Small Structures, Big Droplets: The Role of Nanoscience in Fog Harvesting Bat-El Pinchasik,* Michael Kappl, and Hans-Jürgen Butt Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ABSTRACT: Designing materials for water harvesting has gained much attention in recent years as water scarcity continues to be one of the biggest problems facing mankind. In this issue of ACS Nano, Xu et al. propose a new device for harvesting water from fog. They use conically shaped copper wires with periodic roughness to enhance condensation and transport of water drops. While the periodic roughness enhances drop coalescence and motion, the conical shape of the wires guides the drops in a specific direction. Together, a self-sustained water-harvesting system is described which does not require additional external stimulus but makes use of a smart design and economic production.
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ccording to the United Nations, ca. 700 million people around the world suffer from water scarcity. By 2025, this number is expected almost to triple.1 Although the sub-Saharan region of Africa has the largest concentration of water-stressed countries, water scarcity is a problem in dozens of other countries, as well. In 2005, the UN launched a 10-year action named “Water for Life”, aimed at solving the problem of limited water access that is experienced in numerous regions in the world. One of the goals of this action is to “strengthen international cooperation in the fields of technologies for enhanced water productivity, financing opportunities, and an improved environment”. With large regions having little rainfall and a global climate that is changing, looking for new routes to collect water is one of the most important global challenges. In the absence of sufficient precipitation, collecting water from fog or oversaturated vapor is one promising route to harvest water. In nature, one can find inspiring evolutionary solutions for water deficiency. The Namib desert beetle, living in arid areas, collects water droplets on its back.2 Its bumpy surface consists of alternating hydrophobic and hydrophilic regions.3 Water droplets easily condense on the hydrophilic patches, and once the droplet reaches the boundary of a patch, its volume continues to grow while the contact area remains constant. Eventually, the weight of the drop overcomes capillary adhesion, and the droplet rolls off the inclined back and into the beetle’s mouth, guided by grooves on the surface of the beetle’s back. Such a design allows the beetle to live under harsh desert conditions. Another example can be found in the family of agamid lizards.4 Their skin is capable of collecting and transporting water by means of a capillary system between their scales. Various developments in the field have made use of inspiration from nature, where materials are evolutionarily © 2016 American Chemical Society
Overall, three important aspects contribute to efficient fog harvesting: enhanced condensation, control of lateral adhesion, and guided transport. optimized for the survival of species in their environments.5 From the design point of view, spider silk6 and cactus spikes7,8 gained much attention because of their conical geometry that stimulates capillary propulsion.9−12 From a physicochemical point of view, the desert beetle inspired the use of materials with alternating wettability or wettability gradients.13 Nevertheless, inspiration from nature is only the starting point, while the ultimate goal is to improve materials and design in order to maximize water collection from fog. Overall, three important aspects contribute to efficient fog harvesting: enhanced condensation, control of lateral adhesion, and guided transport. While water easily condenses on hydrophilic surfaces, transport is hindered on such surfaces due to large hysteresis and pinning. The opposite applies to superhydrophobic materials, where pinning is minimized but condensation is hindered. Previous studies have used superhydrophobic and slippery14,15 surfaces for fog harvesting. However, the use of materials with varying wettability offers great promise for both condensation and transport of water droplets.16,17 In this case, one should be careful when using surfaces containing both hydrophobic and hydrophilic areas, namely, biphilic surfaces, as the border between the two areas can cause additional pinning.18 However, condensation of drops on wettability gradients promotes autopropulsion of drops toward hydrophilic areas.19 Such propulsion also boosts Published: December 12, 2016 10627
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Figure 1. Water collector device for efficient fog harvesting. Water droplets condense on the conical hydrophilic copper wires. Coalesced droplets self-propel along the wire due to difference in Laplace pressure. In this way, space for new droplets to condense becomes available. Finally, droplets are collected in the reservoir. Reprinted from ref 23. Copyright 2016 American Chemical Society.
negative influence on the harvesting rate for both positive and negative tilting angles.23 Any deviation from horizontal placement of the wire reduces drop velocity along the wire and therefore the overall collection rate. The reason for that, however, is not thoroughly explained in the study. Finally, a fog-harvesting device made of an array of conical wires in a comb arrangement is tested for different fog impacting velocities and angles. The harvesting rate increases with fog flow velocity. It is maximal when the flow impacts the comb arrangement in the normal direction. This should be taken into account in real life applications. For example, a helical arrangement of wires may overcome the direction dependency. It is also inferred from the study that collection rate is not linear with fog impact velocity. Better understanding of this observation can help to improve future designs of collectors as different regions exhibit different fog characteristics such as droplets’ size and velocity.
