Diffusion and Mobility of Solutes in Viscous and Glassy Systems Studied by Optical Techniques

摘要

After a general introduction, Forced Rayleigh Scattering (FRS) and fluorescence dye bleaching (FRAP) techniques are explained which allow to investigate slow molecular motion, namely, translational diffusion coefficients down to 10~(-21) m~2s~(-1) and rotational correlation times up to the order of 10~3 s. Some representative examples of photoreactive dye probe diffusion in glass forming liquids and colloids are discussed. In particular, we present recent results on the drying process of aqueous latex films. In a concentration range between 18 and 3 weight percent of water we find that the diffusion of a dye probe studied by FRS has a wave vector dependence reflecting the heterogeneity of the film structure. In terms of a two-state model of tracer exchange between the latex spheres and interconnected hydroplasticized regions we obtain a length scale dependent effective diffusion coefficient which is small on scales smaller than the sphere size and increases by up to two orders of magnitude on the scale of the interconnected regions.