A lichen protected by a super-hydrophobic and breathable structure

A species of lichen, Lecanora conizaeoides, is shown to be super-hydrophobic. It uses a combination of hydrophobic compounds and multi-layered roughness to shed water effectively. This is combined with gas channels to produce a biological analogue of a waterproof, breathable garment. The particular lichen grows mostly during wet seasons and is unusually resistant to acid rain [Hauck, M., 2003. The Bryotogist 106(2), 257-269; Honegger, R., 1998. Lichenologist 30(3),193-212]. The waterproof, breathable surface allows this lichen to photosynthesise when other species are covered with a layer of water. In addition, rainwater runs off the surface of the organism, reducing its intake of water from above and probably contributing to its resistance to acid rain

Electrowetting on superhydrophobic SU-8 patterned surfaces

Electrowetting on micro-patterned layers of SU-8 photoresist with an amorphous Teflon (R) coating has been observed. The cosine of the contact angle is shown to be proportional to the square of the applied voltage for increasing bias. However, this does not apply below 40 V and we suggest that this may be explained in terms of penetration of fluid into the pattern of the surface. Assuming that the initial application of a bias voltage converts the drop from Cassie-Baxter to Wenzel regime, we have used this as a technique to estimate the roughness factor of the surface

Self-organization of hydrophobic soil and granular surfaces

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

Critical conditions for the wetting of soils

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 degrees 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 degrees. This difference generates serious errors in determining and modeling soil wetting behavior

Porous materials show superhydrophobic to superhydrophilic switching

Switching between superhydrophobicity and superhydrophilicity in porous materials was predicted theoretically and demonstrated experimentally with the example of thermally induced contact angle change; tunability of this system was also demonstrated

Plastron properties of a superhydrophobic surface

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 the authors 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

An introduction to superhydrophobicity

This paper is derived from a training session prepared for COST P21. It is intended as an introduction to superhydrophobicity to scientists who may not work in this area of physics or to students. Superhydrophobicity is an effect where roughness and hydrophobicity combine to generate unusually hydrophobic surfaces, causing water to bounce and roll off as if it were mercury and is used by plants and animals to repel water, stay clean and sometimes even to breathe. The effect is also known as The Lotus Effect® and Ultrahydrophobicity. In this paper we introduce many of the theories used, some of the methods used to generate surfaces and then describe some of the implications of the effect

Immersed superhydrophobic surfaces: Gas exchange, slip and drag reduction properties

Superhydrophobic surfaces combine high aspect ratio micro- or nano-topography and hydrophobic surface chemistry to create super water-repellent surfaces. Most studies consider their effect on droplets, which ball-up and roll-off. However, their properties are not restricted to modification of the behaviour of droplets, but potentially influence any process occurring at the solid-liquid interface. Here, we highlight three recent developments focused on the theme of immersed superhydrophobic surfaces. The first illustrates the ability of a superhydrophobic surface to act as a gas exchange membrane, the second demonstrates a reduction in drag during flow through small tubes and the third considers a macroscopic experiment demonstrating an increase in the terminal velocity of settling spheres

Effect of Particle Size on Droplet Infiltration into Hydrophobic Porous Media As a Model of Water Repellent Soil

The wettability of soil is of great importance for plants and soil biota, and in determining the risk for preferential flow, surface runoff, flooding,and soil erosion. The molarity of ethanol droplet (MED) test is widely used for quantifying the severity of water repellency in soils that show reduced wettability and is assumed to be independent of soil particle size. The minimum ethanol concentration at which droplet penetration occurs within a short time (≤10 s) provides an estimate of the initial advancing contact angle at which spontaneous wetting is expected. In this study, we test the assumption of particle size independence using a simple model of soil, represented by layers of small (0.2–2 mm) diameter beads that predict the effect of changing bead radius in the top layer on capillary driven imbibition. Experimental results using a three-layer bead system show broad agreement with the model and demonstrate a dependence of the MED test on particle size. The results show that the critical initial advancing contact angle for penetration can be considerably less than 90° and varies with particle size, demonstrating that a key assumption currently used in the MED testing of soil is not necessarily valid

Drop impact behaviour on alternately hydrophobic and hydrophilic layered bead packs

A high level of water repellency in soils has an impact on soil hydrology, plant growth and soil erosion. Studies have been performed previously on model soils; consisting of close packed layers of glass spheres (140–400 μm in diameter), to mimic the behaviour of rain water on water repellent soils. In this study measurements were performed on multi-layered bead packs, to assess the interaction of water drops impacting layers consisting of different hydrophobic and hydrophilic layers. A high speed video camera was used to record the impact behaviour of water droplets on the bead packs focussing on the spreading of the droplet and the subsequent rebound behaviour of the droplet. Observations were made from the videos of the liquid marble effect on the droplet, whereby hydrophobic particles form a coating around the droplet, and how it differed depending on the arrangement of hydrophobic and hydrophilic layers within the bead pack. The droplet release height was varied in order to establish a relationship between impact velocity and the degree to which liquid marbling occurs, with higher impact speeds leading to a greater degree of liquid marbling. Measurements were also made to find the transition speeds between the three rebound conditions; rebound, pinning and fragmentation, showing an overall decrease in pinning velocity as the bead size increased