Direction, magnitude and causes of photophoretic forces

Abstract The conventional theory which explains the photophoretic force as the result of a temperature difference on the surface of the body acted on is confronted with contradictory observations: in particular, motions of large powder particles against the direction of light and a magnitude of the force up to 10 4 times larger than calculated. In this paper, investigations aimed to solve these problems by relating photophoresis and physical properties of particles are presented. Particles of powders 10–100 μm in size are dropped into a horizontal beam of light, separated according to their deflection from the vertical by photophoretic forces, collected and examined by electron microscopy. Evidence is shown that with these particles the dominant type of photophoretic force is a force ( F α ) caused by differences of the thermal accommodation coefficient (α) over the particle surface. The well-known type of force ( F T ) induced by temperature differences nevertheless plays an essential, namely, directing role, as its torque can align a particle acted on by F α relative to the direction of light. In this way, motions of large, strongly absorbing particles towards the light source can be explained. A magnitude of F α up to 10 4 times larger than F T is deduced from differences of α of about 0.1. Surface details suitable to cause these differences of α (smooth and ragged parts of the surface, pockets formed by foliated structures, etc) are demonstrated by electron micrographs.