SELF-DIFFUSION IN CONCENTRATED COLLOID SUSPENSIONS STUDIED BY DIGITALVIDEO MICROSCOPY OF CORE-SHELL TRACER PARTICLES

Optical video microscopy and digital image processing have been used t o study the self-diffusion of colloidal particles with a hard-sphere p otential. The colloid particles consist of cross-linked polymers and a re dispersed in a good solvent to avoid aggregation. To investigate si ngle particle motion in highly concentrated dispersions, a host-tracer system, consisting of two different kinds of polymer particles, has b een designed: the host particles are made of poly-t-butylacrylate (wit h ethanedioldiacrylate as cross-linker) and have the same refractive i ndex as the employed solvent, 4-fluorotoluene. The tracer particles ha ve a core-shell structure with a polystyrene core (cross-linked with m -diisopropenylbenzene) and a shell consisting of cross-linked poly-t-b utylacrylate to match surface properties and interaction potential to those of the ''invisible'' particles. The motion of the strongly scatt ering 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 coefficie nts D-self,D-long are fully consistent with a recent theoretical predi ction 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 a t phi = 0.60, D-self,D-long seems to be still finite, possibly indicat ing the existence of long-time motion of colloidal particles even in t he glassy state.