Development of core-shell colloids to study self-diffusion in highly concentrated dispersions.

To study single particle motion in highly concentrated colloidal dispersions, a host-tracer colloid system was developed, consisting of crosslinked polymer micronetwork spheres placed in a good solvent. The host colloid is made invisible to the experimental probe by matching its refractive index to that of the solvent. For the tracer particles a core-shell structure was chosen to ensure the interaction potential to be identical to that of the host particles. Therefore the shell was made of the same polymer as the host. The core differs in refractive index from the solvent and is therefore visible due to scattered light. The self-diffusive motion of a few strongly scattering tracer particles is then studied by dynamic light scattering (DLS) and optical dark field microscopy. DLS results give a proof for the "hard sphere" character of the particles studied and also show that true single particle motion is detected. Mean squared displacements, determined via optical dark field microscopy, are compared with results calculated from DLS measurements. In combination with a third method, forced Rayleigh scattering, where dye-labeled tracers are used, self-motion was studied for nine decades in time. The results obtained from the three methods are fully consistent. In concentrated dispersions (φ = 0.54) the transition from short-time to long-time diffusion is visible. This paper also presents first results obtained by optical microscopy for a sample in the glassy state (φ = 0.63). [ABSTRACT FROM AUTHOR]