Dynamical correlation length scales and dynamical heterogeneity in colloidal hard-sphere systems approaching the glass transition.

Abstract

Understanding the transition from a liquid to a disordered solid, the so-called glass transition, is a long-standing un-solved problem in the field of condensed matter physics. Recently it has become clear that solving this puzzle requires understanding the cooperative dynamics of clusters of about 100 molecules. æDue to the small, nanometer, size of these clusters in molecular systems, colloidal systems can instead be used to model the molecular behavior in glasses. A colloidal system is a dispersion of particles in a fluid medium. In the case of this experiment, micron-sized polystyrene spheres are dispersed in an aqueous KCl solution. These systems are of interest by many researchers in a field called soft-matter physics, but are also ideal for studying glassy behavior because they undergo a glass transition at a certain volume fraction, ?, of particle volume to water volume„analogous to the glass transition temperature in molecular glasses. This experiment relies on our ability to measure the movements of the particles in the colloidal system using an electrical current passing through a micropore separating two reservoirs of an acrylic cell containing the colloidal suspension. The motion of the particles produces a noisy current. These current fluctuations are then analyzed using sophisticated high-order statistical methods in order to determine the size of cooperative clusters. Of particular interest is how the cluster size changes as the glass transition is approached, a behavior that can be used to test competing theoretical models.