Measurement of the speed of sound by observation of the Mach cones in a   complex plasma under microgravity conditions
  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. 
D. I. Zhukhovitskii (1), V. E. Fortov (1), V. I. Molotkov (1), A. M.   Lipaev (1), V. N. Naumkin (1), H. M. Thomas (2), A. V...
physics.plasm-ph