Anomalous tracer diffusion in film forming colloidal dispersions

Abstract

Film forming colloidal dispersions can be conceived as a material composed of interpenetrating hydrophobic (polymer) and hydrophilic (partially broken interfaces) phases where the transport properties of one phase are influenced by the geometric confinement effect imposed by the other. We studied the transport properties of film forming colloidal dispersions by introducing hydrophobic dye molecules into the colloidal particles and determining their motion with forced Rayleigh Scattering as a function of length scale (grating distance A) and water content. At water contents between 18 and 3 weight percent we find signatures of anomalous tracer diffusion, namely stretched exponential decay curves with relaxation times which significantly deviate from the q 2 -dependence (q 2 = 4π 2 /Λ 2 ) of Fickian diffusion. The form of the q-dependence is contrary to what could be expected from a simple confinement model. Analyzing the results in terms of a length scale dependent effective diffusion coefficient we find that diffusion on large length scales proceeds faster than on small length scales by nearly one order of magnitude. We attempt to interpret our findings in a simple two-state model with enhanced diffusion on large length scales due to the existence of interconnected hydroplasticized regions.