Most insects and spiders drown when submerged during flooding or tidal inundation, but some are able to survive and others can remain submerged indefinitely without harm. Many achieve this by natural adaptations to their surface morphology to trap films of air, creating plastrons which fix the water-vapor interface and provide an incompressible oxygen-carbon dioxide exchange surface. Here we demonstrate how the surface of an extremely water-repellent foam mimics this mechanism of underwater respiration and allows direct extraction of oxygen from aerated water. The biomimetic principle demonstrated can be applied to a wide variety of man-made superhydrophobic materials. feature of these surfaces is the suspension of a droplet of water on surface protrusions so that the water effectively sits upon a composite solid-air surface with a solid fraction of 20% or less. 4-6 The droplet of water is then effectively separated from the solid by a layer of air. We observed that when such a superhydrophobic surface is immersed in water it glistens with a silvery sheen, thus indicating that a sheathing film of air remains on the submerged surface. Such silvery reflections are also observed from the plastrons on some aquatic insects and spiders. [7][8][9][10] Plastrons are an adaptation to the water environment that allows oxygen to be extracted directly from the surrounding water. We therefore considered whether the submerged surface of a superhydrophobic material could be used to mimic plastron respiration rather than be simply a passive waterrepellent surface. Given that these materials gain their superhydrophobicity by dramatically reducing the solid fraction of their surface area, it might be naively expected that submersion would necessarily cause water penetration and a loss of superhydrophobic properties thus preventing their use for plastron respiration. PACS NumbersEven when fully saturated the oxygen concentration in water is around thirty times less than in air. Some diving insects resolve this problem by carrying an air bubble that provides both an air reservoir and a water-vapor interface across which gaseous diffusion occurs, thus allowing oxygen to be replenished and carbon dioxide to hydrophobic open mesh network are also found in the cocoons of some silk-using insects, the egg-shells of terrestrial insects and the silk nests of some terrestrial, ground dwelling spiders. 12-14To investigate whether a superhydrophobic material could mimic the plastron action of biological systems we created a sol-gel foam material using methyltriethoxysilane and a phase separation process. Because all pore facing surfaces are methyl terminated, these materials are intrinsically superhydrophobic. 15 When the foam is submerged a silvery sheen can be observed indicating that a film of air remains at the surface of the just as it does at the surface of an insect's plastron. We hollowed out a cylindrical block of foam to create a gas cavity surrounded by porous superhydrophobic walls, analogous to the diving bell of the wate...

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Soil can become extremely water repellent following forest fires or oil spillages, thus preventing penetration of water and increasing runoff and soil erosion. Here the authors show that evaporation of a droplet from the surface of a hydrophobic granular material can be an active process, lifting, self-coating, and selectively concentrating small solid grains. Droplet evaporation leads to the formation of temporary liquid marbles and, as droplet volume reduces, particles of different wettabilities compete for water-air interfacial surface area. This can result in a sorting effect with self-organization of a mixed hydrophobic-hydrophilic aggregate into a hydrophobic shell surrounding a hydrophilic core.

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The wettability of soil is of great importance for plants and soil biota and in determining whether flooding and soil erosion will occur. The analysis used in common measurements of soil hydrophobicity makes the assumption that water always enters soils if the average contact angle between the soil and water is 90° or lower; these tests have been used for decades. The authors show theoretically and experimentally that water cannot enter many soils unless the contact angle is considerably lower than this, down to approximately 50°. This difference generates serious errors in determining and modeling soil wetting behavior.