Dynamics of Uncharged Colloidal Silica Spheres Confined in Bicontinuous Porous Glass Media

The restricted diffusion of uncharged colloidal silica spheres in an optically matched, porous glass with uncharged internal surfaces is measured by means of heterodyne dynamic light scattering (DLS) and fluorescence recovery after photobleaching (FRAP). Glass and sphere surfaces have been chemically treated such that, in addition to hydrodynamic interactions, only hard particles−wall interactions are present. The wavevector-dependent effective diffusion coefficient inside the porous glasses, obtained from the initial decay of the correlation functions, is always smaller than the bulk diffusion coefficient. In particular the diffusion coefficient at small K, probing diffusion over several pore diameters, is strongly reduced due to increasing hydrodynamic drag at large particle to pore size ratios. The long-time self-diffusion coefficient of fluorescent silica spheres measured by FRAP is in good agreement with the small angle DLS results. The reduction of the diffusion coefficient as a function of the size ratio qualitatively agrees with various models describing the restricted motion of a particle in an infinitely long cylinder.