Enhanced diffusion of tracer microspheres in a temporally fluctuating porous structure

Transport in porous materials is at the center of a wide range of natural and industrial processes. While many studies have focused on the properties of porosity with spatial fluctuations, much less is known about the effects of temporal fluctuations in the medium’s structure. We have designed a simple experimental system aimed at investigating the transport properties of a crystalline porous material made of a hexagonal lattice of 2um spheres, which are either static, or fluctuating in time about an average position (dynamic). Using complementary illumination, we can image and track both diffusive tracers (100-400nm) and lattice spheres. We compare the transport properties of the static and dynamic lattices with similar average densities and find that diffusion is significantly faster within the dynamic lattice. We hypothesize that this might be due to a combination of different effects. First, the motion of lattice spheres could reduce the hydrodynamic hindrance of the tracers, and increase diffusivity within a cavity. Second, fluctuations in pore sizes, induced by the fluctuations in distances between adjacent spheres, could permit a higher transition rate between cavities than a fixed pore size of the same mean value. We present results relating to these two effects.