62 research outputs found
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 --. 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
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