35 research outputs found
Two-dimensional complex (dusty) plasma with active Janus particles
A two-dimensional complex plasma containing active Janus particles was studied experimentally. A single
layer of micrometer-sized plastic microspheres was suspended in the plasma sheath of a radio-frequency discharge in
argon at low pressure. The particle sample used was a mixture of regular particles and Janus particles, which
were coated on one side with a thin layer of platinum. Unlike a suspension consisting of regular particles only,
the suspension with inclusion of Janus particles did not form an ordered lattice in the experimental conditions
used. Instead, the particles moved around with high kinetic energy in a disordered suspension. Unexpectedly,
the mean kinetic energy of the particles declined as the illumination laser power was increased. This is
explained by the competition of two driving forces, the photophoretic force and the oppositely directed ion
drag force. The mean-squared displacement of the particles scaled as t^α with α = 2 at small times t indicating
ballistic motion and α = 0.56±0.27 at longer times due to the combined effect of the Janus particle propensity
to move in circular trajectories and external confinement
Dissipative solitary waves in a two-dimensional complex plasma: Amorphous versus crystalline
The propagation of a dissipative soliton was experimentally studied in a two-dimensional binary complex plasma. The crystallization was suppressed in the center of the particle suspension where two types of particles were mixed. The motions of individual particles were recorded using video microscopy, and the macroscopic properties of the solitons were measured in the amorphous binary mixture in the center and in the plasma crystal in the periphery. Although the overall shape and parameters of solitons propagating in amorphous and crystalline regions were quite similar, their velocity structures at small scales as well as the velocity distributions were profoundly distinct. Moreover, the local structure rearranged drastically in and behind the soliton, which was not observed in the plasma crystal. Langevin dynamics simulations were performed, and the results agreed with the experimental observations
Wave spectra of square-lattice domains in a quasi-two-dimensional binary complex plasma
Domains of square lattice have been observed in a quasi-two-dimensional binary complex plasma. The longitudinal and transverse mode of the wave spectra were measured. To compare with the experiment, Langevin dynamics simulations of a binary mixture were carried out, where the non-reciprocal interactions between different species were modeled with a point-wake Yukawa potential. A strong dependence of the wave spectra on the relative magnitude of the point-wake charge is revealed
Heat transport in a flowing complex plasma in microgravity conditions
Heat transport in a three-dimensional complex (dusty) plasma was experimentally studied in microgravity conditions using a Plasmakristall-4
(PK-4) instrument on board the International Space Station (ISS). An extended suspension of microparticles was locally heated by a shear
flow created by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analyzed,
and from these, using a fluid heat transport equation that takes viscous heating and neutral gas drag into account, the complex plasma’s
thermal diffusivity and kinematic viscosity were calculated. Their values are compared with previous results reported in ground-based
experiments with complex plasmas
Machine learning in the study of phase transition of two-dimensional complex plasmas
Machine learning is applied to investigate the phase transition of two-dimensional complex plasmas. The Langevin dynamics simulation is employed to prepare particle suspensions in various thermodynamic states. Based on the resulted particle positions in two extreme conditions, bitmap images are synthesized and imported to a convolutional neural network (ConvNet) as a training sample. As a result, a phase diagram is obtained. This trained ConvNet model has been directly applied to the sequence of the recorded images using video microscopy in the experiments to study the melting
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
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