Electrophoresis is the motion of a charged body proportional to an electric field. In contrast to Chapter 11, in which electrophoresis of molecules is described; this chapter discusses the motion of charged particles. Particle electrophoresis is a straightforward way to manipulate particles in microfluidic devices, both for positioning and for separation; it is ubiquitous if electric fields are used for any purpose.

Particles and molecules differ in that small molecules can be treated as point charges from the standpoint of how they perturb the surrounding electric field, and small molecules cannot support enough counterions to create a continuum electrical double layer. Particles, in contrast, have a large-enough charge that they are surrounded by a continuum EDL. For large particles, electrophoresis is described by use of analyses similar to electroosmosis, albeit with different boundary conditions. For smaller particles, we account for the breakdown of the thin-EDL assumption, leading to size-dependent electrophoretic velocity. Throughout, we discuss both the velocity distribution of the fluid and the velocity of the particle with respect to the bulk fluid.

INTRODUCTION TO ELECTROPHORESIS: ELECTROOSMOSIS WITH A MOVING BOUNDARY AND QUIESCENT BULK FLUID

Electrophoresis of particles and electroosmosis of fluid are both caused by the same physical phenomena – electrostatic forces on the wall and double layer counteracted at steady state by viscous forces in the fluid or elastic forces in the solid.