Study of nonlinear electrophoresis

We distinguish two kinds of nonlinear electrophoresis that can be observed in strong electric fields. The first is due to the interaction of the applied field E with the field-induced ionic charges, under conditions when the field-induced variation of the electrolyte concentration remains small compared to its equilibrium value. The second one ("superfast electrophoresis") is related to the interaction of a strong applied field with a secondary diffuse layer of counterions ("space charge") which is induced by the external field outside the primary diffuse electrical double layer because of the concentration polarization. In the first case, with increasing field strength, the processes that are nonlinear with respect to E start playing a more important role that can be characterized by the third power term in the expansion of the electrophoretic velocity in powers of E. An expression is derived for "cubic electrophoresis" of spherical particles with a thin double layer, using the procedure of successive approximation in powers of small-parameter E. In line with the theory at E = 100 V/cm and higher substantial nonlinear effects in the electrophoretic velocity of polystyrene latex and aluminum oxide particles as well as yeast cells were observed. In the second case, the interaction of the "space charge" that is formed near the surface of electron-type conducting particles like magnesium or semiconductors like pyrite and molybdenite with a strong electric field (100-500 V/cm) produces electrophoretic velocities that are 1-2 orders of magnitude higher compared with those typical for nonconducting particles. It has been shown that the mobility of particles increases linearly with E and the particle size, in contrast to classical electrophoresis. This can be explained by the Dukhin-Mishchuk theory of superfast electrophoresis. A new method for measuring electrophoresis in strong electric fields has been developed.