Granular flows through vertical pipes controlled by an electric field

The flow of granular nickel particles moving down vertical pipes from a hopper in the presence of a local, horizontal ac electric field is studied experimentally. The flow is initiated by opening the bottom outlet of the pipe after the pipe is fully filled with particles from the hopper. The mass of particles flowing out of the pipe is measured as a function of time by an electronic balance. The time dependence of the steady-state flow rate Q, under each fixed voltage V, is obtained. Depending on the magnitude of V, two types of flow behaviors are observed. For low V (< V-c = 2.0 kV), a downward-moving interface-separating a dense particle region below it from a low-density region above-exists between the hopper and the electrodes. Two prominent peaks exist in the Q(t) curve for V in the range of 1.4 kV <less than or equal to> V < V-c, resulting in two clearly defined flow rates Q(A2) and, later in time, Q(B). The particles measured by Q(A2) originate from the pipe above the electrodes, and those by Q(B) coming initially from the hopper. For high V (<greater than or equal to>V-c), no interface exists and the whole region between the hopper and the electrodes are densely filled; only one constant flow rate Q(A2) is observed. (The precise meaning of Q(A2) and Q(B) are defined in the text.) The steady-state flow rates Q(A2) and Q(B) measured for each V, are plotted as a function of V. The flow rate Q(A2) is a monotonically decreasing function of V, which can be approximately fitted by a power law, with an exponent of -0.8. while Q(B) is found to be voltage independent. These features result from a competition between the blocking effect of the electric-field region and the gravity-driven pushing effect from the hopper outlet. The local electric field is able to retard the downward movement of a dense column existing above it, but is ineffective in doing so when the column above is dilute in density.