# Pearl drops

Original entry: Sung Hoon Kang, APPHY 226, Spring 2009

Title: Pearl drops

Reference: J. Bico, C. Marzolin and D. Quere, Europhys. Lett. 47, 220-226 (1999).

## Contents

## 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 = (f_{sv}-f_{sl})/f_{lv}(1)

where f_{sv}, f_{sl}, f_{lv} 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(f_{sl}-f_{sv})dx+f_{lv}dx cosθ^{*}(2)

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

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 (f_{sv}<f_{sl}). 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. For a crenellated surface as Fig. 2a, the change in surface energy by displacing the contact line dx parallel to the surface is equal to:

dF = φ_{s}(f_{sl}-f_{sv})dx+(1-φ_{s})f_{lv}dx + f_{lv}dx cosθ^{*}(4)

where φ_{s} is the solid fraction of the surface. The thin lines in Fig. 2 a) indicates liquid/vapor interfaces, which occupies a surface fraction of 1-φ_{s} below the liquid drop. All these interfaces are pinned at the corners of the crenellations. From the equation (4), the equilibrium contact angle can be calculated by having minimum F:

cosθ^{*}= -1 + φ_{s}(cos θ + 1) (5)

The previous derivation assumed crenellation for modeling, but in general the fraction φ_{s} is a function of the contact angle θ. If we assume that the solid surface has a hemispherical bump as shown in Fig. 2 b), the liquid/vapor interafaces are positioned where Young's condition is satisfied. Then, similar calculation gives the following relation:

cosθ^{*}= -1 + φ_{B}(cos θ + 1)^{2}(6)

where φ_{B} is the ratio of the surfaces of the spike bases over the total solid surface.

Fig. 3 shows the plot of the equations (5) and (6) (dotted line). In both cases, the contact angle has a discontinuity, which is quite different from Wenzel equation. As sson as the contact angle is above 90°, air trapping occurs below the drop. These results showed good agreement with experimental data of the Kao group [3], which are shown in Fig. 4 for samples with different solid roughness.

Then, the authors made surfaces with a controlled design to verify their model by experiments. The details of the fabrication procedure is described in the paper. Fig. 5 shows the SEM images of the microstructured hydrophobic surfaces to quantify the effect of the surface pattern on the contact angle. For isotropic surfaces (holes and spikes), they observed that the advancing contact angle had very close agreement with equation (5). For the case of the stripes, it showed slightly different results because of anisotropy in wetting. The contact angle was very large for the spike structure and the water drop was nearly spherical as shown in Fig. 6.

The modeling work and the fabricated surface were quite interesting for both scientific and practical aspects. For these reasons, I guess, this paper has been cited 373 times since 1999.

## References

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).

3. S. Shibuichi, T. Onda, N. Satoh and K. J. Tsujii, *Phys. Chem*. 100, 19512-19517 (1996).