Measurements of thermophoretic velocities of aerosol particles in microgravity conditions in different carrier gases

Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrier gases (helium, nitrogen, argon, xenon) were performed in microgravity conditions (the drop tower facility, in Bremen). The experiments permitted the study of thermophoresis in conditions which minimize the impact of gravity. Monodisperse aerosol particles were observed through a digital holographic velocimeter, a device allowing the determination of 3-D coordinates of particles in the viewing volume. Particle trajectories, and consequently particle velocities, were reconstructed by analysing the sequence of particle positions. We successfully observed thermophoretic velocities in low-gravity conditions. The experiments show that the thermophoretic velocity decreases from helium (He) to nitrogen (N2), argon (Ar), and xenon (Xe). Talbot et al. [1980. Thermophoresis of particles in a heated boundary layer. Journal of Fluid Mechanics 101, 737–758] predict thermophoretic velocities that nearly equal the observed values in Xenon, but are larger than observed values in N2 and Ar and smaller than the observed values in He. Yamamoto and Ishihara [1988. Thermophoresis of a spherical particle in a rarefied gas of a transition regime. Physics of Fluids 31, 3618–3624] predict thermophoretic velocities that are smaller than observed values and also predict negative values in N2, Ar and Xe. Beresnevʹs theory [1995. Thermophoresis of a spherical particle in a rarefied gas: Numerical analysis based on the model kinetic equations. Physics of Fluids 7, 1743–1756] fits the experimental data well when the coefficient of tangential momentum accommodation is set to one and the coefficient of energy accommodation is set to a value between 0.4 and 0.9, depending upon the gas.