Making a splash with water repellency
Original entry: Sujit S. Datta, APPHY 225, Fall 2009.
C. Duez, C. Ybert, C. Clanet, and L. Bocquet, Nature Physics 3, 180 (2007).
wetting, contact angle, splashing, superhydrophobic
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 <math>\rho U^2 a/\gamma</math> is large, where <math>\rho</math> is fluid density, <math>U</math> and <math>a</math> are the sphere velocity and size, and <math>\gamma</math> is the fluid surface tension). The answer, of course, lies in exactly how an air pocket is formed after the sphere falls into the water.