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

TL;DR

This paper experimentally investigates how complex plasma flows form dynamic structures around an obstacle, revealing phenomena like dust voids, ring shapes, bow shocks, and wave patterns, influenced by flow speed and electric biases.

Contribution

It provides the first detailed experimental analysis of dynamical structure formation in flowing complex plasma past an obstacle, including shock and wave phenomena.

Findings

Formation of dust voids and ring structures around the obstacle

Observation of bow shocks at supersonic dust flow speeds

Detection of wave structures resembling vortex streets

Abstract

We report the experimental observation of dynamical behavior of flowing complex plasma past a spherical obstacle. The experiment has been carried out in a $\Pi$-shaped DC glow discharge experimental device using kaolin particles as the dust component in a background of Argon plasma. A stationary dust cloud is formed by maintaining the pumping speed and gas flow rate. A spherical obstacle vertically mounted on the cathode tray acts as an obstacle to the flow of dust particles. The controlled dust flow is generated by reducing the mass flow of the neutrals through a mass flow controller. 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 far particles are attracted towards the floating obstacle and reflected back when they have arrived at a minimum 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 dust fluid around a negatively biased obstacle, a bow shock is formed on the upstream side of the sphere, while the generation of wave structures is observed on the downstream side for a particular range of flow velocities. Reynolds numbers in this case is estimated as $R_e \gtrsim 50$. This wave structure reminds of the beginning of the formation of a Von-K\'arm\'an vortex street. A physical picture for the observed structure based on ion-drag, neutral streaming and electric forces is discussed.