Dynamic arrest of colloids in porous environments: disentangling crowding and confinement

TL;DR

This study uses numerical simulations to analyze how crowding and confinement affect the slow dynamics of colloids in porous environments, revealing the role of trapping and dynamic heterogeneity.

Contribution

It introduces a method to separate free and trapped particles using Delaunay tessellation, clarifying their distinct roles in colloidal dynamics near the percolation transition.

Findings

Trapping causes a nonzero long-time plateau in correlation functions.

The subdiffusive exponent z ≈ 0.5 near the percolation transition.

Dynamic heterogeneity affects both free and trapped particles.

Abstract

Using numerical simulations we study the slow dynamics of a colloidal hard-sphere fluid adsorbed in a matrix of disordered hard-sphere obstacles. We calculate separately the contributions to the single-particle dynamic correlation functions due to free and trapped particles. The separation is based on a Delaunay tessellation to partition the space accessible to the centres of fluid particles into percolating and disconnected voids. We find that the trapping of particles into disconnected voids of the matrix is responsible for the appearance of a nonzero long-time plateau in the single-particle intermediate scattering functions of the full fluid. The subdiffusive exponent $z$, obtained from the logarithmic derivative of the mean-squared displacement, is observed to be essentially unaffected by the motion of trapped particles: close to the percolation transition, we determined $z \simeq 0.5$ for both the full fluid and the particles moving in the percolating void. Notably, the same value of $z$ is found in single-file diffusion and is also predicted by mode-coupling theory along the diffusion-localisation line. We also reveal subtle effects of dynamic heterogeneity in both the free and the trapped component of the fluid particles, and discuss microscopic mechanisms that contribute to this phenomenon.