The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Tangential Forces between Spheres of Colloidal Doublets

by Sebastian Vasile Catana



The nature of forces within colloidal doublets were studied using differential electrophoresis. Tangential forces have been found to exist between two non-touching colloidal spheres. Our study focused on the behavior of silica-polystyrene doublet particles under the effects of simultaneous electric and gravitational fields. Several doublets present in electrolyte solutions of different concentrations (3mM, 5mM and 7mM) were examined. The hydrodynamic state of a doublet has been used as a monitoring tool for identifying forces other than the forces already known to exist along the line of centers between the two colloid spheres. A rigid hydrodynamics state occurs as tangential forces between the spheres oppose the hydrodynamic forces which tend to rotate the spheres with respect to each other when an electric field is applied. We observed shifts between a rigid state and a freely-rotating state suggesting time varying tangential forces. "In-between" hydrodynamic states suggest the intermediate values of the tangential forces. The ability to break doublets that have been present in rigid states indicates that tangential forces are not likely to arise from the physical contact between the two spheres but are transmitted across the fluid gap.