Climbing sand droplets

L. caballero, J. Contreras, f. melo

In a recent experiment, we observed "droplets" forming when an inclined surface initially covered by fine powder is vibrated vertically. Contrary to common sense, droplets move uphill in the direction of maximum slope. We find that the speed of droplets is nearly independent on their size, being an increasing function of both the frequency and the amplitude of vibration. By performing experiments in different surface geometries we show that droplets of size smaller than a critical value reach their stable position at the top of a convex surface. In contrast, droplets do not remain at the minimum of the potential well defined by a concave surface, drifting in the direction of increasing slope. The effect of the friction and adhesion of grains at the vibrating surface is investigated. By evacuating the container we show that convection produced by the air flow in interaction with the fine powder is responsible of droplets forming.

Top view of a convex surface covered with fine silica powder when vibrated vertically. Droplets climb the slope reaching a stable equilibrium position at the surface maximum.