Shear flow experiments in microgravity

Kurzfassung

PK-4 is a new facility on board of the International Space Station (ISS) designed to perform experiments with complex plasmas in microgravity conditions. A particular emphasis is on experiments with flowing complex plasmas. In this presentation, an experiment will be presented where a shear flow was created in a microparticle cloud by means of radiation pressure from a powerful optical manipulation laser. The first shear flow experiment was performed during the PK-4 commissioning in June 2015. Melamine-formaldehyde microspheres with a diameter of 3.4 micron were suspended in a neon dc discharge plasma using polarity switching with the frequency of 500 Hz and duty cycle of 50%. Manipulation laser (wavelength of 808 nm) was used in continuous regime; the laser diode current was varied in the range of 8-30 A. The particle observation system (cameras and illumination laser) performed a continuous scan of the particle cloud with the speed of 0.1 mm/s. Experimental video showing particle motion was analyzed using Particle Tracking Velocimetry (PTV). This enabled 3D reconstruction of the shear flow from the scanning data. Next, assuming azimuthal symmetry of the shear flow, the particle velocity profile and shear rate were calculated. The particle pair correlation function g(r) was reconstructed; this showed "stringy" structure of the particle cloud. Shear stress was calculated assuming isotropic Yukawa interparticle potential. From shear stress and shear rate, the shear viscosity of complex plasma can be evaluated. Further steps will include improving statistics (e.g., by performing stepwise scan of the particle cloud), using a more realistic interparticle potential (anisotropic) in the analysis, and corroborating results by Particle Image Velocimetry (PIV) and template matching analysis.