Nature often seems to use more than one length-scale of roughness – the Lotus leaf has small bumps on large bumps. We can replicate this experimentally using photolithographic techniques and electrodeposition to overlay large scale textured surfaces with small-scale roughness.
In one experiment a pattern was created using copper in which “bumps” of the order of 40 microns had surfaces which were themselves rough. The bumps look like chocolate chip cookies, as seen in the images above.
When the multi-textured surfaces have a hydrophobic coating applied to them, they become very effective at shedding water. This is because the contact angle is high, as we can see in the image below.
Read more about the process in the publication below.
Dual-scale roughness produces unusually water repellent surfaces N.J. Shirtcliffe, G. McHale, M.I. Newton, G. Chabrol and C.C. Perry, Adv. Maters. 16 (2004) 1929-1932
Wetting and wetting transitions on copper-based super-hydrophobic surfaces N.J. Shirtcliffe, G. McHale, M.I. Newton, G. Chabrol and C.C. Perry, Langmuir 21 (2005) 937-943