The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Determining the Forces between Colloidal Particles Using Differential Electrophoresis

by Darrell Velegol



Many industrial and natural processes involve colloidal suspensions, and a major challenge in the engineering of colloidal processes is understanding the stability and flow behavior of the suspension. Both properties depend ultimately on the forces between colloidal particles. These forces are seldom known, and the classical theory of colloidal forces, DLVO theory, has gone largely untested for colloidal particles at close separations. This thesis has sought to answer the following questions: 1) Does DLVO theory correctly predict the normal forces between two polystyrene latex particles or between a polystyrene latex particle and a silica particle? 2) Are the tangential forces and restraining torques between colloidal particles zero, as DLVO theory predicts, and if not, do they depend upon the normal forces?
In order to measure the forces between colloidal particles, we have developed the experimental technique of differential electrophoresis. The forces are measured by interpreting experimentally observed particle trajectories with solutions of the hydrodynamic and electrostatic equations for two particles. Both normal and tangential forces (and restraining torques) can be measured with sub-piconewton resolution.
The following are the primary findings of this research: 1) DLVO theory is inadequate to describe the normal forces between polystyrene latex particles or between silica and polystyrene latex particles in aqueous suspension. The data show that the normal forces holding these colloidal particles together were sometimes greater than 50 pN, which is at least an order of magnitude higher than what DLVO theory predicts. 2) Tangential forces play a significant role in the dynamics of colloidal doublets. Doublets can act as single rigid bodies, as loosely coupled spheres (freely rotating doublets), or something in between. Tangential forces from 0 to 4 pN were measured, and these were not always correlated with the normal forces between two particles. This rich behavior of tangential forces and restraining torques could impact the way colloid scientists view the flow behavior of dense colloidal suspensions. The presence of surface heterogeneity provides one hypothesis for why the large discrepancies exist between the force data and the DLVO predictions, and recent theoretical papers support this position.