Article pubs.acs.org/Langmuir
Hierarchical Surface Architecture of Plants as an Inspiration for Biomimetic Fog Collectors M. A. K. Azad,*,† W. Barthlott,† and K. Koch‡ †
Nees Institute for Biodiversity of Plants, Rheinische Friedrich-Wilhelms-University of Bonn, Venusbergweg 22, 53115 Bonn, Germany ‡ Faculty of Life Sciences, Rhine-Waal University of Applied Sciences, Marie-Curie Straße 1, 47533 Kleve, Germany S Supporting Information *
ABSTRACT: Fog collectors can enable us to alleviate the water crisis in certain arid regions of the world. A continuous fog-collection cycle consisting of a persistent capture of fog droplets and their fast transport to the target is a prerequisite for developing an efficient fog collector. In regard to this topic, a biological superior design has been found in the hierarchical surface architecture of barley (Hordeum vulgare) awns. We demonstrate here the highly wettable (advancing contact angle 16° ± 2.7 and receding contact angle 9° ± 2.6) barbed (barb = conical structure) awn as a model to develop optimized fog collectors with a high fog-capturing capability, an effective water transport, and above all an efficient fog collection. We compare the fog-collection efficiency of the model sample with other plant samples naturally grown in foggy habitats that are supposed to be very efficient fog collectors. The model sample, consisting of dry hydrophilized awns (DH awns), is found to be about twice as efficient (fog-collection rate 563.7 ± 23.2 μg/cm2 over 10 min) as any other samples investigated under controlled experimental conditions. Finally, a design based on the hierarchical surface architecture of the model sample is proposed for the development of optimized biomimetic fog collectors.
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INTRODUCTION
unguicularis) demonstrated a better efficiency of the plant species.15 In the last few decades the only technology for collecting fog has been large fog collectors (LFCs) made of polyolefin mesh; it has been used in 40 countries in South America, Europe, Africa, and Asia where suitable regular fog is present.2,3,16 With the aim to increase the amount of fog collection, a multifunnel fog collector has been proposed.17 Although the simulations showed that the multifunnel fog collector is more efficient than the old design of the LFC, the design can be further improved by using elements based on the hierarchical architectures of plants. Some studies have shown the potential of artificial silk fiber,18−24 electrospun nanofibers,25−27 and hydrophilic− hydrophobic cooperative surfaces/systems28−30 to collect fog. Inspired by the cactus spine,11 conical copper31 and zincoxide32 wire or artificial spines27 and fog-collecting impermeable surfaces with cone arrays11,33−35 have been demonstrated. Although the cone arrays were proved to be highly efficient for a continuous fog collection, there were not enough indications in these publications of how the structures can be implemented to build a large fog collector, with one exception,11 where the proposed prototype surface lacks void spaces that may result in the hindrance to flow of the fog-carrying wind. The fog-carrying wind has the tendency to deviate the direction while approaching an impermeable obstacle. This is why fog
Shortages of fresh water are a major problem affecting around 1 billion people worldwide;1 the problem is most acute in arid, semiarid tropical, and subtropical climates with limited or no rainfall at all. Fog is an important source of water that is often ignored. However, fog collection can be an elegant solution to meet the demand of water for afforestation, for irrigation, and above all as a drinking water source for humans and animals in these areas.2−4 In the foggy regions, the dripping of water from the foliage is one of most often seen phenomena. The arrangement, density, type, and size of the foliage play an important role in the amount of fog drip. Plants having “needlelike” structures, e.g., pine, redwood, and fir, are reported to collect a good amount of fog whereas the “leaflike” impermeable structures (no regular void space on the surface) allow the moisture to flow around the surface of the leaf.5−7 Increased branchiness of plant species, such as narrow leaf syndrome, is reported to be an important trait to collect fog.8−10 Tillandsia usneoides, Opuntia microdasys,11 and Cotula fallax,12 all having hierarchical structures, have been demonstrated to collect fog.9,13 Vogel and Müller-Doblies14 did an extensive study on eight monocot families and some Oxalis species in some semidesert regions (Namaqualand and adjacent regions) of South Africa, where they demonstrated that these plants did have “special morphological adaptations” of their aerial parts that facilitate fog and dew collection. A comparison of fog collection between a microstructured Namib grass (Stipagrostis sabulicola) and a Namib beetle (Onymacris © XXXX American Chemical Society
Received: July 1, 2015 Revised: November 11, 2015
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DOI: 10.1021/acs.langmuir.5b02430 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
needle-like structures (NLSs) were arranged vertically in such a way that there were gaps (width + 5−10% of the width of the awns/Ns/ NLSs) between the needles. The samples were prepared in such a way that their effective surface areas remained close to each other. About 50−55% of the surface area of each prepared 2 × 2 cm2 sample was occupied by the Ns/NLSs and the rest was free to get rid of the impedance of the fog flow. Therefore, the samples have equivalent effective surface areas for fog collection. The measurement of the surface area coverage by the Ns/NLSs was calculated from the images of the samples by Photoshop CS3. For the first set of samples, we used intact Ns/NLSs, whereas for the second set, the needles of Abies and Sequoia were cut to reach as close to the diameter of the Hordeum awn as possible. The Tillandsia leaves (NLSs) of smaller diameter close to Hordeum awn were also selected for the second set as well. Samples of 2 × 2 cm2 were prepared as previously described. Hydrophilization. Dry Hordeum awns were dip-coated with a water-based polymer solution containing TiO2 (titanium dioxide) nanoparticles (TA 2202, Nadico Technologie GmbH, Langenfeld, Germany), followed by drying at room temperature for ∼48 h. After evaporation of the solvent, the particles are bonded into a remaining matrix that prevents the release of free TiO2 into the environment. The average particle size of TiO2 is given as
