OBSTRUCTION MODEL OF THE FRACTIONAL STOKES-EINSTEIN RELATION IN GLASS-FORMING LIQUIDS

Diffusion of tracer molecules in glass-forming liquids is modeled in t erms of diffusion in an obstructed space. The obstructions, which mode l heterogeneities in supercooled liquids, are taken to be spherical an d frozen in position for simplicity. This idealized model leads to the widely observed ''fractional Stokes-Einstein (FSE) equation'' (D prop ortional to (eta/T)(-xi), 0 < xi less than or equal to 1) relating the tracer particle diffusion coefficient D and the fluid viscosity eta. The spherical obstruction model indicates the exponent value xi = 3/5, but xi is predicted to vary somewhat with obstruction shape. Experime ntal values of xi are summarized for comparison with our model. (C) 19 98 Elsevier Science B.V. All rights reserved.