coalescence of adjacent droplets, resulting in energy gain from reducing their total interfacial area. While energy gain from coalescence causes droplets to jump from superhydrophobic surfaces,20 motion of surface-anchored droplets is promoted on hydrophilic surfaces as reported by Xu et al. The third important aspect of efficient fog harvesting is directional transport of droplets. Each condensed drop has to move toward the water reservoir and make place for new drops to condense. Therefore, a gradient in curvature is a powerful tool to achieve spontaneous motion of drops in a specific direction. Such asymmetry induces a gradient in Laplace pressure inside the drop and results in capillary autopropulsion even on surfaces with uniform surface tension.10 On a conical geometry, a drop will self-propel toward the base, where the curvature is lower.9,21 Although conical geometry gained the most attention in previous studies, additional geometries were reported, as well.14,22 In this issue of ACS Nano, Xu et al. show that combining these three principles can boost the efficiency of condensers.23 In their study, they use a copper wire with conical shape and alternating roughness. The hydrophilicity of the rough copper wire and its good heat conductance facilitate condensation. The conical shape produces a gradient of curvature along the wire (Figure 1). As the Laplace pressure in the droplet depends on the radius of curvature, the two sides of the condensed droplet experience different pressure. Such difference is the driving force for the droplet motion toward the base of the cone. While the conical shape induces directional motion, Xu et al. added an alternating roughness along the wire. Both conical shape and roughness are fabricated electrochemically. Roughness increases the wettability of a hydrophilic surface by increasing the effective area exposed to water. Therefore, by introducing periodic roughness, Xu et al. introduce alternating wettability along the wire axis. In this way, droplets forming on regions with low roughness (less hydrophilic) will spontaneously move toward a region of high roughness (more hydrophilic). Such motion results in enhancement of droplet coalescence and growth. Once the droplet reaches a critical size, it spontaneously moves along the wire due to the curvature gradient. Xu et al. make use of a rather facile method to achieve alternation in wetting properties by tuning the surface roughness instead of using chemical patterning.24 Such an approach can potentially increase the robustness and long-term stability of the device performance. Xu et al. also study the influence of tilting angle on the harvesting rate. Interestingly, the tilt of the conical wire has a
In this issue of ACS Nano, Xu et al. show that combining these three principles can boost the efficiency of condensers. The fog-harvesting device built by Xu et al. using the copper conical wire with alternating roughness is capable of collecting up to 6 l/(m2 h) water for specific conditions. For comparison, previous studies using woven meshes for fog harvesting reported efficiency in the range of 0.1−1.6 l/(m2 h).25−27 Other studies reported rates of roughly 13 l/(m2 h), double the rate reported in this study, but made use of superhydrophilic surfaces for which long-term high performance is questionable.16
OUTLOOK AND FUTURE CHALLENGES Several fog harvesters entered the market in the last few decades,25 and designing new materials for this purpose has progressed. Nevertheless, some fundamental aspects are yet to be addressed for further development. To solve the problem of performance depending on the fog flow direction, threedimensional structures should be designed to maximize harvesting.28 In addition, hierarchical structures could address condensation of a wide range of fog droplet sizes. To date, the use of curvature gradients to promote direct motion of droplets has focused mainly on conical geometries. Other asymmetric geometries for autopropulsion may be more efficient or easier to fabricate.14 Figure 2 shows suggested architectures that 10628
