Screening and collective effects in randomly pinned fluids: A new theoretical framework

TL;DR

This paper introduces a new theoretical framework to understand how randomly pinned particles affect the dynamics and relaxation processes in supercooled water, combining local and collective effects.

Contribution

It develops a novel theoretical approach to describe the impact of random pinning on relaxation dynamics, validated against simulations and experimental data.

Findings

Relaxation time increases with pinning fraction and density.

The theory accurately predicts simulation results for supercooled water.

Thermal dependence of relaxation matches prior experimental data.

Abstract

We propose a theoretical framework for the dynamics of bulk isotropic hard-sphere systems in the presence of randomly pinned particles and apply this theory to supercooled water to validate it. Structural relaxation is mainly governed by local and non-local activated process. As the pinned fraction grows, a local caging constraint becomes stronger and the long range collective aspect of relaxation is screened by immobile obstacles. Different responses of the local and cooperative motions results in subtle predictions for how the alpha relaxation time varies with pinning and density. Our theoretical analysis for the relaxation time of water with pinned molecules quantitatively well describe previous simulations. In addition, the thermal dependence of relaxation for unpinned bulk water is also consistent with prior computational and experimental data.