Using Magnetic Beads to Poke Dense Colloidal Samples

We are studying a dense suspension of colloids near the colloidal glass transition using magnetic beads as local probes. We experimentally study the displacement of magnetic beads under different magnetic force magnitudes such that the magnetic bead is displaced and returns to its equilibrium position. We vary both the duration and strength of the magnetic forces. We observe that for small forces and/or short durations, the magnetic bead returns to its equilibrium position. However, we find a threshold force at which the magnetic bead plastically deforms the sample and thus does not return to the equilibrium position. We examine how this threshold force increases at larger volume fractions of the colloidal sample.