Nonlinear Thermophoresis beyond Local Equilibrium Criterion

TL;DR

This paper investigates the nonlinear behavior of thermophoresis under strong thermal gradients, revealing a transition from linear to nonlinear drift that correlates with local disequilibrium, thus addressing discrepancies between experiments and theory.

Contribution

It demonstrates the existence of nonlinear thermophoresis beyond the local equilibrium criterion, providing new insights into thermophoretic behavior at large gradients.

Findings

Linear drift at moderate gradients

Nonlinear drift at strong gradients

Nonlinearity coincides with local disequilibrium

Abstract

Thermophoresis (thermodiffusion, Soret effect) moves molecules along thermal gradients. We measure its phenomenological linear drift relation by single particle tracking in convection-free settings. For moderate thermal gradients, drift velocity depends linearly on the gradient. However, for strong thermal gradients, we find a nonlinear dependence of the drift on the applied gradient for large Soret coefficient and particle radius. Interestingly, the onset of the nonlinearity coincides with a local disequilibrium of the particle. Nonlinear thermophoresis resolves several fundamental contradictions between thermophoretic experiments and theory.