Multi-scale texturing

Surface with multiscale levels of roughness.

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.

3D view of copper electrodeposit
Confocal-image-of-30um-electrodeposited-Cu_1
Confocal microscope image of 30µm-electrodeposited Copper
Close-up of "chocolate chip cookies"
Close-up of “chocolate chip cookies”

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.

Water droplet on a surface with multi-scale texturing and with a hydrophobic coating

Read more about the process in the publication below.

Publication

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

Further Reading

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