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.
The PK-3 Plus laboratory on board the International Space Station is used to study the interaction between metallic spheres and a complex plasma. We show that the metallic spheres significantly ...affect both the local plasma environment and the microparticle dynamics. The spheres charge under the influence of the plasma and repel the microparticles, forming cavities surrounding the spheres. The size of the cavity around a sphere is used to study the force balance acting on microparticles at the cavity edge. We show that the ion drag force and pressure force from other microparticles balances with the electric force acting from the sphere to within 20%. At intermediate distances from the sphere surface, the interaction between the microparticles and the metallic spheres is attractive due to the drag force stemming from the ions which are moving towards the highly charged spheres. The spheres thus strongly affect the plasma fluxes. This modification of the plasma flux can lead to an effective surface tension acting on the microparticles, and to the excitation of dust-density waves near the spheres, as the local electric field crosses a threshold.
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.
We propose a low-density discharge plasma model that takes into account the impact of oxygen admixture in typical conditions of complex (dusty) plasmas. Numerical simulations based on this model show ...that the concentration of negative ions turns out to be very high, and they play an important role in the overall kinetics in this particular range of plasma conditions. The ambipolar diffusion electric field drags these negative ions into the center of the plasma. The density of negative ions is high enough to push the negatively charged dust component out of the center, both by weakening the radial electric field and by increasing the thermophoretic force. This phenomenon was observed in the published experiment and qualitatively supports the proposed model. Additionally, the proposed model allows an alternative explanation of the experiment.
Experimental observations are presented of unusual ('abnormal') microparticles, having trajectories very different from those of the majority of microparticles in the particle cloud in PK-3 Plus ...chamber (on board the International Space Station). To quantitatively study the mechanism driving this 'irregular' motion, we performed a series of experiments with quasi-two-dimensional complex plasmas in a modified GEC RF reference cell in a ground-based laboratory. The results show that the average particle velocity increases with illumination laser power, particularly for the 'abnormal' particles. We suggest that the photophoretic force provides an important contribution to the drive, and briefly discuss the mechanism leading to this effect. Optical microscopy results indicate that the 'abnormal' particles could be those having deformations or defects on their surface.
We report the experimental discovery of "electrorheological (ER) complex plasmas," where the control of the interparticle interaction by an externally applied electric field is due to distortion of ...the Debye spheres that surround microparticles (dust) in a plasma. We show that interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments, as well as molecular dynamics simulations, show a phase transition from an isotropic to an anisotropic (string) plasma state as the electric field is increased.
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