Fluctuating Diffusivity Emerges even in Binary Gas Mixtures

TL;DR

This paper reveals that fluctuating diffusivity can naturally emerge in binary gas mixtures due to mass differences and time scale separation, providing a new understanding of diffusivity variability in soft matter systems.

Contribution

It introduces a novel origin for fluctuating diffusivity based on stochastic simulations of binary gas mixtures with small component fractions and mass disparities.

Findings

Fluctuating diffusivity occurs when the minor component's mass is much smaller.

Time scale separation causes the observed diffusivity fluctuations.

Minor component exhibits fluctuating diffusivity under specific mass conditions.

Abstract

Diffusivity in some soft matter and biological systems changes with time, called the fluctuating diffusivity. In this work, we propose a novel origin for fluctuating diffusivity based on stochastic simulations of binary gas mixtures. In this system, the fraction of one component is significantly small, and the mass of the minor component molecule is different from that of the major component. The minor component exhibits fluctuating diffusivity when its mass is sufficiently smaller than that of the major component. We elucidate that this fluctuating diffusivity is caused by the time scale separation between the relaxation of the velocity direction and the speed of the minor component molecule.