Brownian-like motion of a single dust grain in a radio-frequency plasma discharge comparison of experiments and simulations

Bronwnian-like motion of a single dust-grain in a radio frequency plasma has been studied by different research groups. The rise of the particles temperature above “room temperature” is attributed to e.g. random fluctuations of the particle charge and fluctuations of the electrical field. Additional disturbance might occur due to gas density variations, temporal variation of the particles mass and particle interaction with the illuminating laser light. In addition, a nonoptimal frame rate of the optical diagnostic system and pixel locking can lead to an incorrect estimation of the particle kinetic temperature. Our experiments are conducted in a weakly ionized radio-frequency gas discharge at a low neutral gas pressure and power. A single micron sized spherical particle is trapped in a harmonic-like potential trap in the sheath of the lower driven electrode [1]. Its twodimensional planar motion is recorded with a long-distance microscope and a high-resolution camera. From the measured particle positions we derive the probability density function, the velocity autocorrelation function and the mean squared displacement. We obtain a particle kinetic temperature above 350 K, a neutral gas damping time of about 0.5 sec and a resonance frequency of 1-2 Hz. Anisotropic oscillation of the particle occurs, leading to angle dependent temperatures along the x and y direction in the plane of the recorded images, which can be explained by the presence of an asymmetric horizontal potential trap. Experimental observations are compared with our simulations using md simulations and the Ornstein-Uhlenbeck stochastic process

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.Comment: 4 pages, 3 figures, movies available at http://www.mpe.mpg.de/pke/lane-formation

Bubbles, blobs, and cusps in complex plasmas

Complex plasmas are low temperature plasmas containing microparticles in addition to the ions, electrons and neutral particles. Under most laboratory conditions, the microparticles are charged negatively by collecting electrons from the plasma. They then interact with each other via a screened Coulomb potential, forming, for instance, a crystalline phase When the microparticles are illuminated with a laser sheet, their positions can be recorded with a conventional camera. This allows tracing the motion of individual microparticles, therefore making possible an investigation on the atomistic level. In this paper, we present complex plasma experiments on bubbles and blobs such as those shown i

Shear flow in a three-dimensional complex plasma in microgravity conditions

Shear flow in a three-dimensional complex plasma was experimentally studied in microgravity conditions using Plasmakristall-4 (PK-4) instrument on board the International Space Station (ISS). The shear flow was created in an extended suspension of microparticles by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analyzed and from these, using the Navier-Stokes equation, an upper estimate of the complex plasma's kinematic viscosity was calculated in the range of $0.2$--$6.7~{\rm mm^2/s}$. This estimate is much lower than previously reported in ground-based experiments with 3D complex plasmas. Possible reasons of this difference are discussed.Comment: 5 pages, 4 figure

Three-dimensional structure of a string-fluid complex plasma

Three-dimensional structure of complex (dusty) plasmas was investigated under long-term microgravity conditions in the International-Space-Station-based Plasmakristall-4 facility. The microparticle suspensions were confined in a polarity-switched dc discharge. The experimental results were compared to the results of the molecular dynamics simulations with the interparticle interaction potential represented as a superposition of isotropic Yukawa and anisotropic quadrupole terms. Both simulated and experimental data exhibited qualitatively similar structural features indicating the bulk liquid-like order with the inclusion of solid-like strings aligned with the axial electric field. Individual strings were identified and their size spectrum was calculated. The decay rate of the size spectrum was found to decrease with the enhancement of string-like structural features