16 Pages

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 [5], 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.