The effective hydrodynamic radius is not a constant

TL;DR

This study uses molecular dynamics simulations to show that the effective hydrodynamic radius varies with temperature, challenging the common assumption of its constancy in testing the Stokes-Einstein relation.

Contribution

The paper demonstrates that the effective hydrodynamic radius is temperature-dependent, impacting the validity of variants of the Stokes-Einstein relation.

Findings

Effective hydrodynamic radius decreases with temperature.

Stokes-Einstein relation remains valid when considering radius changes.

Assumption of constant radius should be re-evaluated in related tests.

Abstract

The effective hydrodynamic radius is usually assumed to be a constant in testing the Stokes-Einstein relation by its variants. We have performed molecular dynamics simulations with ortho-terphenyl and Kob-Andersen model to examine the assumption and found the effective hydrodynamic radius is not a constant but decreases with decreasing temperature. The variant of Stokes-Einstein relation is not established but Stokes-Einstein relation is valid by considering the changes of the effective hydrodynamic radius. We propose the assumption should be seriously evaluated when using variants to test Stokes-Einstein relation.