Slope Stability of Electropacked Beds

The mechanics of packed or fluidized beds of semi- insulating particles are substantially altered by strong applied electric fields. The angle of repose of a loosely packed bed of glass beads increases with the applied electric field; a sufficiently intense field will freeze the bed. An extensive series of angle of repose measurements are reported which suggest three distinct regimes of critical slope equilibria: infinite slope equilibrium for low electric field intensities; finite slope equilibrium for somewhat stronger fields, and the frozen bed limit for strong electric fields. The failure of an infinite slope equilibrium is characterized by individual particle motions confined to a thin layer near the surface. This regime is amenable to analysis, and the electrically induced cohesion can be inferred from the experimental data. The results are compared to Dietz's published expression for the cohesive electrical force acting between two contacting spherical particles. SEM photographs of the particles used experimentally provide evidence that surface asperities and very fine particles may be more important than previously suspected.