Retardation and transitions of dilute and dense granular flows in a vertical pipe induced by electric fields

Granular flow of nickel particles down a vertical pipe from a hopper is shown to be retarded by a horizontal ac electric field applied to a local region along the pipe. The particles are released from the hopper by pulling out a stopper in the hopper. Two sequences of experiments with different initial flow conditions are performed. In the first sequence, a dilute flow in the pipe is created after a fixed voltage V (≤4.8kV) is applied across two short, vertical copper electrodes. The steady-state flow rate Q remains practically constant for V<V₁ (=2.6kV). At V=V₁, Q drops abruptly; the drop depends on the location of the electrodes. For V>V₁, the flow becomes dense; Q decreases with a power law, Q∼V⁻¹. In the second sequence of experiments, V is first set at 4.8 kV; the flow is allowed to start, and soon becomes a dense flow; then, V is reduced to the desired voltage. The new, steady-state Q vs V curve coincides with the previous Q(V) curve of the first sequence, except for V₂<V<V₁, where V₂=1.0kV. The voltage V₂ is a continuous transition point at which Q changes from a dense flow (V>V₂) to a dilute flow (V<V₂). Our results show that a large enough ac electric field can decrease the flow rate of a dilute or dense flow; the critical voltage that can reduce a dense flow, V₂, is less than that for the dilute flow, V₁.