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The present study is concerned with a theoretical analysis of the photophoresis of a microsized long cylinder in a perpendicular optical field. Different from previous studies of photophoresis, thermal stress slip usually neglected is taken into account in the analysis. The gaseous fluid relative to the microparticle in photophoretic motion falls into slip-flow regime. Asymmetric distribution of the absorbed heat energy within the particle becomes the driving force for photophoretic motion of the cylinder-shaped particle. By evaluating heat source function distributions at various conditions, the study focuses on the effects of particle size and optical properties on the energy distribution and the resultant influences on the photophoresis. The photophoretic mobility is developed by the slip flow model with consideration of thermal stress slip. The results reveal that the photophoretic mobility decreases with the increase of particle thermal conductivity (k*) and increases with Knudsen number (Kn). The thermal stress slip effect on photophoretic velocity is more noticeable at high Kn, but disappears at the continuum limit. A long cylinder-shaped particle has higher photophoretic velocity than a spherical particle at low k*, while the situation reverses at high k*. With thermal stress slip considered, the critical condition for crossing of the photophoretic velocity curves of cylindrical and spherical particles is mildly influenced by Kn.

The present article pertains to a theoretical analysis for thermophoresis of a micro-particle in rarefied gas environment. Different from the conventional analyses, thermal stress effect in velocity slip boundary condition is taken into account. This interfacial thermal characteristic is always ignored in previous theoretical studies. The present theory with consideration of the thermal stress slip provides an appropriate modification and thus agrees well with the measured data of thermophoretic mobility of various particles. Order-of-magnitude analysis also qualitatively demonstrates enhancement of the thermal stress slip with reducing particle size or increasing characteristic temperature gradient. Finally, a parametric analysis shows that the thermal stress slip effect becomes more pronounced at conditions of high particle thermal conductivity and high Knudsen number as well.

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.

This Letter presents a study on photophoresis of a microparticle in gaseous media with focus on the effect of thermal stress slip, which is deemed as one of factors causing deviations of previous theoretical predictions from measurements. The present modified theory agrees well with the measurements and, combining with 1 order-of-magnitude analysis, demonstrates the significance of the thermal stress slip in photophoresis of a particle, especially, of small radius. With the physical mechanisms addressed, the parametric analysis reveals that this interfacial thermal effect becomes more pronounced with reducing thermal conductivity of the particle and increasing Knudsen number as well.

In the present study, combining the conventional photothermal analysis and the concept of interaction of solvent molecules in interfacial layer used for thermophoresis in liquid, a theory for photophoresis of a hydrophobic particle suspended in liquids is developed. To characterize hydrophobicity of the micro-particle, slip length in Navier's formula is used as an index. Analytical expressions are derived and a parametric analysis for photophoretic velocity is performed with emphasis on the influences of particle characteristics such as size, optical properties, hydrophobicity, and thermal conductivity. Heat source function and the corresponding asymmetry factor at various conditions are evaluated to interpret the mechanisms of negative and positive photophoresis and the conditions for transition between them. The present theory discloses that the particle surface hydrophobicity or fluid slippage at particle-liquid interface may lead to a remarkable enhancement in the particle photophoretic velocity in liquids. Higher particle thermal conductivity and larger size of liquid molecules both result in weaker photophoretic motion.