Intermittent granular flow in the presence of an electric field

In this letter experimental observations of vertical-pipe granular flow in the presence of an electric field are reported. Granular nickel particles flowing down a vertical pipe are found to be retarded by a local horizontal ac electric field when the field voltage V is greater than a critical voltage V_c. For V < V_c, the steady-state flow rate is practically unaffected by V. The flow rate decreases discontinuously at V_c, and then decreases gradually for V > V_c. Physically, particle clusters are formed above an arch interface, which exists in the electric-field region when V > V_c. The vertical flow consists of clusters intermittently dropping off from the interface. A transient density wave with wavelength lengthened by electric field is also observed.