First in situ determination of confined Brownian tracer motion in dense random sphere packings

The long-time self-diffusion of fluorescent molecular and colloidal tracers in a dense random packing of large (nondiffusing) spheres has been studied for the first time in situ, using fluorescense recovery after photobleachi ng. The long-time tracer diffusion coefficient depends uniquely on the rati o beta of the tracer size to the packing sphere size. This dependence is mu ch stronger than that predicted by theories which neglect hydrodynamic inte ractions, emphasizing the importance of hydrodynamics on confined diffusion of finite-sized tracers. Hydrodynamic friction cah be described qualitativ ely by a mapping model of the pore space, which reproduces experimental dif fusivities fairly well.