Tracer Self-Diffusion in Porous Silica. A Dynamical Probe of the Structure

We have studied the diffusion of a fluorescent molecule in porous silica of various porosities. The measured diffusion coefficient depends on the length scale. At small scales we observe free diffusion; at large scales the diffusion is slowed down because of the obstruction effects created by the porous structure. The crossover region begins at scales comparable to the pore size. This is to our knowledge the first measurement of scale-dependent diffusion where the limits of free diffusion and large-scale diffusion are clearly evidenced. The analysis of the data leads to an original description of the structure of the porous media.