Measuring diffusion in supercooled liquids: The effect of kinetic inhomogeneities

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Abstract
There is growing evidence to suggest that cooperativity in glasses is characterized by a spatial distribution of relaxation times. In this paper we examine the consequences of such inhomogeneities in the measurement of translational and rotational diffusion constants. We find that the decay of a concentration profile (the basis of transient grating experiments) is accelerated by the development of additional gradients parallel to the relaxation time gradients. This may account for recently observed differences in the variation of rotational and translational diffusion constants with viscosity below the glass transition temperature. The additional gradients may also provide the first direct means of examining the length scale associated with these inhomogeneities.