Foam Structures

A common approach to creating super water-repellent surfaces is to make a rough structure and then coat it with a hydrophobic surface chemistry. If the surface is rubbed or abraded, the effect is worn away as the coating with the surface chemistry is removed. In the sol-gel technique, a foam is produced by a phase separation method and by varying the solution properties, the pore sizes can be controlled. If the foam is produced using MTEOS (methyltriethoxysilane) then all the pores have hydrophobic surfaces.

A droplet on top of a foam block
A droplet on top of a foam block

When this foam is abraded the surface is naturally superhydrophobic. If it becomes dirty, and loses its super water-repellency, abrading it renews the surface and recreates the superhydrophobicity. Read more about the process in the publication below.

Publication

Intrinsically super hydrophobic organo-silica sol-gel foams
N.J. Shirtcliffe, G. McHale, C.C. Perry and M.I. Newton, Langmuir 19 (14) (2003) 5626-5631

Further Reading

Porous materials show superhydrophobic to superhydrophilic switching
N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry and P/ Roach, Chem. Comm. 25 (2005) 3135-3137

Superhydrophobic to superhydrophilic transitions of sol-gel films for temperature, alcohol or surfactant measurement
N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry and P. Roach, Maters. Chem. & Phys. 103 (1) (2007) 112-117