We report the first observation of the Mach cones excited by a larger microparticle (projectile) moving through a cloud of smaller microparticles (dust) in a complex plasma with neon as a buffer gas under microgravity conditions. A collective motion
of the dust particles occurs as propagation of the contact discontinuity. The corresponding speed of sound was measured by a special method of the Mach cone visualization. The measurement results are incompatible with the theory of ion acoustic waves. The estimate for the pressure in a strongly coupled Coulomb system and a scaling law for the complex plasma make it possible to derive an evaluation for the speed of sound, which is in a reasonable agreement with the experiments in complex plasmas.
We employ the approximation of overlapped scattering potentials of charged dust particles exposed to streaming ions to deduce the equation of state for a stationary dust cloud in the radio frequency discharge apart from the void dust boundary. The ob
tained equation defines the potential of a dust particle as a function of the ion number density, the mass of a carrier gas atom, and the electron temperature. A scaling law that relates the particle number density to the particle radius and electron temperature in different systems is formulated. Based on the proposed approach the radius of a cavity around a large particle in the bulk of a cloud is estimated. The results of calculation are in a reasonable agreement with the experimental data available in literature.
Subsonic motion of a large particle moving through the bulk of a dust crystal formed by negatively charged small particles is investigated using the PK-3 Plus laboratory onboard the International Space Station. Tracing the particle trajectories show
that the large particle moves almost freely through the bulk of plasma crystal, while dust particles move along characteristic alpha-shaped pathways near the large particle. In the hydrodynamic approximation, we develop a theory of nonviscous dust particles motion about a large particle and calculate particle trajectories. A good agreement with experiment validates our approach.
