Making a splash with water repellency

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Revision as of 05:02, 28 September 2009 by Datta (Talk | contribs) (Key Points)

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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

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 <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).