Phys. Rev. Lett. 112, 016101 (2014)

The superhydrophobicity of lotus leaves has inspired scientists to recreate similar surfaces for self-cleaning and anti-fouling purposes, and to extend the super-repellent properties to nonpolar solvents. However, such superamphiphobic surfaces are flat or only slightly curved. Hans-Jürgen Butt and co-workers at the Max Planck Institute for Polymer Research in Mainz have now explored why this is the case and set physical limitations to the curvature of a surface for superamphiphobicity.

The researchers first coated a microsphere with soot-based nanoparticles to make it superamphiphobic. The coated microparticle was then attached to a cantilever and brought into contact with a liquid surface. With this set-up, the force to detach the microparticle from different solutions could be measured; this adhesion force contains information about the interaction between the two surfaces.

Under the experimental conditions, the coating fails to repel oil, even though the same nanostructuring would make a flat surface superoleophobic. Through modelling of the experimental results, the researchers are able to explain that the capillary pressure exerted by the liquid as the particle moves into it is greater than the pressure of the entrapped air at the liquid/air interface, causing the oil to wet the microparticle. Because the capillary pressure depends on the curvature of the microparticle, there is a critical radius below which superoleophobicity, and more generally superamphiphobicity, is lost.