Inspired by the surface features observed in nature – in both plants and animals – researchers have made a range of surfaces which manipulate solid/liquid interface properties. These have many potential uses, such as in the dirt repelling StoCoat™ Lotusan® exterior building paint, hydrophobic concrete designed to reduce rusting of the reinforcing bars, self-cleaning clothes that resist wine stains, water-repellant swimwear, mPhase Technologies long lasting reserve batteries, Gore-Tex® fabric in breathable clothing, solar panel and radar dome coatings to prevent build-up of scattering droplets, as lens coatings in high tech optics (e.g. Zeiss LotuTec®)
Here we show a selection of man-made surfaces that are either hydrophobic or highly water-shedding.
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 observed 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 these surfaces.
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.
Surfaces with multi-scale texturing
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.
Granular Materials: superhydrophobic sand
Grains of sand can be given a hydrophobic coating to create a superhydrophobic surface which water rolls off.
Slippery liquid-infused porous surfaces (SLIPS)
Nano or microstructured substrates can be infused with a lubricating liquid to create a surface that is very slippery to most things.