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Results and perspectives of the investigation of traditional and thermal stress induced thermophoresis of particles in gas in microgravity

par Vedernikov, Andrei Alexeievitch ;Balapanov, Daniyar ;Beresnev, Sergey;Queeckers, Patrick
Référence 40th COSPAR Scientific Assembly (2-10 August 2014: Moscow, Russia)
Publication Non publié, 2014

Communication à un colloque

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Résumé :

Thermophoretic motion of particles suspended in a gas has been a subject of extensive theoretical and experimental investigations for many years because of its wide spread in nature, significance for fundamental and applied aerosol physic. Negative thermophoresis, i.e. solid particle motion towards hotter region in a gas and not as usually from hotter region, was predicted more than 40 years ago and remains an unsolved problem for a choice between different models treating main driving mechanisms -- thermal slip and thermal stress induced gas motion. For a problem of negative thermophoresis, we present experimental evidences in favor of the latter mechanism based on direct observation of particle motion at microgravity; Knudsen particle number 2\cdot 10(-3) (Kn being the ratio of the molecular mean free path to the particle size); particle-to-gas heat conductivity ratios 2\cdot 10(4) for copper solid particles and 1.8 for glass bubbles. For both types of particles the experimental results fit well the gas kinetic model of Beresnev and Chernyak [1]. We present characteristics of a set-up and procedures that are able to provide sufficient accuracy and volume of experimental data for testing any model of particle thermophoresis. High quality microgravity is a necessity for such investigations. The short duration microgravity of drop towers suits well this requirement. The sign and value of the thermophoretic force strongly depends on the Knudsen number, particle-to-gas heat conductivity ratio and accommodation coefficients, all of which vary within several decimal orders of magnitude. In order to make crucial conclusions on the choice of the adequate model, there should be hundreds of short duration microgravity experiments. The European Space Agency scientific project Interaction in Cosmic and Atmospheric Particle Systems (ICAPS) [2] planned for the International Space Station, provides complementary opportunities for the investigation of thermophoresis at large and very large Knudsen numbers for single particles and large clusters of particles under wide range of experimental parameters, i.e. different particle sizes, shapes, materials; different properties of gases; several types of additional forces and their time-space variation. ESA PRODEX Program, Belgian Federal Science Policy Office and Bremen Drop Tower Operation and Service Company ZARM FABmbH (Germany) are greatly acknowledged for their support. [1] Beresnev S., Chernyak V. Thermophoresis of a spherical particle in a rarefied gas: Numerical analysis based on the model kinetic equations // Phys. Fluids. 1995. V.7. P.1743. [2] Blum, J. et al. "Dust in Space", Europhysicsnews, Vol. 39, pp. 27-29, 2008.