Low-Knudsen-number photophoresis of aerosol spheroids

The photophoretic motion of a freely suspended aerosol spheroid exposed to a radiative heat flux that is oriented arbitrarily with respect to its axis of revolution is analytically studied. The Knudsen number is assumed to be so small that the fluid flow can be described by a continuum model with a thermal slip at the particle surface. In the limit of small Peclet and Reynolds numbers, the appropriate energy and momentum equations are solved using the bifocal-coordinate transformations. Expressions for the photophoretic velocity and force are obtained in closed form for various cases of prolate and oblate spheroidal particles. The average photophoretic velocity and force for an ensemble of identical, noninteracting spheroids with random orientation distribution are also determined. The results indicate that the aspect ratio and relative thermal conductivity of a spheroidal particle and its orientation with respect to the incident light can have significant effects on its photophoretic behavior.