Negative photophoresis of smoke particles observed under microgravity

We observed the photophoresis of smoke particles in a small focal laser beam under micro-gravity, exerted in an airplane, to avoid air convectional flow due to buoyancy. We measured the motion of smoke particles and found that the particles exhibit negative photophoresis under the suppression of the convection. Based on the distribution of velocity of negative photophoresis, it is shown that the photophoretic velocity is proportional to the laser power applied for the particle. We discuss the mechanism in terms of the radiometric force.