Slowing of acoustic waves in electrorheological and string-fluid complex plasmas

M Schwabe and SA Khrapak and SK Zhdanov and MY Pustylnik and C Rath and M Fink and M Kretschmer and AM Lipaev and VI Molotkov and AS Schmitz and MH Thoma and AD Usachev and AV Zobnin and GI Padalka and VE Fortov and OF Petrov and HM Thomas, NEW JOURNAL OF PHYSICS, 22, 083079 (2020).

DOI: 10.1088/1367-2630/aba91b

The PK-4 laboratory consists of a direct current plasma tube into which microparticles are injected, forming a complex plasma. The microparticles acquire many electrons from the ambient plasma and are thus highly charged and interact with each other. If ion streams are present, wakes form downstream of the microparticles, which lead to an attractive term in the potential between the microparticles, triggering the appearance of microparticle strings and modifying the complex plasma into an electrorheological form. Here we report on a set of experiments on compressional waves in such a string fluid in the PK-4 laboratory during a parabolic flight and on board the International Space Station. We find a slowing of acoustic waves and hypothesize that the additional attractive interaction term leads to slower wave speeds than in complex plasmas with purely repulsive potentials. We test this hypothesis with simulations, and compare with theory.

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