Forced-induced Diffusion in Dilute Colloidal Suspensions*

We use fluorescence microscopy to study the dynamics of a Brownian probe particle driven by a constant external force through dilute colloidal suspensions with hard-sphere interparticle potential. As the probe particle moves through the colloidal suspension it collides with colloidal particles deforming the equilibrium suspension microstructure. The shape and the range of this distortion is determined by the volume fraction of the colloidal suspension and by the relative strength of the external force with respect to the entropic restoring force of the colloidal particles. We find that the average velocity of the probe particle is linear with the external force and it decreases with the volume fraction. Standard deviation of the distribution of the position of the probe particle perpendicular to the direction of motion decreases with the forcing strength and saturates at strong forcing for all volume fractions studied. Colloidal particles in the vicinity of the probe particle exhibit non-Gaussian dynamics. We compare our results with force-induced diffusion measured by Brownian dynamics simulations.