  We present experimental observations of the onset of flow (liquefaction) for
horizontally vibrated granular materials. As the acceleration increases above
certain value, the top layer of granular material liquefies, while the
remainder of the layer moves with the shaker in solid body motion. With
increasing acceleration, more of the layer becomes fluidized. The initial
bifurcation is backward, and the amount of hysteresis depends mainly on
frictional properties of the granular media. A small amount of fluidization by
gas flow lifts the hysteresis. Modest differences in the frictional properties
of otherwise identical particles leads to rapid segregation.
