Dynamic arrest of colloids in porous environments: disentangling crowding andconfinement

Abstract

Using numerical simulations we study the slow dynamics of a colloidal hard-sphere fluidadsorbed in a matrix of disordered hard-sphere obstacles. We calculate separately thecontributions to the single-particle dynamic correlation functions due to free and trappedparticles. The separation is based on a Delaunay tessellation to partition the spaceaccessible to the centres of fluid particles into percolating and disconnected voids.We find that the trapping of particles into disconnected voids of the matrix isresponsible for the appearance of a nonzero long-time plateau in the single-particleintermediate scattering functions of the full fluid. The subdiffusive exponentz, obtained from the logarithmic derivative of the mean squared displacement, is essentiallyunaffected by the motion of trapped particles: close to the percolation transition, wedetermined for both the full fluid and the particles moving in the percolating void. Notably, the same value ofz is found in single-file diffusion and is also predicted by mode-coupling theory along thediffusion–localization line. We also reveal subtle effects of dynamic heterogeneity in boththe free and the trapped component of the fluid particles, and discuss microscopicmechanisms that contribute to this phenomenon.