Manipulation of Colloids by Nonequilibrium Depletion Force in Temperature Gradient

TL;DR

This study experimentally investigates how temperature gradients and polymer solutions influence colloid distribution, revealing a non-equilibrium depletion force that can trap colloids regardless of their thermophoretic behavior.

Contribution

It demonstrates a novel method to manipulate colloids using non-equilibrium depletion forces driven by temperature gradients and polymer thermophoresis.

Findings

Colloids are driven towards hot regions by a laser-induced temperature gradient.

Polymer concentration amplifies colloid trapping density.

Hydrodynamic theory quantitatively explains the observed phenomena.

Abstract

The non-equilibrium distribution of colloids in a polymer solution under a temperature gradient is studied experimentally. A slight increase of local temperature by a focused laser drives the colloids towards the hot region, resulting in the trapping of the colloids irrespective of their own thermophoretic properties. An amplification of the trapped colloid density with the polymer concentration is measured, and is quantitatively explained by hydrodynamic theory. The origin of the attraction is a migration of colloids driven by a non-uniform polymer distribution sustained by the polymer's thermophoresis. These results show how to control thermophoretic properties of colloids.