Phoretic behavior of asymmetric particles in thermal nonequilibrium with the gas: Two-sphere aggregates

Abstract

An investigation into the phoretic behavior arising from thermal slip of an asymmetrical aerosol particle in thermal nonequilibrium with the carrier gas is presented. The asymmetrical particle is modeled as an aggregate of two spheres unequal in size and/or composition, with gas/particle thermal nonequilibrium arising from radiative transfer between the particle and an isotropic background. The particle and gas conduction equations and the creeping flow equation are solved for the binary sphere system using a spherical-harmonics-based method. In high temperature (e.g., combustion) environments, results indicate that the thermal slip forces arising from radiative cooling of an asymmetric aggregate can lead to significant phoretic velocities. The velocity is body-fixed, i.e., directed along the aggregate axis, and in the absence of alignment forces the aggregate motion is stochastic. The “effective” diffusion resulting from this motion can be orders of magnitude larger than ordinary Brownian diffusion for a volume-equivalent sphere. Conventional thermophoresis, resulting from a temperature gradient in the bulk gas, can act to align the aggregate axis with the temperature gradient. Under these conditions, the body-fixed motion of the aggregate will become deterministic, and can lead to a considerable increase or decrease in the “apparent” thermophoretic diffusivity of the aggregate.