Manifestation of the collective drift of molecules in argon according to their mean square displacements

TL;DR

This study investigates the mean square displacement of argon molecules, revealing deviations from classical diffusion laws linked to collective effects, and introduces a new method to determine the Maxwell relaxation time.

Contribution

It provides detailed analysis of MSD deviations in argon, quantifies the collective contribution to self-diffusion, and proposes a novel method for measuring Maxwell relaxation time.

Findings

Collective contribution to self-diffusion varies from 0.23 to 0.4 near critical conditions.

Deviations from asymptotic MSD law are mainly due to square-root contributions.

A new method for determining Maxwell relaxation time is introduced.

Abstract

The mean square displacement (MSD) of an argon molecule as a function of time is studied. Its deviations from the standard asymptotic law for intermediate times are analyzed in details. It is shown that these deviations are mainly connected with the square-root contribution to the MSD which is proportional the ratio of the collective part to the full self-diffusion coefficient. It is established that the relative value of the collective contribution to the self-diffusion coefficient of argon changes from 0.23 near the triple point up to 0.4 at approaching the critical point. A new method for the determination of the Maxwell relaxation time is proposed. Its temperature dependence on the coexistence curve and one of isochors is investigated.