The thermophoretic force acting on a spherical non-evaporating or evaporating particle suspended in a rarefied plasma

Abstract

Previous thermophoretic force studies for a spherical particle suspended in a rarefied plasma have been improved upon on the basis of a more detailed analysis concerning the interaction between a charged sphere and ions and electrons. Revised analytical expressions for the thermophoretic force acting on a non-evaporating or evaporating spherical particle for the extreme case of the free-molecule regime and a thin plasma sheath are presented. Besides for the metallic limiting case, analytical expressions are also obtained for the non-metallic limiting cases of particulate materials (with zero electrical conductivity). It has been shown that the ion and electron components of the thermophoretic force are appreciably different from the previous results. The thermophoretic force is directly proportional to the square of the particle radius. Evaporation may greatly enhance the thermophoretic force acting on a particle due to the additional contribution to the thermophoretic force caused by the reaction force of non-uniform evaporated mass flux flowing out from the evaporating particle. This evaporation-added thermophoretic force is more significant for particulate materials with comparatively low latent heat of evaporation and at high plasma temperatures. For the case without evaporation, there is only a small difference between the metallic and non-metallic particles in their thermophoretic forces. However, marked difference may exist at high plasma temperatures between the metallic and non-metallic evaporating particles in terms of their total thermophoretic forces because the different floating potential distributions on the surfaces of the two types of particles cause different distributions of the local heat flux and evaporated-mass flux along the particle surface.