ABSTRACT
We consider the conditions under which a granular surface becomes unstable under vibration. A simple Coulomb friction model suggests three stability boundaries that depend upon the relative phases of the horizontal and vertical driving forces and the gradient of the surface. The experimental data from sand and ballotini do indeed exhibit three stability lines, but differ in detail from the Coulomb model, the “coefficient of friction” being a function of frequency and gradient. It is clear that surface instability depends upon the dynamical motion of individual grains in their local potential wells. This motion and thus the surface stability is dependent upon collisional and viscous dissipation. This is confirmed by comparing the stability lines for ballotini in vacuum, air and water.
