PARTICLE AGGREGATION DUE TO COMBINED GRAVITATIONAL AND ELECTROPHORETIC MOTION

The stability and pairwise aggregation rates of small spherical partic les in a heterogeneous suspension under the collective effects of grav itational motion and electrophoretic migration are analyzed. The parti cles are assumed to be non-Brownian, with thin, unpolarized double lay ers and different zeta potentials. The gravity vector and the electric field are assumed to be oriented in either the same direction or oppo site directions. The particle aggregation rates are always enhanced by the presence of an electric field for parallel alignment of the gravi tational and electrophoretic velocities. For antiparallel alignment wi th the magnitude of the gravitational relative velocity exceeding the magnitude of the electrophoretic relative velocity between two widely separated particles, the particle aggregation rates are reduced by the presence of the electric field, and there is a ''collision-forbidden' ' region in parameter space due to stronger hydrodynamic interactions of the particles for gravitational motion than for electrophoretic mot ion. For antiparallel alignment with the magnitude of the electrophore tic relative velocity exceeding the magnitude of the gravitational rel ative velocity between two widely separated particles, the particle ag gregation rates are enhanced by the presence of the electric field. (C ) 1997 Academic Press.