Modification of wave velocity in a string fluid

Abstract

Complex or dusty plasmas consist of microparticles embedded in a low-temperature plasma. The microparticles acquire high charges by collecting plasma particles and strongly interact with each other. Since they are suspended in a low pressure background gas, the damping of their movement is much weaker than, for instance, in colloids, so that studies of complex plasmas complement those of colloids. Here we present complex plasma experiments performed in the PK-4 Laboratory on board the International Space Station. The heart of this laboratory is formed by a U-shaped glass tube in which a direct current (DC) plasma is produced with a driven and a grounded electrode. The microparticles can be trapped by switching the polarity of the driven electrode at a time scale that is much smaller than that of the particle dynamics. The polarity switching also induces ion fluxes which follow the electric field almost instantaneously. These ion fluxes cause the formation of space charges downstream from the microparticles, which lead to the formation of microparticle strings, analogous to strings forming in electrorheological fluids. Here, we numerically demonstrate that the velocity of waves moving through this string fluid depends on the Mach number of the ions. For this purpose, we perform a Molecular-Dynamics simulation of the (negatively charged) microparticle movement with a potential that includes, in addition to the usual interaction via Yukawa potential, a positive space charge in downstream direction mimicking the wake formation. We excite a shock wave travelling through the system by pushing it on one side with a piston, and observe the velocity of the travelling wave. We compare the results of this simulation to a slow wave mode observed in experiments in the PK-4 Laboratory: In these experiments, the microparticles were trapped in the discharge and formed a string fluid. Self-excited waves appeared which had an unusual waveform and propagated slowly compared with the usual dust acoustic wave mode.