Factors Affecting Separations of Suspensions in Nonuniform Electric Fields

Abstract

A study of the more quantitative aspects of the behavior of suspended polymer particles in a real dielectric is presented. While it is recognized that nonuniform fields exert an attractive force of appreciable magnitude upon particles of high dielectric constant suspended in a liquid medium of lower dielectric constant, the limits of such behavior are not precisely known. This report presents some quantitative data on the subject. The results show that in real dielectrics there is an appreciable effect due to the very small currents, about 1013 amperes at applied voltages in the kilovolt range [R. Coelho, Progress Report No. 22, pp. 13–17, Laboratory for Insulation Research, Massachusetts Institute of Technology (December, 1957)] which even a good liquid insulator supports. It is shown that the initial attraction to the central electrode due to the nonuniform field and its polarizing induction is gradually overcome by the repulsive effects of charge accumulated on the particles resulting from ionic conduction and related events in the liquid. The experimental observations include the study of the effects of field strength, electrode size, cell size, and particle size. Field strength, electrode size, and particle size show criticality. At certain upper limiting voltages, smaller central electrode diameters and small particle sizes, all precipitation appears to be prohibited. Use of the newly discovered ``sluff-off'' voltage for making particle size separations is discussed.