Conducting particles can display electrophoretic velocities hundreds of times larger than those expected for nonconducting particles. For ion-exchange particles in which colons are excluded from the interior, superfast electrophoresis occurs when the externally applied electric field exceeds that required for producing the overlimit current through the particle. Then a secondary diffuse cloud of counterions is induced outside the primary diffuse cloud (the latter is associated with the electric double layer). This extra induced charge, which increases with the electric field strength, causes the much larger electrophoretic velocities observed. Using multiple-exposed videoimaging and a new inclined flowcell to separate the effects of sedimentation and electrophoresis, we measure the electrophoretic velocity of electronically conducting particles (Al/Mg alloy, graphite, or activated carbon; 250-500 mu m diameter) which are then compared to earlier measurements with ionically conducting particles. For ionic strengths less than 1 mM, the electrophoretic mobility (velocity/electric field) of electronically conducting particles increases significantly with the electric field and the particle size, but is almost independent of the ionic strength. These trends are inconsistent with Smoluchowski's equation for the mobility of a dielectric particle, but instead are consistent with the theory (and earlier measurements on ion-exchange particles) for superfast electrophoresis. Although the electronically conducting particles move much faster than expected for dielectric particles, the velocity is not quite as high as that for ionically conducting particles. Smaller superfast electrophoresis for electronic conductors could be caused by the overpotentials which drive the redox reactions necessary to exchange electrons for ions at the particle surfaces; also both positive and negative secondary charge clouds are induced on opposite sides of an electronic conductor particle, which partially neutralizes the "superfast" effect. (C) 1998 Academic Press.
