Microparticle Cloud Research Articles

Observation of Le Sage gravity analog in complex plasma.

Fragmentation of a suspension of micron-sized plastic microparticles and their contraction into dense globules was experimentally obtained in a gas discharge plasma, when the plasma density was deliberately and abruptly increased. The globules took up spherical shapes 0.14-1.1mm in diameters and contained from tens to thousands microparticles. The fragmentation and globule formation appears to be similar to the development of gravitational instability. This process is attributed to the Le Sage's like attraction among microparticles in a dense plasma due to the plasma losses inside a globule hypothesized theoretically in the middle of the 1990s. The key role of plasma flows in the attraction was prominently demonstrated in the same experiment by the distinctly visible disintegration of the globules when we reduced the density of the surrounding plasma to the initial one. Also molecular dynamics simulations of fragmentation of microparticle clouds and globules formation qualitatively resemble typical patterns of the fragmentation and collapse of interstellar nebulae.

Surface Tension of a Cloud of Charged Microparticles in a Gas-Discharge Plasma

Surface Tension of a Cloud of Charged Microparticles in a Gas-Discharge Plasma

Dust Cloud Convections in Inhomogeneously Heated Plasmas in Microgravity

Convection is a phenomenon that often occurs in the presence of temperature gradients. In microgravity, free convection can not occur due to the lack of buoyancy. However, during parabolic flights we observed convections of microparticles in a gas discharge within the cylindrical plasma chamber of the setup PK-4. The microparticles and the plasma were exposed to a thermal gradient. There, the cloud convections and dust waves were observed. Analysis by tracking the microparticles’ trajectories showed that the vortices were induced by thermal creep, a gas flow that commonly occurs in gases with low pressures at inhomogeneously heated solid interfaces. This effect has driven a gas convection which in turn caused the convection of the microparticle cloud.

Experimental Investigation of the High-Rate Interaction of a Cloud of Microparticles with a Metal with the Use of a Rail Accelerator

Experimental Investigation of the High-Rate Interaction of a Cloud of Microparticles with a Metal with the Use of a Rail Accelerator

Ion confinement efficiency and ionization balance in a complex DC discharge plasma

We consider the efficiency of an ion confinement inside a cloud of charged microparticles in a low-pressure DC discharge. To describe the ion confinement efficiency in such complex plasma, we propose the indicators calculated taking into account the processes responsible for the generation, the losses, and the accumulation of ions in a cloud of charged microparticles in a plasma using a fluid model. The efficiency of ion accumulation by a microparticle cloud shows the ratio of the average ion densities in discharge with microparticles and without them. The efficiency of ion accumulation by a microparticle shows the difference of average ion densities in a discharge with microparticles and without them, related to microparticle number density. The specific power costs of the existence of one ion in a microparticle cloud determines the linear power costs of the discharge in a cloud related to the linear number of ions in it. The power efficiency of ion accumulation by a microparticle cloud is defined as a ratio of specific power costs in a discharge without microparticles, to specific power costs of ion existence in a cloud. A strong dependence of indicators on the microparticle number density has been revealed. Inefficient conditions of ion confinement inside a cloud are found. Experimental data on dynamic instabilities of a discharge with microparticles was analyzed. It is found that efficiency of ion confinement is connected with dynamic processes in complex plasma. The limiting microparticle number density is shown to serve as the criterion of the occurrence of plasma instability. Exceeding the limiting microparticle number density results, generally, in the development of dynamic instability of complex plasma, and, in inefficient states, in quenching of the discharge.

Dispersion relation for the dust ionization and dust acoustic waves in the gas discharge complex plasma

A theoretical approach is developed for the dust ionization (DIW) and dust acoustic waves (DAW) propagating in the cloud of microparticles in the low-pressure gas discharge under microgravity conditions. The theory explores the fluid approximation for the microparticle subsystem of complex plasma combined with the kinetic equation for the ions. In the one-dimensional approximation, the wave equation is obtained whose solution defines the dispersion relation for the waves in complex plasma involving the oscillations of microparticles. Obtained dispersion relation unifies both the dust ionization and the dust acoustic waves (DIW and DAW, respectively). According to this dispersion relation, the effect of microparticles on the recombination rate leads to a number of peculiarities. Among them are existence of the minimum frequency, above which the wave propagation is possible, a weak dependence of the DIW wave number on the frequency, and a high phase velocity of DIW as compared to DAW. It is demonstrated that no instability of DIW is possible, whereas DAW can reveal instability under proper conditions. Calculation results correlate with those obtained in a recent experiment.

Spatial distribution of dust density wave properties in fluid complex plasmas.

Complex plasmas consist of microparticles embedded in a low-temperature plasma and allow investigating various effects by tracing the motion of these microparticles. Dust density waves appear in complex plasmas as self-excited acoustic waves in the microparticle fluid at low neutral gas pressures. Here we show that various properties of these waves depend on the position of the microparticle cloud with respect to the plasma sheath and explain this finding in terms of the underlying ion-drift instability. These results may be helpful in better understanding the propagation of dust density waves in complex plasmas and beyond, for instance, in astrophysical dusty plasmas.

