Effects of gas species on pressure dependence of thermophoretic velocity

Microgravity experiments are conducted to measure the thermophoretic velocity, and effects of gas species are investigated. Particles adopted are PMMA spheres of 2.91 μm in mean diameter, and atmospheric gases chosen are pure gases of argon, nitrogen, and carbon dioxide. The temperature gradient is set at 10 K/mm, and the pressure is set at several conditions in the range from 20 kPa to 100 kPa. Terminal velocities of particles suspended in a gas are individually measured during 0.25 s of the microgravity condition, which is achieved by a free-fall. The accuracy of the measurement is attained by accumulating data from repeated trials. Obtained experimental results are compared with theoretical predictions; a notable discrepancy is found for carbon dioxide, while the results for other two gases are consistent with predictions. Some attempts are made to fix the discrepancy: first by modifying constants and second by modifying two empirical coefficients in the theory.