Glass formation in
colloidal suspensions has many of the
hallmarks of
glass formation in molecular materials. For hard-sphere
colloids, which interact only as a result of
excluded volume,
phase behaviour is controlled by
volume fraction, phi; an increase in phi drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When phi increases above phi* approximately 0.53, the
viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the
glass transition, phi(g) approximately 0.58, where particle crowding greatly restricts structural relaxation. The large particle size makes it possible to study both structure and dynamics with
light scattering and imaging;
colloidal suspensions have therefore provided considerable insight into the
glass transition. However, hard-sphere
colloidal suspensions do not exhibit the same diversity of behaviour as molecular
glasses. This is highlighted by the wide variation in behaviour observed for the
viscosity or structural relaxation time, tau(alpha), when the glassy state is approached in
supercooled molecular
liquids. This variation is characterized by the unifying concept of fragility, which has spurred the search for a 'universal' description of
dynamic arrest in glass-forming
liquids. For 'fragile'
liquids, tau(alpha) is highly sensitive to changes in T, whereas non-fragile, or 'strong',
liquids show a much lower T sensitivity. In contrast, hard-sphere
colloidal suspensions are restricted to fragile behaviour, as determined by their phi dependence, ultimately limiting their
utility in the study of the
glass transition. Here we show that deformable
colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular
liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual
colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility.
Colloidal suspensions may thus provide new insight into
glass formation in molecular systems.
