Analysis of transition and mobility of microparticle photophoresis with slip-flow model

The objective of the present study is to investigate photophoretic motion of a spherical microparticle in slip-flow regime of gaseous medium. Energy from incident light absorbed by the particle is calculated by employing Mie scattering theory. Temperature and relative velocity distributions of the gaseous flow around the microparticle are developed using a slip-flow model with consideration of thermal stress slip effect. It is demonstrated that the present results agree well with previous measurements and theoretical predictions. Heat source function and asymmetry factor indicating, respectively, the level and the uneven characteristics of the energy distribution within the particle are evaluated. At low, intermediate, and high absorptivities, three different patterns of asymmetry factor versus size parameter are found and named negative photophoresis prevailing, normal switching of photophoresis, and positive photophoresis dominant. Influences of particle optical properties on the critical size for transition of negative–positive photophoresis are analyzed. The results demonstrate that increasing absorptivity or refractivity of the particle leads to a reduction in critical size for photophoresis transition. Both increase in Knudsen number and reduction in particle-to-gas thermal conductivity ratio enhance the photophoretic mobility.