Impact of random obstacles on the dynamics of a dense colloidal fluid

TL;DR

This study uses molecular dynamics simulations to explore how randomly placed obstacles affect the slow dynamics and arrest scenarios of a dense colloidal fluid confined in a porous matrix, revealing complex relaxation behaviors.

Contribution

It provides detailed insights into the impact of obstacle density on colloidal fluid dynamics, including the observation of both continuous and discontinuous arrest phenomena, and tests predictions of mode-coupling theory.

Findings

Obstacles significantly alter relaxation patterns.

Both continuous and discontinuous arrest scenarios observed.

Subdiffusive behavior linked to matrix structure.

Abstract

Using molecular dynamics simulations we study the slow dynamics of a colloidal fluid annealed within a matrix of obstacles quenched from an equilibrated colloidal fluid. We choose all particles to be of the same size and to interact as hard spheres, thus retaining all features of the porous confinement while limiting the control parameters to the packing fraction of the matrix, {\Phi}m, and that of the fluid, {\Phi}f. We conduct detailed investigations on several dynamic properties, including the tagged-particle and collective intermediate scattering functions, the mean-squared displacement, and the van Hove function. We show the confining obstacles to profoundly impact the relaxation pattern of various quantifiers pertinent to the fluid. Varying the type of quantifier (tagged-particle or collective) as well as {\Phi}m and {\Phi}f, we unveil both discontinuous and continuous arrest scenarios. Furthermore, we discover subdiffusive behavior and demonstrate its close connection to the matrix structure. Our findings partly confirm the various predictions of a recent extension of mode-coupling theory to the quenched-annealed protocol.