Three-dimensional direct imaging of structural relaxation near the colloidal glass transition

Confocal microscopy was used to directly observe three-dimensional dynamics of particles in colloidal supercooled fluids and colloidal glasses. The fa stest particles moved cooperatively; connected clusters of these mobile par ticles could be identified: and the cluster size distribution, structure, a nd dynamics were investigated. The characteristic cluster size grew markedl y in the supercooled fluid as the glass transition was approached, in agree ment with computer simulations; at the glass transition, however, there was a sudden drop in their size. The clusters of fast-moving particles were la rgest near the alpha-relaxation time scale for supercooled colloidal fluids , but were also present, albeit with a markedly different nature, at shorte r beta-relaxation time scales, in both supercooled fluid and glass colloida l phases.