ICPAL · Investigation of Cooperative Phenomena on the Atomistic Level

Investigation of Cooperative Phenomena on the Atomistic Level

Summary of project context and objectives Physics on small scales is becoming more and more important. An especially interesting scale is the mesoscale - the scale on which the individual movement of particles transforms into cooperative movement. Of course, in ordinary fluids, this scale is out of reach of direct visualization, as the movement of individual molecules usually cannot be observed. One system that is well suited for the study of effects on the mesoscale is a “complex” or “dusty” plasma. Complex plasmas consist of a plasma – an ionized gas with ions, lectrons and neutralparticles – in which micrometer sized particles (“dust”) are embedded. The microparticles collect electrons and ions from the surrounding gas and thus acquire a high charge. They interact with each other and with the surrounding plasma. The goal of the project ICPAL was to study effects on the mesoscale occurring in complex plasmas, with an emphasis on the onset of cooperative behaviour. To do so, we employed a mix of computer simulations and experiments. The simulations were developed at Prof. David Graves’ group at the University of California (UC) at Berkeley, CA, USA. The experiments were performed in PK-3 Plus laboratory on board the International Space Station and on ground, at Dr. Hubertus Thomas’ group at the Max-Planck-Institute for extraterrestrial Physics in Garching, Germany (now at the German Aerospace Center (DLR) in Oberpfaffenhofen, Germany). The joint Russian-German PK-3 Plus microgravity laboratory was deorbited in 2013, but experimental data is still available for analysis. Description of work performed and main results achieved During the outgoing phase at UC Berkeley we developed a computer model of the complex plasma. In detail, we adapted a two-dimensional hybrid fluid/analytical plasma model that was previously developed at UC Berkeley (Kawamura, Graves, & Lieberman, 2011) to the geometry of the plasma chamber used in the PK-3 Plus laboratory. The output of the plasma model is then used to calculate the forces acting on the microparticles in the plasma chamber. The microparticles themselves are simulated using the open source Molecular Dynamics (MD) code LAMMPS (Plimpton, 1995), which we extended for our purpose. Using this computer model, we were able to reproduce many experimental findings, for instance the formation of vortices, lane formation, Mach cones, and vortex formation (Schwabe & Graves, 2013). We studied in detail the collective effects of microparticles inside a cloud displaying vortex motion. These vortices are an ideal test case for studying the onset of collective effects on the kinetic level. One of the most intriguing effects of fluid physics is turbulence. Complex plasmas are ideal to study turbulence, as the particles that transmit the interaction can be visualized directly. We demonstrated that signatures of turbulence are present in complex plasmas (Schwabe et al., 2014). We compared the velocity structure functions obtained via our simulations to those redicted by classic Kolmogorov theory (Kolmogorov, 1991) and found excellent agreement. This result is significant as it demonstrates the emergence of turbulent pulsations at low Reynolds numbers at the kinetic level – a result that has applications in many other physical, biophysical and biological applications (Schwabe et al., 2014). Final results and their potential impact and use In summary, we studied complex plasmas using a hybrid simulation of the plasma bulk, sheath and microparticles. We reproduced many experimental results, demonstrating how elementary processes in complex plasmas lead to cooperative behaviour of the microparticles. In particular, we first showed that in the simulation, signatures of turbulence are present. Using the analysis methodology developed for the simulation data, we demonstrated that turbulent behavior occurs in experimental complex plasmas as well, in particular inside waves excited by the heartbeat instability. Thus, the combination of experiments and simulations allowed us to study the onset and manifestation of cooperative behaviour, with an emphasis on the new research direction of turbulence in complex plasmas. Bibliography Kawamura, E., Graves, D. B., & Lieberman, M. A. (2011). Plasma Sourc. Sci. Techn. (20), 035009. Kolmogorov, A. N. (1991). Proc. Roy. Soc. A (434), 15. Plimpton, S. (1995). J. Comp. Phys. (117), 1. Schwabe, M., & Graves, D. B. (2013). Phys. Rev. E (88), 023101. Schwabe, M., et al. (2014). Phys. Rev. Lett. (112), 115002. Zhdanov, et al. (2015). EPL (110), 35001. Wider societal implications While the research done in this project is basic research, it paves the way towards using complex plasmas as model systems for turbulence. Turbulence is one of the remaining unsolved problems of modern theoretical physics and is usually studied with tracer particles or simulations. Complex plasmas allow studying turbulence on the level of the carriers of the turbulent interactions. This may make it possible to study in detail the energy and vorticity transfer across scales and space, which might be an important leap towards finally solving the problem of turbulence.

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