Vertical and horizontal vibration of granular materials: Coulomb friction and a novel switching state

Experiments for simultaneous horizontal (h) and vertical (v) vibration of granular materials show novel flow dynamics. We focus on moderate dimensionless accelerations 0 < Gamma(h,v) < 1.6. Phenomena include the spontaneous formation of a static heap at Gamma(hs)(Gamma(v)) when Gamma(v) < 1, convective flow for Gamma(hc) > Gamma(hs), and a novel switching state for frequencies omega(v) not equal omega(h). A simple friction model provides an approximate, but not exact, description of the steady states and the transition to convection. [S0031-9007(98)06699-X].