Probing particle dynamics in a fully opaque porous network using X-ray differential dynamic radiography (XDDR)

Being able to follow in real-time the motion of particles of various nature, shape and size in porous media clearly represents a major research and societal issue. Currently, the leading technique used for tracing particle dynamics in porous media is based on optical microscopy. However, such methods require working with partially transparent samples, which is not the case of many naturally occurring porous media. The latter can indeed be fully opaque, with soil being the most salient example. We propose an alternative method based on recording the time evolution of X-ray radiographs in pure absorption mode. We show that a specific analysis of such a dataset can provide a quantitative determination of the intermediate scattering function (ISF) of these particles in various opaque porous media. The potential of our approach, named X-ray differential dynamic radiography (XDDR), was first checked by simulating random walk dynamics of light colloids inside a porous SiO₂ RCP (random close packing) network saturated with water. Potential perturbation induced by Fresnel diffraction is analyzed. Finally, two experiments are performed on the beamline ANATOMIX at the SOLEIL synchrotron, demonstrating the possibility to probe μm SiO₂ particle sedimentation either in bulk water or inside a RCP of poly(methyl methacrylate) (PMMA) spheres. XDDR appears to fill a relevant "niche" between DDM (differential dynamics microscopy) and XPCS (X-ray photon correlation spectroscopy), allowing to cover a time scale from 0.2 μs to several minutes and a range (<i>q</i>-range) from 0.1 μm^-1 to 5 μm^-1.