii (e) The volume of water collected by each MR sample normalized to the volume of water collected by the flat surface


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Table 1: Statistical water collection data

From the flow simulation result in Figure 3 (a), the velocity of air that flows in a direction orthogonal to the base of the SCB structures can be seen to decelerate as it flows towards the structure. Air velocity in the SCB curls within the cavity (as seen from the arrows highlighting the flow direction), resulting in low speed air flows within the cavity. The air velocity in the cavity can be seen to transit from negative values to small positive values a magnitude of 10 times lower than the original air velocity. This transition is due to the changing direction of the air within the cavity, further cycling the moisture in the air within the cavity. Curling increases the probability of airborne moisture coming into contact with the walls of the MRs, also helping to prevent water droplets in the SCBs from being ejected by incoming air flow, by presenting a certain amount of flow resistance. These effects work together to allow water to be accumulated within the


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cavity to relatively large volumes. Figure 3 (b) shows the 3D surface plot of the vorticity, which is the tendency of air to rotate around the structure and Figure 3 (c) shows a slice plot of the same flow simulation. The simulation was performed with the SCB arranged in a hexagonal array, where a single SCB was isolated for better visibility in Figure 3. Consequently, the influence of the other SCB in the array can be seen from Figure 3 (b) and (c), as evidenced by the decaying velocity around the base of the SCB. From both figures, it can be observed that the air tends to experience a larger magnitude of curling near the tips of the SCB structure, enhancing the rate of accumulation around that area. This is simulation is supported by the images in Figure 2 (d) and Figure 2 (e), where it can be observed that water tends to condense preferentially on the tips of the SCB structures, before rolling into the MRs where they have a higher chance of survival or rolling out of the SCB where there is a higher chance of re-evaporation. Low magnitudes of vorticity occur along the outer slopes of the reservoir, creating a boundary layer of lower air velocity due to friction with the surface. Incoming air would flow past the surface without forming any significant accumulation in the process. After water droplets have been accumulated within the cavities, the accumulated water will have to be transported to the collection point, freeing up the structures for the next cycle of accumulation. Three SCB samples were prepared, with 1 mm, 1.5 mm and 4 mm radii, to study the dependence of water collection efficiency on structure size. All samples had SCB structures arranged in hexagonal arrays on the surface. The average half-angles of the fabricated structures are 15 °. The dependence of accumulation rate on the size of the SCB structures were studied as summarized in Figure 3 (d), with a magnified version in


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Figure S6. Table 1 shows the statistical data of the water collection experiments plotted in Figure 3 (d). The surface area of the condensed droplets was measured and plotted against time. This image data was taken from the video recording of the water collection testing. The amount of water collected is calculated taking the measured area into account. It can be seen that the surface area of the condensed water droplets is insignificant on the flat surface within the first 100 seconds of water collection. This is due to the lack of nucleation sites on the flat surface

21,22

. The water collection started

with a radial collection of micro-droplets accumulating on the flat surface, resulting in the eventual formation of a breath figure

23–26

. There is an increase in the surface area of

water nucleated as the radii of the SCB structures decrease, with the 1 mm radii SCB structures showing a faster rate of water accumulation as compared to the rest of the samples (Supplementary Video 3). This observation can be attributed to the fact that the SCB structure is small enough to be packed together more densely than the larger radii structures, allowing coalescence to occur at high efficiencies, resulting in extremely high water collection rates. Hence, it is hypothesized that the smaller the surface structure, the higher the efficiency. As water is confined to an even smaller area, droplets are allowed to coalesce faster. The amount of water collected was normalized against the flat surface to compare their efficiencies as shown in Figure 3 (e). The normalized volume for the 1 mm radii SCB sample, , is calculated by the equation, =

,

where is the volume of water collected by the flat surface sample and is the volume of water collected by the 1 mm radii sample. The normalized volume of the other samples


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was calculated using the same equation, substituting the volume collected by the 1 mm radii sample with the volume collected by each sample. As all the samples are of the same dimensions (3 cm by 3 cm), the normalized volume can be interpreted as the increase in efficiency of water collection as compared to the flat surface. Overall, the 1 mm radii sample performed the best, supporting the hypothesis that smaller structures would increase the efficiency of water collection. It can be seen that the 1 mm radii sample shows a 120% increase in water collection efficiency as compared the flat sample in the initial 15 minutes of water collection. This high efficiency is a result of the initial high rate of accumulation as compared to the other samples. Subsequently, the 1 mm radii sample showed a mean increase of efficiency of 50%, which is higher than the 20% increase in efficiency observed in the 1.5 mm radii SCB structures sample. The initial water collection ability of the 1.5 mm radii sample was comparable to the flat surface, followed by a mean increase of efficiency of 20%. For the 4 mm radii SCB structure sample, the size of the structures was too large for the water to be collected efficiently, as seen by the almost 40% decrease in efficiency as compared to the flat surface. Due to the large size of the SCB on the surface of the sample, the volume of water required to fill the reservoir is much larger than the others. Assuming that the amount of water required to overflow a single MR would be the volume of a spherical droplet of water, the volume of a sphere can be calculated by the equation = 4⁄3 . This means that the 4 mm radii cavity requires 63 times more water to fill than the 1 mm radii cavity and the 1.5 mm radii cavity requires 2.375 times more water to fill as compared to the 1 mm radii cavity. The 1 mm radii cavity can be filled with 15 µl of water (Figure S2). The 1 mm radii sample performed the best out of all the samples may be due to enhanced


