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We report observation of the dust ionization waves (DIWs) excited by an external oscillating electric field on the Plasma Kristall-4 facility under microgravity conditions. It is shown that at the smallest excitation amplitude, the waves are linear, and the dispersion relation can be deduced from the experimental data. The microparticle oscillations are represented as a superposition of two longitudinal waves propagating in the opposite directions. In the investigated range of excitation frequency, the wavenumber is not directly proportional to the frequency, and the phase velocity is almost proportional to the frequency. We propose an interpretation of DIW assuming that the microparticle effect on the recombination rate rather than the microparticle subsystem compressibility is responsible for the wave propagation. The calculated phase velocity of DIW is compatible with the experimental one. [ABSTRACT FROM AUTHOR]

The PK-3 Plus laboratory onboard the International Space Station is used to form complex plasma with a liquidlike particle subsystem in a metastable state and to observe the propagation of crystallization fronts corresponding to the surfaces of crystal domains. We propose the 'axis' algorithm of solidlike particles identification, which makes it possible to isolate different domains and their surfaces as well. Determination of the three-dimensional front velocity is based on its definition implying that there exists a small area of the domain surface propagating along some line perpendicularly to it. The velocity measured in this way is an important characteristic of the plasma crystallization kinetics. It proves to be almost independent of time and the direction of front propagation and amounts to ca. 60–80 μm s−1. [ABSTRACT FROM AUTHOR]

Complex (dusty) plasmas are composed of weakly ionised gas and charged microparticles and represent the plasma state of soft matter. Due to the 'heavy' component - the microparticles - and the low density of the surrounding medium, the rarefied gas and plasma, it is necessary to perform experiments under microgravity conditions to cover a broad range of experimental parameters which are not available on ground. The investigations have been performed onboard the International Space Station (ISS) with the help of the 'Plasma Crystal-3 Plus' (PK-3 Plus) laboratory. It was perfectly suited for the formation of large stable liquid and crystalline systems and provided interesting insights into processes like crystallisation and melting, laning in binary mixtures, electrorheological effects due to ac electric fields and projectile interaction with a strongly coupled complex plasma cloud. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a direct measurement of the speed of sound in a three-dimensional complex plasma --a room-temperature plasma that contains micrometer-sized particles as fourth component. In order to obtain an undisturbed system, the setup was placed under microgravity conditions on board the International Space Station. The speed of sound was measured with the help of Mach cones excited by a supersonic probe particle moving through the extended particle cloud at Mach numbers M ≲ 3. We use the Mach cone relation to infer the particle charge and compare with that predicted by standard theories. In addition, we compare our results with a numerical simulation. In both experiment and simulation, we observe a double Mach cone structure. [ABSTRACT FROM AUTHOR]

Oscillation of particles in a dust crystal formed in a low-pressure radio-frequency gas discharge under microgravity conditions is studied. Analysis of experimental data obtained in our previous study shows that the oscillations are highly isotropic and nearly homogeneous in the bulk of a dust crystal; oscillations of the neighboring particles are significantly correlated. We demonstrate that the standard deviation of the particle radius vector along with the local particle number density fully define the coupling parameter of the particle subsystem. The latter proves to be of the order of 100, which is two orders of magnitude lower than the coupling parameter estimated for the Brownian diffusion of particles with the gas temperature. This means significant kinetic overheating of particles under stationary conditions. A theoretical interpretation of the large amplitude of oscillation implies the increase of particle charge fluctuations in the dust crystal. The theoretical estimates are based on the ionization equation of state for the complex plasma and the equation for the plasma perturbation evolution. They are shown to match the results of experimental data processing. Estimated order of magnitude of the coupling parameter accounts for the existence of the solid-liquid phase transition observed for similar systems in experiments. [ABSTRACT FROM AUTHOR]

