On Stokes-Einstein and Debye behaviour in colloidal fluids

We discuss recent experiments on tracer sphere diffusion in colloidal host suspensions. Stokes-Einstein behaviour (tracer diffusion inversely proportional to host viscosity) is observed at low host viscosities, i.e. the host suspension behaves as an effective fluid. At high host viscosities, however, tracer diffusion decouples from the viscosity and the host suspension behaves more like a static confining geometry