Self-diffusion in concentrated colloid suspensions studied by digital video microscopy of core-shell tracer particles

Optical video microscopy and digital image processing have been used to study the self-diffusion of colloidal particles with a hard-sphere potential. The colloid particles consist of cross-linked polymers and are dispersed in a good solvent to avoid aggregation. To investigate single particle motion in highly concentrated dispersions, a host-tracer system, consisting of two different kinds of polymer particles, has been designed: the host particles are made of poly-t-butylacrylate (with ethanedioldiacrylate as cross-linker) and have the same refractive index as the employed solvent, 4-fluorotoluene. The tracer particles have a core-shell structure with a polystyrene core (cross-linked with m-diisopropenylbenzene) and a shell consisting of cross-linked poly-t-butylacrylate to match surface properties and interaction potential to those of the "invisible" particles. The motion of the strongly scattering core-shell particles ("tracer" particles) was observed by dark-field light microscopy. From the obtained particle trajectories, mean squared displacements, van Hove autocorrelation functions, and vector-vector correlation functions were calculated, yielding a direct real-space image of the "cage effect" at phi = 0.52 and of the transition to a glassy state between phi = 0.56 and phi = 0.60, as expected for a hard sphere system. The extracted long-time self-diffusion coefficients D-self,D-long are fully consistent with a recent theoretical prediction using full many-body hydrodynamics at phi less than or equal to 0.56 and a colloid glass transition at phi(g) = 0.583. However, even at phi = 0.60, D-self,D-long seems to be still finite, possibly indicating the existence of long-time motion of colloidal particles even in the glassy state.