A heterogeneous picture of alpha relaxation for fragile supercooled liquids

We examine some of the consequences, and their connection to experiments on supercooled liquids, of a scaling model of heterogeneous relaxation that i s based on the theory of frustration-limited domains. In particular, we foc us on what appears to be the two slowest components of structural relaxatio n, the one usually described by a stretched exponential or a Cole-Davidson function and the somewhat faster, apparently power-law decay known as von-S chweidler relaxation. Based on our model we study the alpha-relaxation acti vation free energy, the imaginary part of the dielectric frequency-dependen t susceptibility, the susceptibility-mastercurve of Dixon [Phys. Rev. Lett. 65, 1108 (1990)], and the breakdown of the Stokes-Einstein relation for tr anslational diffusion at low temperatures. We also obtain estimates for the characteristic domain sizes as a function of temperature. As with all meso scopic approaches, a number of assumptions must be introduced, but they all fit the overall scaling picture that motivates this approach. The good agr eement with experimental dielectric relaxation data on two representative s upercooled liquids, salol and glycerol, though necessarily dependent upon a djustable parameters, gives support to the theory. (C) 2000 American Instit ute of Physics. [S0021-9606(00)50823-X].