Nonlinear behavior of a driven probe in a colloidal suspension near the Glass Transition.

By applying a force to isolated microscopic magnetic particles embedded in a PMMA colloidal suspension a nonlinear relationship between the applied force and the measured velocity was measured with the assistance of a confocal microscope.~ Over short time scales it is possible to observe a threshold force, below which no apparent motion is detected.~ The value of this threshold force is highly dependent upon the concentration of the sample.~ At stronger forces, the velocity of a driven magnetic particle displays a power law dependence upon the applied force.~ In particular, the behavior becomes increasingly nonlinear as the concentration is raised toward the colloidal glass transition point. This behavior was consistent across numerous samples with different concentrations and a varying size ratio of PMMA to magnetic particle.