Measuring colloidal forces using differential electrophoresis

Differential electrophoresis is a technique for studying the forces holding two Brownian particles in a doublet configuration. The technique is based on the concept that particles having different zeta potentials (xi) try to move at different velocities in an electric field but the colloidal forces holding the particles together frustrate the electrophoretic motion. A doublet with a dipole moment of xi will rotate in an electric field; by measuring the angular velocity or stationary-state angular distribution, one is able to make conclusions about the rigidity of the doublet (i.e., whether or not there are forces acting tangentially to the surfaces of the two particles). At a sufficiently high electric field, the doublet aligns with the field and an electrophoretic displacement force tries to pull apart the doublet; this force, which is typically in the range 0.1-50 pN for micron size particles, can be calculated precisely from the electric field and known properties of the system. The doublet breaks if the electrophoretic displacement force exceeds the maximum attractive colloidal force. The electrophoretic rotation velocity and the displacement force are both proportional to the applied electric field and the difference in the xi potentials between the two particles. Experimental results are reported for mixed doublets of a silica/polystyrene latex, which demonstrate the existence of tangential forces between the particles' surfaces and a time-dependent nature of the colloidal interactions.