When liquids are supercooled, viscosity increases dramatically. One possible explanation relies on a calculation by Einstein of the viscosity of a suspension of solid spheres. An extension of his calculation suggests that when the concentration of spheres reaches a critical value, the viscosity should become infinite. This cannot be what actually happens in a cold liquid. What is more likely is that regions of the liquid fluctuate in time between fluid- and solid-like behavior. The theory of this kind of dynamical disorder is under investigation. Molecular rearrangements sometimes look as if they are affected by internal friction. A model illustrating this phenomenon has been studied by computer simulation. This model is a molecule with two minimum energy states coupled to a number of internal normal modes.
