(162g) Probing Cooperative Motion And Activated Hops In Super-Cooled Colloidal Suspensions

The physics of the glass transition remains inadequately understood despite its broad technological relevance. The anomalous divergence of viscosity without apparent structural change as a liquid is cooled has been attributed to the existence of growing dynamic length scales of ?cooperatively rearranging regions? (CRR). However, experiments to determine the length scales of CRR have been scarce. In this work, we use ultra-fast fluorescence correlation spectroscopy (FCS) combined with high-speed imaging to determine the CRR sizes by measuring single-particle dynamics of fluorescent tracer particle suspended in super-cooled ?hard-sphere? colloidal suspensions. Fluorescently labeled poly-(methyl methacrylate) (PMMA) tracer particles of radii ranging from r=0.1-0.5 μm are mixed with un-labeled PMMA particles of radii r=0.6 μm. The bulk PMMA volume fraction,φ, varies from 0.52-0.58, which ranges from the super-cooled regime up to the glassy regime, according to the hard-sphere phase diagram. We find that the correlation length, measured by the mean free paths of fluorescent probes, increases as the colloidal glass transition of φ=0.58 is approached, in agreement with Adam-Gibbs theory.

A unique platform to probe how the sizes of CRR change dynamically is through confinement, where a glass transition can be observed ?sooner? as film thickness approaches a critical value while temperature and volume fraction are kept constant. By using a custom-built compression apparatus combined with FCS, we examine the effect of film thickness on the sizes and shapes of CRR of confined PMMA suspensions. Recent experimental evidence has confirmed that the sizes of CRR grow dramatically as the film thickness of colloidal PMMA thin films approaches approximately 17 particle layers. Multiple relaxation processes of confined suspensions have been observed and analyzed from the measured correlation functions.