Shocks propagate in a 2D dusty plasma with less attenuation than due to gas friction alone

In a dusty plasma, an impulsively generated shock, i.e., blast wave, was observed to decay less than would be expected due to gas friction alone. In the experiment, a single layer of microparticles was levitated in a radio frequency glow-discharge plasma. In this layer, the microparticles were self-organized as a 2D solid-like strongly coupled plasma, which was perturbed by the piston-like mechanical movement of a wire. To excite a blast wave, the wire's motion was abruptly stopped, so that the input of mechanical energy ceased at a known time. It was seen that, as it propagated across the layer, the blast wave's amplitude persisted with little decay. This result extends similar findings, in previous experiments with 3D microparticle clouds, to the case of 2D clouds. In our cloud, out-of-plane displacements were observed, lending support to the possibility that an instability, driven by wakes in the ion flow, provides energy that sustains the blast wave's amplitude despite the presence of gas damping.

Microparticle cloud imaging and tracking for data-driven plasma science

Oceans of image and particle track data encountered in plasma interactions with microparticle clouds motivate development and applications of machine-learning (ML) algorithms. A local-constant-velocity tracker, a Kohonen neural network or self-organizing map, the feature tracking kit, and U-Net are described and compared with each other for microparticle cloud datasets generated from exploding wires, dusty plasmas, and atmospheric plasmas. Particle density and the signal-to-noise ratio have been identified as two important factors that affect the tracking accuracy. Fast Fourier transform is used to reveal how U-Net, a deep convolutional neural network developed for non-plasma applications, achieves the improvements for noisy scenes. Viscous effects are revealed in the ballistic motions of the particles from the exploding wires and atmospheric plasmas. Subdiffusion of microparticles satisfying Δr2∝tk (k=0.84±0.02) is obtained from the dusty plasma datasets. Microparticle cloud imaging and tracking, when enhanced with data and ML models, present new possibilities for plasma physics.

Numerical simulation of the suppression of cellular detonation by inert particles

The simulation of a cellular detonation wave propagating along a hydrogen-air mixture with a cloud of alumina microparticles was carried out. The gas dynamics was modeled by the system of Navier-Stokes equations describing the motion of a viscous compressible heat-conducting gas, with allowance for multicomponent nature of the gas mixture and chemical kinetics. A methodology has been developed for calculating detonation currents in ANSYS Fluent using the reduced kinetics. The reduced kinetics was verified by the size of the detonation cell. The interaction with inert particles with a diameter of 0.3÷100 μm and volume fraction of 10−4÷10−2 was investigated using a continuum approach. Values of the volume fraction resulting in a change of the detonation cell size, weakening of the detonation wave, and detonation suppression have been obtained.

Dust density waves in a dc flowing complex plasma with discharge polarity reversal

We report on the observation of the self-excited dust density waves in the dc discharge complex plasma. The experiments were performed under microgravity conditions in the Plasmakristall-4 facility on board the International Space Station. In the experiment, the microparticle cloud was first trapped in an inductively coupled plasma and then released to drift for some seconds in a dc discharge with constant current. After that, the discharge polarity was reversed. DC plasma containing a drifting microparticle cloud was found to be strongly non-uniform in terms of microparticle drift velocity and plasma emission in accordance with [Zobnin et al., Phys. Plasmas 25, 033702 (2018)]. In addition to that, non-uniformity in the self-excited wave pattern was observed: In the front edge of the microparticle cloud (defined as head), the waves had larger phase velocity than in the rear edge (defined as tail). Also, after the polarity reversal, the wave pattern exhibited several bifurcations: Between each of the two old wave crests, a new wave crest has formed. These bifurcations, however, occurred only in the head of the microparticle cloud. We show that spatial variations of electric field inside the drifting cloud play an important role in the formation of the wave pattern. Comparison of the theoretical estimations and measurements demonstrate the significant impact of the electric field on the phase velocity of the wave. The same theoretical approach applied to the instability growth rate showed agreement between estimated and measured values.

Turbulence in an Auto-Oscillating Complex Plasma

We present an experiment on the development of turbulence in a microparticle cloud in a low-temperature plasma laboratory onboard the International Space Station. First, the complex plasma cloud was stabilized with an alternating electric field. Once the stabilization was switched OFF, a so-called heartbeat instability developed in which the microparticles are oscillating radially. We show that the kinetic energy spectra develop a double cascade once the cloud becomes unstable. A double cascade has been predicted by Kraichnan and Leith for forced 2-D turbulence, but has only seldomly been observed in experiments. We also show that the reduced rates of energy and enstrophy transfer can be used to infer the excitation wavenumber $k_{\text {exc}}$ , in good agreement with $k_{\text {exc}}$ obtained from the energy spectrum.