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coalescence efficiency and the smaller cavity volume. The overflow of water accumulated in the MRs is the main contributor to the volume of water collected by each sample. The large volume of water required to overflow the 4 mm radii reservoir is the main handicap for the sample’s inability to perform as well as the flat surface. However, this measurement is only in the first hour of water accumulation. It is expected that the water accumulated in the large cavities of the 4 mm radii sample will overflow later in the water accumulation process. This is undesirable because a long time is required for water to be accumulated in the collection trough placed at the base of the sample.


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Figure 4. (a) A schematic of the water accumulation mechanism as observed experimentally (b) Dual-tier growth mechanism: microdroplets grow on the peaks of the structure before rolling into the central collection area where further growth

From the collective set of data discussed above in the results and discussion, the experimentally observed water collection mechanism of the SCB structure is schematically presented in Figure 4 (a). Incoming water droplets were observed to first condense on the sharp ridges of the SCB structure due to the higher air retention at the tips as predicted by the simulation results in Figure 3. This phenomenon was also observed experimentally as seen in Supplementary Video 2. This allows a lot of water droplets to condense on the peak of the structure in a short time. As more and more incoming water droplets impinge on the droplets already on the structure, the water droplets grow in size while maintaining a constant base diameter according to constant base (CB) growth model (more details in Figure S7). This droplet will first grow via diffusion from its surroundings until it is large enough to touch a neighbouring droplet. This contact results in coalescence, further droplet growth and often spontaneous motion of the droplet into the MR. The coalescence of the water droplets occurs both on the sharp ridge and within the cavity. For droplets that coalesce in the cavity, eventually forming a single large droplet in the center of the cavity, where the droplets grow once again by the CB growth model. The large droplet will rest at the base of the reservoir until other droplets roll in to coalesce with it. The growth of this large droplet will occur until the volume of water accumulated reaches a spherical cap with a radius equal to that of the MR. The collected water droplets in the MR occurs when gravity overcomes adhesion of the water droplet to the surface of the structure, resulting in the droplet dripping off into the collection trough. Water flows from higher up in the array, circling


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around the SCBs, along the valleys between the structures. An alternate droplet removal mechanism that was observed is that passing water droplet on the way down to the collection point coalesce with the droplets in the cavities, extracting it from the reservoir. 15

. It was experimentally observed that coalescence aids in the transport of water droplets

out of the cavity before it is fully filled. The combination of gravitational force and coalescence-induced ejection of the accumulated water ensures that the surface is continually cleared of droplets. As discussed previously, to facilitate droplet growth, reevaporation of micro-droplets has to be reduced. Figure 4 (b) shows a schematic illustration of a rapid growth mechanism based on a CB growth model on hydrophobic surfaces. It was reported that the CB growth model has a higher rate of heat transfer which encourages the droplets to grow, therefore this growth model is applied to our SCB structure on two tiers. The water droplets firstly grow according to this CB growth model on the microscopic scale with a constant base of width , on the peak of the structure before coalescing with a neighbour and rolling into the MR. As the droplets accumulate in the MR, they continue to grow via the CB growth model on the macroscopic scale with a constant base width of , until the droplets grow big enough to touch one another as highlighted by the dotted circles as shown on the right of Figure 4 (b). When the droplets touch, coalescence will occur resulting in simultaneous droplet growth and ejection from the SCB, freeing the structure for the next cycle of accumulation. Experimental results are largely in agreement with the hypothesized mechanism from Figure 1 (c) and Figure 4 as seen from the videos (Supplementary Videos 1 to 3). It has also been observed that even after the accumulated water droplet has achieved the same radius as the MR, the droplet can still grow to an elongated shape before flowing out.


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Due to the larger surface area in contact with the droplet, larger cavities tend to accumulate larger volumes of water before overflowing (in cases where coalescence has yet to occur on the second tier between neighbouring cavities). Such large droplets are useful in “bulldozing” other droplets out of the cavities in its way to the collection point.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. It contains SEM images, water contact angle measurement, experimental methods and supplementary information for water collection.

Author information Corresponding Author *E-mail: [email protected] (Junmin Xue), [email protected] (Wee Siang Vincent Lee). Tel./fax +65 65164655. Notes The authors declare no competing financial interest.

Acknowledgements

The authors would like to thank Gardens by the Bay for graciously donating several species of Tillandsia seedling from their nursery for our research. This work was supported by Singapore MOE Tier 1 funding R-284-000-162-114.

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Bioinspired Dual-Tier Coalescence for Water-Collection Efficiency

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