Measurements of phoretic velocities of aerosol particles in microgravity conditions.

.Small particles suspended in non-isothermal gas and/or in an isothermal gas mixture with concentrations gradients of chemical species, experience a force, that is induced by thermal or concentration gradients. The particle motion is commonly termed thermophoresis and diffusiophoresis, respectively. Such phenomena are of considerable importance in a variety of engineering fields (coating of surfaces, gas cleaning, optical fiber production, advanced ceramics, nanophase materials, combustion processes, microcontamination control in the electronics industry, etc.). They certainly play a crucial role in the scavenging of aerosol particles in clouds, when droplets and/or ice crystals grow or evaporate, and below clouds, during the fall of hydrometeors. Experiments to measure the thermophoretic force or velocity have mainly been performed in normal gravity employing several different experimental methods: the modified Millikan cell with heated upper and cooled lower electrodes, the deflection of a particle suspended from a small wire, the penetration method, the electrodynamic levitation and the deflection of particles moving in a jet formed by a slit. 1),While in the free-molecule regime (Knudsen Waldmann’s theory (Waldmann, 1959) seems to be in good agreement with the experimental results, in the 0.1) and in the transition regionslip-flow regime (Kn 10) previously published measurementsKn (0.1 are contradictory. Talbot et al.(1980) observed that Brock’s thermal , is identical toforce (Brock, 1962), when Kn Waldmann’s equation, except for the multiplicative factor cs/cm, where cs is the thermal-slip coefficient and cm is the velocity-slip coefficient. As this ratio should be about unity, Talbot proposes an equation that should be valid throughout the entire interval 0<Kn<, and suggests for cs, ct, cm the values 1.17, 2.18 and 1.14, respectively. The transition regime is difficult to analyse. Numerical solutions of the integral transport equation for the BGK model have been attempted. The solutions of Yamamoto and Ishihara (1988), Loyalka (1992), and Ohwada and Sone (1992), all predict negative thermophoresis for sufficiently large values of the ratio between thermal conductivities of the particle and gas and for Kn<0.2. In normal gravity the thermophoretic effect cannot be studied alone, as particles move due to gravity force and eventually owing to natural convection. These difficulties can be avoided by utilizing a microgravity environment. Up to now only Toda et al. (1996, 1998) and Oostra (1998) have performed experiments in microgravity, in the drop tower facilities of JAMIC (10-5 g0 , t=10s) and Zarm, in Bremen (10-6 g0, t=4.7s), respectively. Concerning diffusiophoresis, a case of great interest occurs when vapour molecules are diffusing through resting air, i.e. when a Stephan flow exists. This phenomenon was studied theoretically and experimentally by Waldmann (1959), Schmitt and Waldmann (1960), Derjaguin et al. (1966), Kousaka and Endo (1993). As in the case of thermophoresis, the microgravity environment simplifies the problem, as it eliminates the continuous renewal of the vapour concentration field. So far there are no published data concerning diffusiophoretic velocities in microgravity.