CAREER: Measuring the Effect of Nano-Scale Charge Nonuniformity on Colloidal Forces

The current understanding of colloidal forces generally assumes that the charges on particles are uniformly distributed. A nonuniform charge distribution will significantly alter interparticle forces. The research goals are: the nanoscale charge nonuniformity will be measured on individual colloidal particles of micron size; and the charge nonuniformity on colloidal particles will be related to the measured normal and tangential colloidal forces between the particles. This research was prompted in part by recent data from the PI that shows time-dependent tangential colloidal forces acting between two non-touching particles that could result from a nonuniform charge distribution. Charge nonuniformity could have a major impact on the modeling of bulk suspension stability and rheology.

The charge nonuniformity on individual particles will be measured by combining microelectrophoresis experiments, electrokinetic theory, and Monte Carlo simulations. The charge nonuniformity on particles will be related to the forces between particles. Using the recently-developed technique of 'differential electrophoresis', the normal forces can be measured between two particles (those forces acting along the line of centers) and the tangential forces between them (those forces acting laterally along adjacent surfaces of the particles) with sub-piconewton resolution. Differential electrophoresis can measure interparticle forces between two Brownian particles at gaps less than 100 nanometers, where van der Waals forces are important.