Electrophoresis of randomly charged particles

The technique of "rotational electrophoresis" has been developed recently to measure the charge nonuniformity on Brownian particles in a suspension. Measurements of the angular velocity in electrophoresis are interpreted with electrokinetic theory to give a standard deviation of zeta potential among N regions on a particle. A key question is how various molecular mechanisms of charging could lead to charge nonuniformity on a length scale of O (100 nm). Simple statistical physics is used to show that although Poisson charging with univalent charges cannot explain recent measurements showing significant charge nonuniformity, random charging with multivalent charges can better explain the data. In addition, the result shows that a small number of charged "visualization" molecules can be adsorbed to a particle without impacting the experimental technique of rotational electrophoresis.