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(5) Ebner, M.; Miranda, T.; Roth-Nebelsick, A. Efficient Fog Harvesting by Stipagrostis sabulicola (Namib Dune Bushman Grass). J. Arid Environ. 2011, 75, 524−531. (6) Zheng, Y.; Bai, H.; Huang, Z.; Tian, X.; Nie, F.-Q.; Zhao, Y.; Zhai, J.; Jiang, L. Directional Water Collection on Wetted Spider Silk. Nature 2010, 463, 640−643. (7) Liu, C.; Xue, Y.; Chen, Y.; Zheng, Y. Effective Directional SelfGathering of Drops on Spine of Cactus with Splayed Capillary Arrays. Sci. Rep. 2015, 5, 17757. (8) Peng, Y.; He, Y.; Yang, S.; Ben, S.; Cao, M.; Li, K.; Liu, K.; Jiang, L. Magnetically Induced Fog Harvesting via Flexible Conical Arrays. Adv. Funct. Mater. 2015, 25, 5967−5971. (9) Liang, Y.-E.; Tsao, H.-K.; Sheng, Y.-J. Drops on Hydrophilic Conical Fibers: Gravity Effect and Coexistent States. Langmuir 2015, 31, 1704−1710. (10) Lv, C.; Chen, C.; Chuang, Y.-C.; Tseng, F.-G.; Yin, Y.; Grey, F.; Zheng, Q. Substrate Curvature Gradient Drives Rapid Droplet Motion. Phys. Rev. Lett. 2014, 113, 026101. (11) Lorenceau, É.; Quéré, D. Drops on a Conical Wire. J. Fluid Mech. 1999, 510, 29−45. (12) Li, E. Q.; Thoroddsen, S. T. The Fastest Drop Climbing on a Wet Conical Fibre. Phys. Fluids 2013, 25, 052105. (13) Thickett, S. C.; Neto, C.; Harris, A. T. Biomimetic Surface Coatings for Atmospheric Water Capture Prepared by Dewetting of Polymer Films. Adv. Mater. 2011, 23, 3718−3722. (14) Park, K.-C.; Kim, P.; Grinthal, A.; He, N.; Fox, D.; Weaver, J. C.; Aizenberg, J. Condensation on Slippery Asymmetric Bumps. Nature 2016, 531, 78−82. (15) Lalia, B. S.; Anand, S.; Varanasi, K. K.; Hashaikeh, R. FogHarvesting Potential of Lubricant-Impregnated Electrospun Nanomats. Langmuir 2013, 29, 13081−13088. (16) Zhu, H.; Yang, F.; Li, J.; Guo, Z. High-Efficiency Water Collection on Biomimetic Material with Superwettable Patterns. Chem. Commun. 2016, 52, 12415−12417. (17) Wang, Y.; Wang, X.; Lai, C.; Hu, H.; Kong, Y.; Fei, B.; Xin, J. H. Biomimetic Water-Collecting Fabric with Light-Induced Superhydrophilic Bumps. ACS Appl. Mater. Interfaces 2016, 8, 2950−2960. (18) White, B.; Sarkar, A.; Kietzig, A.-M. Fog-Harvesting Inspired by the Stenocara beetleAn Analysis of Drop Collection and Removal from Biomimetic Samples with Wetting Contrast. Appl. Surf. Sci. 2013, 284, 826−836. (19) Chaudhury, M. K.; Whitesides, G. M. How to Make Water Run Uphill. Science 1992, 256, 1539. (20) Wisdom, K. M.; Watson, J. A.; Qu, X.; Liu, F.; Watson, G. S.; Chen, C.-H. Self-Cleaning of Superhydrophobic Surfaces by SelfPropelled Jumping Condensate. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 7992−7997. (21) Michielsen, S.; Zhang, J.; Du, J.; Lee, H. J. Gibbs Free Energy of Liquid Drops on Conical Fibers. Langmuir 2011, 27, 11867−11872. (22) Comanns, P.; Buchberger, G.; Buchsbaum, A.; Baumgartner, R.; Kogler, A.; Bauer, S.; Baumgartner, W. Directional, Passive Liquid Transport: The Texas Horned Lizard as a Model for a Biomimetic “Liquid Diode. J. R. Soc., Interface 2015, 12, 20150415. (23) Xu, T.; Lin, Y.; Zhang, M.; Shi, W.; Zheng, Y. High-Efficiency Fog Collector: Water Unidirectional Transport on Heterogeneous Rough Conical Wires. ACS Nano 2016, DOI: 10.1021/acsnano.6b05595. (24) Sun, M.; Liang, A.; Watson, G. S.; Watson, J. A.; Zheng, Y.; Jiang, L. Compound Microstructures and Wax Layer of Beetle Elytral Surfaces and Their Influence on Wetting Properties. PLoS One 2012, 7, e46710. (25) Klemm, O.; Schemenauer, R. S.; Lummerich, A.; Cereceda, P.; Marzol, V.; Corell, D.; van Heerden, J.; Reinhard, D.; Gherezghiher, T.; Olivier, J.; Osses, P.; Sarsour, J.; Frost, E.; Estrela, M. J.; Valiente, J. A.; Fessehaye, G. M. Fog as a Fresh-Water Resource: Overview and Perspectives. Ambio 2012, 41, 221−234. (26) Wang, Y.; Zhang, L.; Wu, J.; Hedhili, M. N.; Wang, P. A Facile Strategy for the Fabrication of a Bioinspired Hydrophilic−Super-
extend the conical geometry to gradients in additional directions.
Figure 2. Conical geometry (top), extended to additional geometries (middle and bottom) using gradients in curvature for guided directional transport of condensed droplets.
Finally, contamination alters the wetting properties of surfaces dramatically. As many of the water-stressed regions are desert-like, one should consider long-term performance of such devices. Hybrid systems that include the use of selfcleaning superhydrophobic materials should be considered. The authors would also like to point out the variability of measures to estimate the efficiency of materials for fog harvesting. It would be beneficial to standardize the conditions used to evaluate efficiency in order to compare different watercollection systems directly and fairly.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Hans-Jürgen Butt: 0000-0001-5391-2618 Notes
The authors declare no competing financial interest.
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