Local structure-mobility relationships of confined fluids reverse upon supercooling

TL;DR

This study investigates how the relationship between local structure and particle mobility in confined fluids changes upon supercooling, revealing a reversal in diffusion behavior at high densities using computer simulations.

Contribution

It demonstrates that local diffusivity trends reverse in supercooled confined fluids and introduces a predictive approach based on available space distribution for particle insertion.

Findings

Diffusivity is higher in dense regions at moderate packing fractions.

Reversal of diffusivity trends occurs in supercooled, dense states.

Average dynamics can be predicted from space availability distributions.

Abstract

We examine the structural and dynamic properties of confined binary hard-sphere mixtures designed to mimic realizable colloidal thin films. Using computer simulations, governed by either Newtonian or overdamped Langevin dynamics, together with other techniques including a Fokker-Planck equation-based method, we measure the position-dependent and average diffusivities of particles along structurally isotropic and inhomogeneous dimensions of the fluids. At moderate packing fractions, local single-particle diffusivities normal to the direction of confinement are higher in regions of high total packing fraction; however, these trends are reversed as the film is supercooled at denser average packings. Auxiliary short-time measurements of particle displacements mirror data obtained for experimental supercooled colloidal systems. We find that average dynamics can be approximately predicted based on the distribution of available space for particle insertion across orders of magnitude in diffusivity regardless of the governing microscopic dynamics.