Single-Particle Forcing in Colloidal Suspensions with Hard-Sphere Interparticle Potential

Using fluorescence microscopy, we study the dynamics of a Brownian magnetic probe driven by a constant external force through a colloidal suspension. The probe collides with colloidal particles disturbing the equilibrium microstructure of the suspension. The magnitude and shape of this distortion is determined by both the ratio of the imposed motion to thermal motion and the volume fraction of the colloidal suspension. We calculate viscoelastic properties such as the microscopic viscosity of the colloidal suspension near the probe from its average velocity and the effective diffusivity of the probe from the time rate of change of its mean-square displacement. We find that at relatively high volume fractions and external forces, the microstructure surrounding the probe "melts", decreasing the viscosity in this region. The force-induced diffusivity of the probe is found to depend on the spatial configuration of the colloidal particles and the strength of the probe forcing. We compare these results with computer simulations measuring viscoelastic properties of similar systems.