Pre-suffused surfaces make them slippery (Vol. 43 No. 2)

Liquid drops most often stick to solids, which contributes to degrade these solids and affects their transparency. Slip can be induced by coating solids with hydrophobic textures: then, liquids only contact the texture tips, which dramatically decrease adhesion. On these super-hydrophobic materials, water nicely recovers the mobility expected from its low viscosity.

Another way to make liquids mobile was proposed in this letter and by Wang et al. (Nature 2011). It uses textures in a oleophilic situation: a solid coated with posts contacting oil can be spontaneously invaded by a film of this oil, the network of pillars acting as a kind of porous medium. At the texture scale (10 µm, typically), gravity is negligible compared to surface forces, so that the film gets trapped by the pillars, even when tilted. If now a drop contacts this substrate, it lands on a substrate mostly wet, and pinning can be strongly reduced. As an example, a coffee drop evaporating on a standard substrate leaves behind a coffee stain, primarily arising from the ability of the liquid to stick, while the coffee powder gets localized on these new slippery materials - making it easy to remove afterwards.

The condition for achieving these "floating" states was explored: the pre-suffused oil must wet the substrate with air above, but also with water (or another oil) above. Apart from its potential applications, this system is one of the very first explored where four phases (instead of three, in classical wetting) meet. It also has the interesting capacity to dissolve incoming liquid contaminants, again taking advantage of the mostly-liquid nature of the substrate.