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

M Schwabe,A S Schmitz,A D Usachev,M H Thoma,S K Zhdanov,V E Fortov,O F Petrov,V I Molotkov,G I Padalka,M Y Pustylnik,S A Khrapak,M Fink,A M Lipaev,A V Zobnin,M Kretschmer,C Räth,H M Thomas

doi:10.1088/1367-2630/aba91b

Abstract

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.

Highlights

Electrorheological (ER) and magnetorheological (MR) fluids change their rheology drastically when electric resp. magnetic fields are applied [1,2,3,4]
We studied the propagation of waves in weightless complex plasmas forming strings using experiments during a parabolic flight and on board the International Space Station
Strings form in a direct current (DC) plasma with unipolar wakes

Summary

Introduction

Electrorheological (ER) and magnetorheological (MR) fluids change their rheology drastically when electric resp. magnetic fields are applied [1,2,3,4]. When an external field is applied, dipoles are induced, and the particles arrange in strongly coupled chains (‘strings’), sheets, or compressed crystalline structures. This leads to a reversible increase in effective viscosity. ER fluids transmit sound better (i.e., with less loss) when a higher voltage is applied [8], and the velocity of sound in ER fluids increases with electric field strength when the longitudinal wave propagates in parallel with the electric field [9,10,11]. In an MR slurry of iron particles in glycerine, two longitudinal modes of sound have been identified, with the speed of sound depending on the strength of the applied magnetic field [7, 12]. A ‘second’ sound mode can occur as a wave of density of collective quasiparticle excitations [13]

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Discussion

Conclusion