Pearl drops

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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 [1]. 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).