Microcanonical studies of turbulence in complex plasmas under microgravity conditions

Abstract

A complex plasma (CP) consists of charged micrometer-sized particles embedded in a low-temperature plasma. The dust particles interact electrostatically with the mean particle separation being typically 100 times the particle size. Hence these systems are optically thin and can be probed three-dimensionally. CPs are (among others) ideal model systems for studying fluid dynamics and turbulence on the single particle level. Here, we report on simulations of vortices in a typical laboratory setup (PK3-Plus chamber), as it was used aboard the International Space Station (ISS). Vortices in complex plasmas are an ideal test bed for studying the onset of turbulence and collective effects on the microcanonical level. Specifically, driven turbulence in complex Plasmas turns out to be of low Reynolds numbers. The opportunity to study turbulence in fluids with low Reynolds numbers at the kinetic level is especially interesting in many other physical, biophysical, and industrial applications as diverse as water flow through porous media, viscoelastic liquid polymer flows, chaotic mixers, self-propelled stochastic microdevices, and bacterial quasiturbulence. The velocity structure functions and the energy and enstrophy spectra of our simulations show that vortex flow turbulence is present which is in essence of the classical Kolmogorov type [1]. Further, we report on a complex plasma under microgravity conditions which is autooscillating due to a so-called heartbeat instability and contains quasi-solitary wave ridges i.e. oscillons. We demonstrate that this system can serve as a nearly ideal model system to mimic weak Kolmogorov-Zakharov-type wave turbulence. The slopes of the structure functions agree reasonably well with power laws assuming extended selfsimilarity. The energy spectrum displays, however, multiple cascades, which we attribute to the influence of friction, the heartbeat instability and a modulational instability [2]. The onset of turbulence is again studied in detail by e.g. analyzing the changes in the energy spectrum [3]. Finally, we give a brief outlook on current and future research on fluid dynamics and turbulence using data from the current experiment aboard the ISS (PK4) and from future plasma chambers (e.g. PlasmaLab). We further briefly point out, how novel nonlinear analysis techniques (e.g. phase walk statistics [4]) can give a refined microcanonical view of the dynamics of many particle systems showing turbulent behaviour. [1] M. Schwabe, S. Zhdanov, C. Raeth, D. B. Graves, H. M.Thomas and G. E. Morfill, PRL, 112, 115002 (2014). [2] S. Zhdanov, M. Schwabe, C. Raeth, H. M. Thomas and G. E.Morfill, EPL 110, 35001 (2015). [3] M. Schwabe, S. Zhdanov, C. Raeth, H. M. Thomas and G. E.Morfill, PRL (submitted) [4] K. Schreiber, H. Modest and C. Raeth, Sci. Rep. (in preparation)