One-dimensional Diffusion of Colloids in Colloid-Polymer Mixtures

Colloid-polymer mixtures serve as model systems to study complex fluids. The challenge in this study lies in the fact that these are systems where the relevant physical processes often occur over a wide range of characteristic length and time scales which can be strongly correlated. The addition of soluble non-adsorbing polymer to colloidal dispersions, even in small amounts, has a significant effect on transport properties (mainly because it induces a new type of interaction between colloidal particles, the depletion interaction). When non-adsorbing polymers are added to a suspension of hard-sphere-like colloidal particles, an effective, generally attractive interaction is induced between the colloidal particles. The origin of this interaction is the depletion effect: the exclusion of polymer from the depletion zone between the colloids give rise to an unbalanced osmotic pressure. Alternatively, one can view the depletion induced attraction as arising from an increase in the entropy of the polymers. When colloid-polymer systems are confined by surfaces, boundary and wall effects become important. Diffusion in quasi one-dimensional geometries is of particular interest not only because of practical applications for diffusion processes in microporous solids and in biological channels but also because of the emergence of a new regime (anomalous diffusion) in the long-time dynamics of colloids.