Dynamics of highly supercooled liquids: heterogeneity, rheology and diffusion
Particle motions in supercooled liquids are severely re stricted or jammed, thus giving rise to slow structural relax ations and highly viscoelastic behavior [1,2]. Recently much attention has been paid to the mode-coupling theory [3,4], which is an analytic scheme describing the onset of slow structural relaxations considerably above Tg. There, the den sity fluctuations with wave numbers around the first peak position of the structure factor are of the most importance and no long-range correlations are predicted. For a long time, however, it has been expected [5-8] that rearrangements of particle configurations in glassy materials should be coopera tive, involving many molecules, owing to configuration re strictions. In other words, such events occur only in the form of clusters whose sizes increase at low temperatures. In nor mal liquid states, on the contrary, they are frequent and un correlated among one another in space and time. Such an idea was first put forth by Adam and Gibbs , who in vented a frequently used jargon, cooperatively rearranging regions (CRR). However, it is difficult to judge whether or not such phenomenological models are successful in describ ing real physics and in making quantitative predictions.