Hydrodynamic boundary effects on thermophoresis of confined colloids

TL;DR

This paper investigates how hydrodynamic effects near walls influence thermophoresis of colloids, revealing a logarithmic increase in trapping efficiency at close distances and aligning well with recent experimental data.

Contribution

It provides a quantitative model for hydrodynamic slowing-down and trapping enhancement of colloids near walls under temperature gradients, supported by experimental comparisons.

Findings

Trapping efficiency increases logarithmically as particles approach the wall.

Lubrication approximation accurately describes particle velocity at small distances.

Model predictions agree with recent experimental observations.

Abstract

We study hydrodynamic slowing-down of a particle moving in a temperature gradient perpendicular to a wall. At distances much smaller than the particle radius, $h\ll a$, lubrication approximation leads to the reduced velocity $u/u_{0}=3\frac{h}{a}\ln \frac{a}{h}$, with respect to the bulk value $u_{0}$. With Brenner's result for confined diffusion, we find that the trapping efficiency, or effective Soret coefficient, increases logarithmically as the particle gets very close to the wall. This provides a quantitative explanation for the recently observed enhancement of thermophoretic trapping at short distances. Our discussion of parallel and perpendicular thermophoresis in a capillary, reveals a very a good agreement with five recent experiments on charged polystyrene particles.