# Making a splash with water repellency

Original entry: Sujit S. Datta, APPHY 225, Fall 2009.

## Reference

C. Duez, C. Ybert, C. Clanet, and L. Bocquet, Nature Physics 3, 180 (2007).

## Keywords

wetting, contact angle, splashing, superhydrophobic

## Key Points

What determines how strongly a solid body rushing into a liquid will splash? (For example, consider a sphere being dropped into water). This splash occurs when a pocket of air gets trapped in the fluid after the sphere rushes in, and intuitively, one imagines (correctly) that inertia plays a key role. The purpose of this work was to demonstrate that the surface wettability of the 'intruding' sphere also plays a fundamental role.

This is illustrated by a simple experiment: Duez and co-workers took two identical glass beads, coated one with a very thin layer of silane chains (to make it hydrophobic), and oxidized the other one in a strong etchant (to make it hydrophilic). When both were dropped in the same manner, the hydrophobic one made a larger splash, both visually and in the amplitude of the resulting sound.

But why does surface wettability matter? After all, these experiments are done in a regime where inertia dominates over any capillary effects (that is, the Weber number $\rho U^2 a/\gamma$ is large, where $\rho$ is fluid density, $U$ and $a$ are the sphere velocity and size, and $\gamma$ is the fluid surface tension).