Droplets Scoot Like Caterpillars

F rom swells in an ocean to ripples in a puddle, the shearing effect of wind blowing over a liquid is visible at all scales. This shear determines the interactions between Earth’s atmosphere and its surface water and, researchers now explain, the movement of liquid droplets that crawl up and down the window of a moving car in the rain. In a series of experiments, Antonio Chahine of the University of Toulouse, France, and his colleagues show that airflow triggers surface waves that cause such droplets to crawl like caterpillars before they break apart [1].