Colloidal attraction induced by a temperature gradient

TL;DR

This paper demonstrates a novel long-range attractive force between colloidal particles induced by a temperature gradient, enabling reversible formation of stable 2D crystals through hydrodynamic slip flow.

Contribution

It introduces a new thermal-gradient-induced attraction mechanism in colloids, supported by experimental force measurements and theoretical analysis.

Findings

Thermal gradient induces long-range hydrodynamic attraction.

Reversible formation of 2D colloidal crystals observed.

Force law matches predictions based on Marangoni-driven Stokes flow.

Abstract

Colloidal crystals are of extreme importance for applied research, such as photonic crystals technology, and for fundamental studies in statistical mechanics. Long range attractive interactions, such as capillary forces, can drive the spontaneous assembly of such mesoscopic ordered structures. However long range attractive forces are very rare in the colloidal realm. Here we report a novel strong and long ranged attraction induced by a thermal gradient in the presence of a wall. Switching on and off the thermal gradient we can rapidly and reversibly form stable hexagonal 2D crystals. We show that the observed attraction is hydrodynamic in nature and arises from thermal induced slip flow on particle surfaces. We used optical tweezers to directly measure the force law and compare it to an analytic prediction based on Stokes flow driven by Marangoni forces.