We propose a method of determination of the dust particle spatial distribution in dust clouds that form in three-dimensional (3D) complex plasmas under microgravity conditions. The method utilizes the data obtained during the 3D scanning of a cloud, and it provides reasonably good accuracy. Based on this method, we investigate the particle density in a dust cloud realized in gas discharge plasma in the PK-3 Plus setup onboard the International Space Station. We find that the treated dust clouds are both anisotropic and inhomogeneous. One can isolate two regimes in which a stationary dust cloud can be observed. At low pressures, the particle density decreases monotonically with the increase of the distance from the discharge center; at higher pressures, the density distribution has a shallow minimum. Regardless of the regime, we detect a cusp of the distribution at the void boundary and a slowly varying density at larger distances (in the foot region). A theoretical interpretation of the obtained results is developed that leads to reasonable estimates of the densities for both the cusp and the foot. The modified ionization equation of state, which allows for violation of the local quasineutrality in the cusp region, predicts the spatial distributions of ion and electron densities to be measured in future experiments. [ABSTRACT FROM AUTHOR]

Subsonic motion of a large particle moving through the bulk of a dust crystal formed by negatively charged small particles is investigated using the PK-3 Plus laboratory onboard the International Space Station. Tracing the particle trajectories shows that the large particle moves almost freely through the bulk of the plasma crystal, while dust particles move along characteristic α-shaped pathways near the large particle. In the hydrodynamic approximation, we develop a theory of nonviscous dust particle motion about a large particle and calculate particle trajectories. Good agreement with experiment validates our approach. [ABSTRACT FROM AUTHOR]

Phase behavior of large three-dimensional (3D) complex plasma systems under microgravity conditions onboard the International Space Station is investigated. The neutral gas pressure is used as a control parameter to trigger phase changes. Detailed analysis of structural properties and evaluation of three different melting-freezing indicators reveal that complex plasmas can exhibit melting by increasing the gas pressure. Theoretical estimates of complex plasma parameters allow us to identify main factors responsible for the observed behavior. The location of phase states of the investigated systems on a relevant equilibrium phase diagram is estimated. Important differences between the melting process of 3D complex plasmas under microgravity conditions and that of flat 2D complex plasma crystals in ground based experiments are discussed. [ABSTRACT FROM AUTHOR]

Using two-dimensional (2D) complex plasmas as an experimental model system, particle-resolved studies of flame propagation in classical 2D solids are carried out. Combining experiments, theory, and molecular dynamics simulations, we demonstrate that the mode-coupling instability operating in 2D complex plasmas reveals all essential features of combustion, such as an activated heat release, two-zone structure of the self-similar temperature profile ("flame front"), as well as thermal expansion of the medium and temperature saturation behind the front. The presented results are of relevance for various fields ranging from combustion and thermochemistry, to chemical physics and synthesis of materials. [ABSTRACT FROM AUTHOR]

Freezing and melting of large three-dimensional complex plasmas under microgravity conditions is investigated. The neutral gas pressure is used as a control parameter to trigger the phase changes: Complex plasma freezes (melts) by decreasing (increasing) the pressure. The evolution of complex plasma structural properties upon pressure variation is studied. Theoretical estimates allow us to identify the main factors responsible for the observed behavior.

We describe the first observation of a void closure in complex plasma experiments under microgravity conditions performed with the Plasma-Kristall (PKE-Nefedov) facility on board the International Space Station. The void--a grain-free region in the central part of the discharge where the complex plasma is generated--has been formed under most of the plasma conditions and thought to be an inevitable effect. However, we demonstrate in this Letter that an appropriate tune of the discharge parameters allows the void to close. This experimental achievement along with its theoretical interpretation opens new perspectives in engineering new experiments with large quasi-isotropic void-free complex plasma clouds in microgravity conditions.

A jump of dust density propagating through the dusty plasma structure has been observed. To excite the disturbance an impulse of axial magnetic field to the dusty plasma in a dc glow discharge striation has been applied. This impulse resulted in the dynamical stretching of the dusty plasma structure. During the reconstruction of the structure a ramp-shaped perturbation of dust density appeared. The perturbation was steepening and formed into a dust-acoustic shock. The anomalously high shock compression is observed.