Onset of flow in a horizontally vibrated granular bed: Convection by horizontal shearing

– We present experimental observations of the onset of flow for horizontally vibrated 3D granular systems. For accelerations Γ above Γ, the top layers of granular material flow, leading to convection with motion both parallel and transverse to the shaking; the lower part of the layer moves with the shaker in solid body motion. With increasing Γ, more of the layer becomes liquefied. The initial bifurcation is backward, but a small amount of fluidization by gas flow lifts the hysteresis. A new convective mechanism, which we explore both experimentally and computationally, associated with horizontal shearing at the walls, is identified as the mechanism driving the transverse convective flow. Although granular materials are common in nature and industrial applications, the complete understanding of their dynamical behavior is still an open problem. Consequently, the dynamics of granular materials have attracted considerable interest in recent years (for comprehensive reviews see [1] and citations therein). Granular materials can exhibit both fluid-like and solid-like properties depending on the circumstances: they resist shearing up to a point, but flow freely under strong enough shear or at low enough density. These materials also display a number of different dynamical states including liquefaction, heap formation and convection under vibration, the spontaneous formation of stable arches, segregation, a variety of density waves, stick-slip motion during avalanches, etc. Much recent attention has been focused on the dynamics of vertically vibrated granular materials. Although there have been some studies [2-7], much less is known about the corresponding dynamics of granular materials subject to horizontal vibration, and of the existing work much is very recent. Very recently, several authors [5, 6] have investigated 2D systems; the present study focuses on the dynamics of horizontally vibrated 3D systems, particularly the transition to flow —sometimes referred to as liquefaction. A better understanding of this second case is of interest scientifically because it gives insight into phenomena associated with shearing and into the competition between dilation and friction —i.e. a dense granular layer resists shearing both because of friction, and because it must dilate to deform. Horizontally driven flow is also of interest because both horizontal and vertical vibration are commonly used in industries as an aid to mixing, segregating and transporting granular materials. Soil liquefaction during earthquakes is a common and destructive phenomenon associated with horizontal shaking. Finally, the present experiments show a novel shear-induced flow which is the motor for convection transverse to the shaking direction; this mechanism is likely to be present in other shear flows as well. This transverse flow is explored carefully, both experimentally and by molecular dynamics (MD) simulations.