Bow shock formation in a dusty plasma flowing around an obstacle under microgravity

Dust flows around an obstacle in three-dimensionally extended dust clouds have been investigated on parabolic flights. As the obstacle, a tungsten wire has been installed in the midplane between the electrodes of a parallel-plate radio frequency discharge. A periodic dust motion was generated by superimposing a low-frequency modulation on the electrodes, shifting the dust cloud between the electrodes. Due to the periodic nature of the dust motion, several shocks could be observed at varying dust densities. When the dust flow was transonic or supersonic, bow shocks were formed upstream of the wire and propagated away from it at a constant speed. However, at the dust densities needed for the shock formation, dust-density waves also occurred and the two phenomena often intermingled. The role of the streaming ions in driving the shock is therefore discussed. At carefully chosen parameters, it was possible to observe only the bow shock while suppressing the waves. It is reasoned that the shock is excited independently from dust-density waves by a density increase upstream of the obstacle.