The Colloidal Glass Transition: Rotational & Translational Decoupling and the Confinement Effect

We study the microscopic properties of two phenomena related to the glass transition: the decoupling of diffusion from a glass-forming material's viscosity as it is cooled and the effect of confinement on the volume fraction that the glass transition occurs. We use colloidal suspensions of microspheres to physically model the glass transition. Colloids are a good approximation of hard-sphere fluids, where particle concentration effectively models a fluid's temperature. Using a high-speed confocal microscope, we rapidly visualize microscopic structural and dynamical processes in three dimensions.;We probe the colloidal fluid's rotational and translational dynamics with ordered clusters of microspheres. Far from the fluid's glass transition, both rotational and translational motion of the clusters are purely Brownian. However, in the liquid's supercooled regime, we observe a decoupling between the two types of motion: as the glass transition is approached, rotational diffusion slows down even more than translational diffusion. Our observation supports the notion that supercooled fluids are not merely fluids with large viscosities but that diffusion takes place by fundamentally changed mechanisms.;The effect of confinement on a fluid's glass transition temperature is the focus of our other experimental investigation. Confining a fluid to a small volume can either increase or decrease the glass transition temperature; in some cases confinement has no effect at all. The effect is strongly dependent on the properties of the boundaries confining the material. We directly observe the three-dimensional dynamical processes of confined colloidal suspensions of microspheres, while systematically varying the confinement volume and the suspension's concentration. The experiments find that confinement induces glassy behavior in a sample that is a fluid when not confined. Like particles in an unconfined near-glassy system, groups of particles in our confined system move together cooperatively. Normally these groups would be spatially isotropic. However, confinement induces a layering of the particles, which modifies the shape of the mobile groups so that they are more planar. The planar restriction helps to explain the sample's glassiness.