Enhancing tracer diffusivity by tuning interparticle interactions and coordination shell structure

TL;DR

This paper demonstrates that by optimizing interparticle interactions, specifically adding soft repulsions, the diffusivity of tracer particles in dense fluids can be significantly enhanced, achieving over threefold increases in diffusion rates.

Contribution

The study introduces a combined optimization and theoretical approach to design interactions that enhance tracer diffusivity by disrupting local cage structures.

Findings

Optimized soft repulsive interactions increase tracer diffusivity.

Enhanced tracers diffuse over three times faster than hard-sphere tracers.

Disruption of coordination shells correlates with increased mobility.

Abstract

This study uses a combination of stochastic optimization, statistical mechanical theory, and molecular simulation to test the extent to which the long-time dynamics of a single tracer particle can be enhanced by rationally modifying its interactions--and hence static correlations--with the other particles of a dense fluid. Specifically, a simulated annealing strategy is introduced that, when coupled with test-particle calculations from an accurate density functional theory, finds interactions that maximize either the tracer's partial molar excess entropy or a related pair-correlation measure (i.e., two quantities known to correlate with tracer diffusivity in other contexts). The optimized interactions have soft, Yukawa-like repulsions, which extend beyond the hard-sphere interaction and disrupt the coordination-shell cage structure surrounding the tracer. Molecular and Brownian dynamics simulations find that tracers with these additional soft repulsions can diffuse more than three times faster than bare hard spheres in a moderately supercooled fluid, despite the fact that the former appear considerably larger than the latter by conventional definitions of particle size.