Latest Results on Complex Plasmas with the PK-3 Plus Laboratory on Board the International Space Station

Complex plasmas are low temperature plasmas that contain microparticles in addition to ions, electrons, and neutral particles. The microparticles acquire high charges, interact with each other and can be considered as model particles for effects in classical condensed matter systems, such as crystallization and fluid dynamics. In contrast to atoms in ordinary systems, their movement can be traced on the most basic level, that of individual particles. In order to avoid disturbances caused by gravity, experiments on complex plasmas are often performed under microgravity conditions. The PK-3 Plus Laboratory was operated on board the International Space Station from 2006 – 2013. Its heart consisted of a capacitively coupled radio-frequency plasma chamber. Microparticles were inserted into the low-temperature plasma, forming large, homogeneous complex plasma clouds. Here, we review the results obtained with recent analyzes of PK-3 Plus data: We study the formation of crystallization fronts, as well as the microparticle motion in, and structure of crystalline complex plasmas. We investigate fluid effects such as wave transmission across an interface, and the development of the energy spectra during the onset of turbulent microparticle movement. We explore how abnormal particles move through, and how macroscopic spheres interact with the microparticle cloud. These examples demonstrate the versatility of the PK-3 Plus Laboratory.

Capacitively coupled rf discharge with a large amount of microparticles: Spatiotemporal emission pattern and microparticle arrangement.

The effect of micron-sized particles on a low-pressure capacitively coupled rf discharge is studied both experimentally and using numerical simulations. In the laboratory experiments, microparticle clouds occupying a considerable fraction of the discharge volume are supported against gravity with the help of the thermophoretic force. The spatiotemporally resolved optical emission measurements are performed with different arrangements of microparticles. The numerical simulations are carried out on the basis of a one-dimensional hybrid (fluid-kinetic) discharge model describing the interaction between plasma and microparticles in a self-consistent way. The study is focused on the role of microparticle arrangement in interpreting the spatiotemporal emission measurements. We show that it is not possible to reproduce simultaneously the observed microparticle arrangement and emission pattern in the framework of the considered one-dimensional model. This disagreement can be attributed to the two-dimensional effects (e.g., radial diffusion of the plasma components) or to the lack of the proper description of the sharp void boundary in the frame of fluid approach.

Measurement of the speed of sound by observation of the Mach cones in a complex plasma under microgravity conditions

We report the first observation of the Mach cones excited by a larger microparticle (projectile) moving through a cloud of smaller microparticles (dust) in a complex plasma with neon as a buffer gas under microgravity conditions. A collective motion of the dust particles occurs as propagation of the contact discontinuity. The corresponding speed of sound was measured by a special method of the Mach cone visualization. The measurement results are incompatible with the theory of ion acoustic waves. The estimate for the pressure in a strongly coupled Coulomb system and a scaling law for the complex plasma make it possible to derive an evaluation for the speed of sound, which is in a reasonable agreement with the experiments in complex plasmas.

Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields.

When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, the size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in detail. In experiments, a dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50 μm) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces-the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter α≥2), the Brownian motion seems not to affect the cloud behavior.

Autowaves in a dc complex plasma confined behind a de Laval nozzle

Experiments to explore stability conditions and topology of a dense microparticle cloud supported against gravity by a gas flow were carried out. By using a nozzle-shaped glass insert within the glass tube of a dc discharge plasma chamber a weakly ionized gas flow through a de Laval nozzle was produced. The experiments were performed using neon gas at a pressure of 100 Pa and melamine-formaldehyde particles with a diameter of 3.43 μm. The capturing and stable global confining of the particles behind the nozzle in the plasma were demonstrated. The particles inside the cloud behaved as a single convection cell inhomogeneously structured along the nozzle axis in a tube-like manner. The pulsed acceleration localized in the very head of the cloud mediated by collective plasma-particle interactions and the resulting wave pattern were studied in detail.

Light-field camera snaps dusty plasma

A novel camera that can refocus images after they have been captured proves suitable for observing clouds of microparticles.

Microparticles deep in the plasma sheath: Coulomb “explosion”

A cloud of microparticles was trapped deep in the sheath of a radio-frequency (rf) discharge, very close to the lower (grounded) electrode of the plasma chamber. This was achieved by employing a specifically designed rf-driven segment integrated in the lower electrode, which provided an additional confinement compressing the cloud to a very high density. After switching the rf-driven segment off, the cloud “exploded” due to mutual interparticle repulsion. By combining a simple theoretical model with different numerical simulation methods, some basic properties of complex plasmas in this highly non-equilibrium regime were determined.

Simulations of one- and two-dimensional complex plasmas using a modular, object-oriented code

In a complex plasma, charged microparticles (“dust”) are added to a background of ions, electrons, and neutral particles. This dust fully interacts with the surrounding plasma and self-consistently alters the plasma environment leading to the emergence of new plasma behavior. Numerical tools that complement experimental investigations can provide important insights into the properties of complex plasmas. This paper discusses a newly developed code, named DEMON (dynamic exploration of microparticle clouds optimized numerically), for simulating a complex plasma. The DEMON code models the behavior of the charged particle component of a complex plasma in a uniform plasma background. The key feature of the DEMON code is the use of a modular force model that allows a wide variety of experimental configurations to be studied without varying the core code infrastructure. Examples of the flexibility of this modular approach are presented using examples of one- and two-dimensional complex plasmas.