Sensitive force measurements in a sheared granular flow with simultaneous imaging

Friction measurements have been made on a layer of glass particles (70-110 μm in diameter) with simultaneous microscopic imaging. The shear force is applied to a transparent cover plate via an elastic beam, which is connected to a stage translating at constant speed V. Fast force measurements are made by means of an eddy current displacement sensor that detects the deformation of the beam. The character of the resulting stick-slip motion is described as a function of V and other variables. The amplitude of the stick-slip motion is nearly constant at low V, and then increases until a transition to continuous motion is reached at a critical speed Vc. Imaging measurements reveal precursors just before the slipping events occur.