Experimental investigation of dynamical structures formed due to complex plasma flowing past an obstacle

Abstract

We report experimental observations of the dynamical behavior of a complex plasma flowing past a spherical obstacle. The investigation has been carried out in a Pi-shaped direct current glow discharge experimental device where a dust cloud of kaolin particles in a background argon plasma is made to flow in a controlled fashion by regulating the mass flow of the neutrals. A spherical metal object vertically mounted on the cathode tray acts as an obstacle to the flow. The flowing dust particles are repelled by the electrostatic field of the negatively charged sphere, and a microparticle free region (dust void) is formed surrounding the obstacle. The distant dust particles are attracted towards the floating obstacle and reflected back when they get to a certain distance, causing a ring shaped structure around the obstacle. We characterize the shape of this structure over a range of dust flow speeds and obstacle biases. For a supersonic flow of the dust fluid, a bow shock is seen to form on the upstream side of the negatively biased sphere, while the downstream side shows the generation of wave structures for a particular range of flow velocities when the Reynolds number Re>~50. The flow generated structures can be physically understood in terms of the dust dynamics under the combined influence of the ion-drag force, the neutral streaming, and the electric force.