Surface instability driven by dipole-dipole interactions in a granular layer.

In a layer of grains that interact through a tunable dipolar force, we show that an instability occurs due to the competition between gravity and the interaction between grains. Above a critical intensity of the interaction, the flat surface of the layer is unstable and peaks are generated. Fluctuations are shown to have conflicting effects. On the one hand, they favor the instability process and for large enough fluctuations the peak amplitude behaves similarly to the order parameter of a supercritical instability. On the other hand, the spatial organization of the peaks is affected by fluctuations that postpone the appearance of spatial ordering.