By Sung Hoon Kang
Title: Pearl drops
Reference: J. Bico, C. Marzolin and D. Quere, Europhys. Lett. 47, 220-226 (1999).
Soft matter keywords
hydrophobic, surface roughness, contact angle, Young's equation, surface tension, Wenzel's equation
Abstract from the original paper
If deposited on a hydrophobic rough substrate, a small drop of water can look like a pearl, with a contact angle close to 180. We examine the conditions for observing such a phenomenon and show practical achievements where the contact angle can be predicted and thus quantitatively tuned by the design of the surface microstructure.
Soft matter example
For a flat uniform solid, the contact angle of a drop is given by Young's equation:
cosθ = (fsv-fsl)/flv
where fsv, fsl, flv are surface tensions for solid/vapor, solid/liquid and liquid/vapor, respectively.
The contact angle can be changed by introducing textured surface. In nature, the contact angles of water drops on some leaves such as lotus or lily-water are around 160° by having rough surfaces . Synthetically, it was found that a hydrophobic surface with surface texture had a contact angle around 170° for water. In this paper, the authors analyzed parameters affecting the contact angle and discussed data obtained from model surfaces.
1. C. Neinhuis and W. Barthlott, Planta 202, 1-8 (1997).