Electric-field induced retardation of granular flows in a pipe

A local horizontal electric field is able to retard the flow of nickel granule down a vertical pipe. In the first sequence of our experiments, the granule is released from a hopper after a voltage difference V is applied across two short, vertical copper electrodes. As V is increased from zero to 4.8 kV, Q remains practically constant for V < V₁, At V = V₁, Q drops abruptly; the drop depends on the location of the electrodes. Q then decreases gradually for V > V₁. In the second sequence of our experiments, V is first set at 4.8 kV; the flow is allowed to start; then V is reduced to the desired voltage. The new Q vs. V curve coincides with the previous Q(V) curve of first sequence, except for V₂ < V < V₁. A simplified one-dimension model is able to explain all the observed features.