Photophoresis on particles hotter/colder than the ambient gas in the free molecular flow

TL;DR

This paper develops a new approximation method for calculating photophoretic forces on aerosol particles in low-pressure gases, accounting for particles hotter or colder than the gas, which improves accuracy over previous assumptions.

Contribution

It introduces a general approximation for photophoretic forces that remains accurate even when particle temperatures differ significantly from the gas temperature.

Findings

Previous models overestimate forces by up to three magnitudes.

New approximation reduces average error to 1%.

Applicable to optically thin gases with temperature variations.

Abstract

Aerosol particles experience significant photophoretic forces at low pressure. Previous work assumed the average particle temperature to be very close to the gas temperature. This might not always be the case. If the particle temperature or the thermal radiation field differs significantly from the gas temperature (optically thin gases), given approximations overestimate the photophoretic force by an order of magnitude on average with maximum errors up to more than three magnitudes. We therefore developed a new general approximation which on average only differs by 1 % from the true value.