抄録
Motion of metal spheres in gas with imposed temperature gradient was analyzed both theoretically and experimentally in the range of parameters favorable for the existence of negative thermophoresis. Polydisperse copper spherical particles with mean diameter of 74 μm were injected into nitrogen at normal pressure and particle-to-gas heat conductivity ratio (Λ being around 2.0・ 104) were the most extreme values reported until now. Particle sedimentation and gravity-induced convection were suppressed in microgravity conditions during free fall in the Bremen drop tower. Observed particle velocities have rather high spread, which we attribute to the presence of non-gravitational convection driven by thermal creep on the non-uniformly heated wire and its holders. Experimental estimates of the thermophoretic velocity normalized on the temperature gradient are positive, however, being far below the values reported in literature. Estimates taking into account perturbations from non-gravitational convection meet better predictions of gas-kinetic theories, which include thermal stress slip flow. Ways to improve experimental procedure and accuracy are identified, so that they will make it feasible to get crucial answer on the question of existence of negative thermophoresis in gases - a new physical phenomenon predicted theoretically and yet not observed experimentally.
