The Spatial Crossover between Far-From-Equilibrium and Near-Equilibrium Dynamics in Locally Driven Suspensions

TL;DR

This study investigates how a colloidal suspension transitions from far-from-equilibrium to near-equilibrium behavior under localized optical driving, revealing a sharp spatial crossover characterized by particle migration and a consistent length scale around Pe=1.

Contribution

We experimentally observe a clear spatial crossover in colloidal suspensions under localized driving, linking far- and near-equilibrium dynamics with a novel inhomogeneous steady state.

Findings

Particles migrate from high to low Péclet number regions.

Steady-state density profiles are strongly inhomogeneous.

A characteristic length scale emerges at Pe ≈ 1.

Abstract

We examine the response of a quasi-two-dimensional colloidal suspension to a localized circular driving induced by optical tweezers. This approach, a microscale version of rheometry, allows us to resolve over three orders of magnitude in P\'eclet number ($Pe$) and provide a direct observation of a sharp spatial crossover from far- to near-thermal-equilibrium regions of the suspension. In particular, particles migrate from high to low $Pe$ regions and form strongly inhomogeneous steady-state density profiles with an emerging length scale that does not depend on the particle density and is set by $Pe\approx1$. We show that the phenomenological two phase fluid constitutive model is in line with our results.