Matthew Gratale

Tunable depletion potentials driven by shape variation of surfactant micelles.

Depletion interaction potentials between micron-sized colloidal particles are induced by nanometer-scale surfactant micelles composed of hexaethylene glycol monododecyl ether (C_{12}E_{6}), and they are measured by video microscopy. The strength and range of the depletion interaction is revealed to arise from variations in shape anisotropy of the surfactant micelles. This shape anisotropy increases with increasing sample temperature. By fitting the colloidal interaction potentials to theoretical models, we extract micelle length and shape anisotropy as a function of temperature. This work introduces shape anisotropy tuning as a means to control interparticle interactions in colloidal suspensions, and it shows how the interparticle depletion potentials of micron-scale objects can be employed to probe the shape and size of surrounding macromolecules at the nanoscale.

Correlated Rearrangements of Disordered Colloidal Suspensions in the Vicinity of the Reentrant Glass Transition

We experimentally investigate correlated rearrangements (dynamic heterogeneity) in disordered colloidal suspensions as a function of increasing inter-particle attraction strength across the reentrant glass transition. Attractive interactions between constituent spheres are induced by polymer depletion forces at fixed particle volume fraction. We vary sample inter-particle attraction strength and concurrently measure particle trajectories by confocal microscopy at each attraction strength. The reentrant transition from repulsive glass to ergodic fluid and from ergodic fluid to attractive glass is readily observed. As the inter-particle attraction increases, we find that inter-particle bonding causes an increasing number of particles to undergo cooperative or correlated displacements; the length scale associated with these correlated rearrangements exhibits reentrant behavior. Other dynamical quantities such as the mean square displacement, the long-time diffusion constant, and the non-Gaussian parameter also exhibit changes as a function of attraction strength. Notably, the arrested (glass) states show small particle displacements at long lag times, whereas the fluid states exhibit fast dynamics over large length scales.