Nature of the Breakdown in the Stokes-Einstein Relationship in a Hard Sphere Fluid

TL;DR

This study uses molecular dynamics simulations to investigate the breakdown of the Stokes-Einstein relation in dense hard sphere fluids, revealing that hopping particles cause deviations while sedentary particles follow the classical relation.

Contribution

It demonstrates that the violation of the Stokes-Einstein equation is due to hopping particles, while sedentary particles still obey the relation, clarifying the microscopic origin of the breakdown.

Findings

Hopping particles exhibit deviations from the SE relation.

Sedentary particles obey the SE relation even at high densities.

Breakdown of SE is linked to the presence of mobile, hopping particles.

Abstract

Molecular Dynamics simulations of high density hard sphere fluids clearly show a breakdown of the Stokes-Einstein equation (SE). This result has been conjectured to be due to the presence of mobile particles, i.e., ones which have the propensity to "hop" distances which are integer multiples of the interparticle distance. We conclusively show that, even though the whole liquid violates the SE equation at high densities, the sedentary particles, i.e., ones complementary to the "hoppers", obey the SE relationship. These results strongly support the notion that the unusual dynamics of fluids near vitrification are caused exclusively by the presence of hopping particles.