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               (1)

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.

The effect of the surface roughness on contact angle was first formulated by Wenzel [2]. If we consider a small displacement dx of the contact line parallel to the surface as shown in Fig. 1, the new contact angle θ* can be calculated using the following approach. In this case, the change in the surface energy (per unit length of the contact line),

                 dF = r(fsl-fsv)dx+flvdx cosθ*     (2)

(r: the solid roughness, defined as the ratio between the real surface and the projected one)

Fig. 1 Infinitesimal spreading of a liquid wedge on a rough surface.

The equilibrium contact angle of the rough surface can be calculated by the minimum of F, which gives Wenzel's equation:

                cosθ* = r cos θ                   (3)

(θ: Young's angle given from equation (1))

This model shows that the surface roughness amplifies the wetting or dewetting behavior because r > 1.

When θ > 90°, the surface energy of a dry solid is lower than the surface energy of the wet solid (fsv<fsl). So, the contact line does not follow the solid surface and the drop styas on a composite surface with a patchwork of solid and air as shown by Barthlott and Neinhuis [1]. Then, the contact angle changes by the presence of the air pockets. We can consider this case by using the model surface shown in Fig. 2.

Fig. 2 Liquid deposited on a model surface with holes (a) crenellated surface; b) hemispherical bumps): for contact angles larger than 90°, air is trapped below the liquid, inducing a composite interface between the solid and the drop.

Reference

1. C. Neinhuis and W. Barthlott, Planta 202, 1-8 (1997).

2. R. N. Wenzel, Ind. Eng. Chem. 28, 988-994 (1936); J. Phys. Colloid Chem. 53, 1466-1467 (1949).