A thermodynamic theory of supercooled liquids
The salient feature defining a supercooled liquid is the extraordinary increase o f its characteristic structural (a) relaxation times as the liquid is cooled below its melting point T m. For instance, the coefficient of shear viscosity (η) increases sm oothly by as much as 15 orders of m agnitude over a temperature ( T ) range, often less than 100 K . As a consequence, the range of temperatures over which locally equilibrated supercooled liquids can be studied is bounded below by the glass transition, Tg, which is that temperature below which the local equilibration time becom es longer than the experim entally observable time. (If the liquid is successfully supercooled, the more stable crystalline phase is dynam ically inaccessible.) Despite the dramatic change in the dynamics, therm odynam ic evidence of a growing length scale or a diverging susceptibility has been weak .