Validity of Stokes-Einstein Relation in Soft Colloids up to the Glass Transition

TL;DR

This study demonstrates that the Stokes-Einstein relation remains valid in soft colloids up to the glass transition, contrasting with hard colloids, and combines experiments with simulations for comprehensive insights.

Contribution

It provides the first combined experimental and simulation evidence that the Stokes-Einstein relation holds in ultrasoft colloids near the glass transition, highlighting the role of softness.

Findings

Stokes-Einstein relation remains valid up to the glass transition in soft colloids.

Simulations accurately reproduce experimental dynamic structure factors.

Validity of the relation is linked to ultrasoft interactions, unlike in hard colloids.

Abstract

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q, t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.