Inspired by the surface features observed in nature – in both plants and animals – researchers have made a range of surfaces with manipulate solid/liquid interface properties. Here is a selection of such surfaces, along with the properties they manipulate.
Photolithography can be used to create pillars like a ‘bed of nails’ but on a much smaller length scale. These surfaces are useful for comparing superhydrophobic effects to theoretical predictions.
Surfaces can be grown in which deposition follows a process called diffusion limited aggregation. When treated with a hydrophobic coating, droplets of water completely ball up and roll off.
Common methods of producing man-made hydrophobic surfaces suffer from the fact that if the surface is abraded (rubbed), the effect is worn away. However, foams can be made that will renew their water repellency under abrasion.
‘Craters’ can be etched into a Copper surface until they join up to create a landscape of sharp pinnacles.
Just as the lotus leaf has small bumps on large bumps, photolithographic techniques can overlay large-scale textured surfaces with small-scale roughness. These surfaces can be used to investigate whether surfaces with multiple length scales shed water more easily.
A fire in a forest with sandy soil can volatise waxes that cause the soil to become extremely water-repellant. This can lead to erosion and stop new vegetation from becoming established.
Counter-intuitive example – Liquid Marbles
‘Hydrophobic’ means ‘water-fearing. We would expect a water droplet placed on a hydrophobic surface to be repelled from the surface. However, if the surface consists of hydrophobic grains that are loose rather than fixed. the grains can stick to the surface of the water droplet rather than being repelled. This creates a ‘liquid marble’ that can roll freely on solid (and water) surfaces.