K. R. Sütterlin

Dynamics of Lane Formation in Driven Binary Complex Plasmas

The dynamical onset of lane formation is studied in experiments with binary complex plasmas under microgravity conditions. Small microparticles are driven and penetrate into a cloud of big particles, revealing a strong tendency towards lane formation. The observed time-resolved lane-formation process is in good agreement with computer simulations of a binary Yukawa model with Langevin dynamics. The laning is quantified in terms of the anisotropic scaling index, leading to a universal order parameter for driven systems.

Direct measurement of the speed of sound in a complex plasma under microgravity conditions

We present a direct measurement of the speed of sound in a three-dimensional complex plasma —a room-temperature plasma that contains micrometer-sized particles as fourth component. In order to obtain an undisturbed system, the setup was placed under microgravity conditions on board the International Space Station. The speed of sound was measured with the help of Mach cones excited by a supersonic probe particle moving through the extended particle cloud at Mach numbers M3. We use the Mach cone relation to infer the particle charge and compare with that predicted by standard theories. In addition, we compare our results with a numerical simulation. In both experiment and simulation, we observe a double Mach cone structure.

Shear flow instability at the interface among two streams of a highly dissipative complex plasma

The shear instability was experimentally studied in a highly dissipative cloud of complex plasma. The cloud of microparticles formed a toroidal vortex with poloidal flow and a stagnation zone in the center. The flow was studied at the kinetic level by resolving the trajectories of the individual microparticles. Hydrodynamic quantities such as flow velocity, vorticity and enstrophy density were determined. The effects of interfacial surface tension were explored. It was found that the two-stream interface between flow and stagnation zone breaks down into a well-developed multi-stream network due to Rayleigh-Taylor–type perturbations.

Experimental investigation on lane formation in complex plasmas under microgravity conditions

A series of experiments dedicated to probing the phenomenon of lane formation in binary complex plasmas over a broad range of parameters has been performed with the PK-3 Plus laboratory on board the International Space Station (ISS) under microgravity conditions. In the experiments, bunches of small particles were driven through a background of big particles. We show that the dynamics of lane formation varies considerably with the density of the background and the size ratio between small and big particles. For consecutive injections of small particles a memory effect of the previous penetration was discovered for the first time. This memory effect was investigated quantitatively with respect to the structure formation and the penetration speed. We show that the memory effect in lane formation is linear. In addition, we studied the crossover from lane formation to phase separation driven by the nonadditive interactions between small and big particles. We found that during this transition the small penetrating particles effectively cage the background particles.

Model experiment for studying lane formation in binary complex plasmas

We present the first experimental realisation on lane formation in binary complex plasmas under gravity conditions. The amount of penetrating particles can be controlled. The experiment can be operated with stable conditions continuously, which allows to study steady state. The driving force is independent of time and position and the background density is homogeneous and isotropic. This provides an ideal model system for comparison with numerical experiments and observations in colloidal suspensions. The experiment setup is based on PK-3 Plus laboratory operated on the International Space Station. Gravitation is compensated by thermophoretic force and penetrating particles are controlled via a toroidal vortex with poloidal flow. The evolution of lane formation along the penetration direction is illustrated by lane order parameter.

Lane formation in binary complex plasmas: role of non-additive interactions and initial configurations

In this letter, we study the influence of non-additive interactions on lane formation, using Langevin dynamics simulations. Lane formation and positive non-additivity have recently been observed in binary complex plasmas on board the International Space Station (ISS). Positive non-additivity of particle interactions is known to stimulate phase separation (demixing), but its effect on lane formation is unknown. We show that there is a non-additivity–stimulated crossover from the normal laning mode to a demixing-dominated laning mode. To analyze this crossover on the individual particle level we applied a very sensitive order parameter for lane formation based on anisotropic scaling indices. Extensive numerical simulations enabled us to identify a critical value of the non-additivity parameter Δ for the crossover. In addition the simulations revealed that the dynamics of lane formation is strongly influenced by the exact spatial configurations at the very moment of contact between two different complex plasmas.

Initial stages in phase separation of binary complex plasmas: Numerical experiments

Numerical experiments are performed to investigate the initial stages in phase separation induced by interaction non-additivity Δ in binary complex plasmas. A characteristic length scale obtained from time-dependent pair correlation functions is used to monitor the domain growth. It is found that the domain growth follows a power law with an exponent α of around 1/3, which is in a good agreement with the Lifshitz-Slyozov growth law for the initial diffusive regime of phase separation. It is also found that α is almost independent of Δ as long as the system is within the spinodal region. The coupling strength is also found to have almost no influence on α, unless the coupling is so large that the background large-particle phase starts crystallizing.

Initial stages in phase separation of binary complex plasmas: numerical experiments

Numerical experiments are employed to study the initial stages in phase separation induced by interaction non‐additivity in binary complex plasmas (plasmas consisting two different sized dust particles). A characteristic length scale obtained from time‐dependent pair correlation functions is used to monitor the domain growth. It is found that the domain growth follows a power law with an exponent α of about 1/3, which is in a good agreement with the initial diffusive regime of phase separation.

New Directions of Research in Complex Plasmas on the International Space Station

PK‐3 Plus is the second generation laboratory for investigations of complex plasmas under microgravity conditions on the International Space Station. Compared to its pre‐cursor PKE‐Nefedov, operational 2001–2005, it has an advanced hardware and software. Improved diagnostics and especially a much better homogeneity of the complex plasma allow more detailed investigations, helping to understand the fundamentals of complex plasmas. Typical investigations are performed to observe the structure of homogeneous and isotropic complex plasmas and instabilities occurring at high particle densities. In addition, the new setup allows the tuning of the interaction potential between the microparticles by using external ac electric fields. Thus, we are able to initiate electrorheological phenomena in complex plasma fluids in the PK‐3 Plus laboratory, and observe the phase transition from a normal fluid to a string fluid state at the individual particle level for the first time. Such new possibilities open up new direct...