Drag coefficient of the weakly ionized plasma in the high Knudsen number regime

Abstract

The drag force acting on a micron-sized polystyrene particle in the high Knudsen number regime is investigated. Analysis of the particle trajectories in stationary neutral argon gas environment suggests the damping time constant τ∝p−1.20±0.04 and Epstein drag force coefficient δ=1.40. The neutral drag coefficient is compared with the drag coefficient measurement in dust-free plasma. The phenomena of the reduced drag in weakly viscous and weakly ionized rf plasma are also observed in this report. It is shown that the slight changes in rf power and pressure would enhance the reduced drag effect, which suggests that there is an additional electrostatic force acting along the particle motion in the plasma.