Direct measurement of thermophoretic forces

TL;DR

This paper presents a novel experimental method to measure thermophoretic forces on colloidal particles with high precision, revealing the temperature dependence of the Soret coefficient and extending thermophoresis studies to larger particles and new solvent conditions.

Contribution

It introduces a generalized potential approach to quantify thermophoretic forces from particle distribution data, enabling measurements at 10 fN resolution and expanding the scope of thermophoresis research.

Findings

Thermophoretic forces can be extracted with 10 fN resolution.

The temperature dependence of the Soret coefficient matches previous smaller colloid results.

Hydrodynamic effects in confined geometries are characterized.

Abstract

We study the thermophoretic motion of a micron sized single colloidal particle in front of a flat wall by evanescent light scattering. To quantify thermophoretic effects we analyse the nonequilibrium steady state (NESS) of the particle in a constant temperature gradient perpendicular to the confining walls. We propose to determine thermophoretic forces from a 'generalized potential' associated with the probability distribution of the particle position in the NESS. Experimentally we demonstrate, how this spatial probability distribution is measured and how thermophoretic forces can be extracted with 10 fN resolution. By varying temperature gradient and ambient temperature, the temperature dependence of Soret coefficient $S_T(T)$ is determined for $r = 2.5 \mu m$ polystyrene and $r = 1.35 \mu m$ melamine particles. The functional form of $S_T(T)$ is in good agreement with findings for smaller colloids. In addition, we measure and discuss hydrodynamic effects in the confined geometry. The theoretical and experimental technique proposed here extends thermophoresis measurements to so far inaccessible particle sizes and particle solvent combinations.