Gas-liquid crossover in the Lennard-Jones system.

It is demonstrated that the crossover between gas- and liquid-like regions on the phase diagram of the Lennard-Jones system occurs at a fixed value of the density divided by its value at the freezing point, ρ/ρfr ≃ 0.35. This definition is consistent with other definitions proposed recently. As a result, a very simple practical expression for the gas-to-liquid crossover line emerges.

Freezing Temperature and Density Scaling of Transport Coefficients.

It is demonstrated that the freezing density scaling of transport coefficients in fluids, similar to the freezing temperature scaling, originates from the quasi-universal excess entropy scaling approach proposed by Rosenfeld. The freezing density scaling has a considerably wider applicability domain on the phase diagram of Lennard-Jones and related systems. As an illustration of its predictive power, we show that it reproduces with an excellent accuracy the shear viscosity coefficients of saturated liquid argon, krypton, xenon, and methane.

Excess entropy and Stokes-Einstein relation in simple fluids.

The Stokes-Einstein (SE) relation between the self-diffusion and shear viscosity coefficients operates in sufficiently dense liquids not too far from the liquid-solid phase transition. By considering four simple model systems with very different pairwise interaction potentials (Lennard-Jones, Coulomb, Debye-Hückel or screened Coulomb, and the hard sphere limit) we identify where exactly on the respective phase diagrams the SE relation holds. It appears that the reduced excess entropy s_{ex} can be used as a suitable indicator of the validity of the SE relation. In all cases considered the onset of SE relation validity occurs at approximately s_{ex}≲-2. In addition, we demonstrate that the line separating gaslike and liquidlike fluid behaviours on the phase diagram is roughly characterized by s_{ex}≃-1.

Transport properties of Lennard-Jones fluids: Freezing density scaling along isotherms.

It is demonstrated that properly reduced transport coefficients (self-diffusion, shear viscosity, and thermal conductivity) of Lennard-Jones fluids along isotherms exhibit quasi-universal scaling on the density divided by its value at the freezing point. Moreover, this scaling is closely related to the density scaling of transport coefficients of hard-sphere fluids. The Stokes-Einstein relation without the hydrodynamic diameter is valid in the dense fluid regime. The lower density boundary of its validity can serve as a practical demarcation line between gaslike and liquidlike regimes.

Simple estimation of thermodynamic properties of Yukawa systems.

A simple analytical approach to estimate thermodynamic properties of model Yukawa systems is presented. The approach extends the traditional Debye-Hückel theory into the regime of moderate coupling and is able to qualitatively reproduce thermodynamics of Yukawa systems up to the fluid-solid phase transition. The simplistic equation of state (pressure equation) is derived and applied to the hydrodynamic description of the longitudinal waves in Yukawa fluids. The relevance of this study to the topic of complex (dusty) plasmas is discussed.

Classical scattering in strongly attractive potentials.

Scattering in central attractive potentials is investigated systematically, in the limit of strong interaction, when large-angle scattering dominates. In particular, three important model interactions (Lennard-Jones, Yukawa, and exponential), which are qualitatively different from each other, are studied in detail. It is shown that for each of these interactions the dependence of the scattering angle on the properly normalized impact parameter exhibits a quasiuniversal behavior. This implies simple scaling of the transport cross sections with energy in the considered limit. Accurate fits for the momentum transfer cross section are suggested. Applications of the obtained results are discussed.

Particle flows in a dc discharge in laboratory and microgravity conditions.

We describe a series of experiments on dust particles' flows in a positive column of a horizontal dc discharge operating in laboratory and microgravity conditions. The main observation is that the particle flow velocities in laboratory experiments are systematically higher than in microgravity experiments for otherwise identical discharge conditions. The paper provides an explanation for this interesting and unexpected observation. The explanation is based on a physical model, which properly takes into account main plasma-particle interaction mechanisms relevant to the described experimental study. A comparison of experimentally measured particle velocities and those calculated using the proposed model demonstrates reasonable agreement, both in laboratory and microgravity conditions, in the entire range of discharge parameters investigated.

Fluid-solid phase transitions in three-dimensional complex plasmas under microgravity conditions.

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.

Freezing and melting of 3D complex plasma structures under microgravity conditions driven by neutral gas pressure manipulation.

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.

Influence of a polarization force on dust acoustic waves.

The effect of the polarization force acting on the grains in a nonuniform plasma background on the propagation of low-frequency waves in complex (dusty) plasmas is analyzed. It is shown that polarization interaction leads to a renormalization (decrease) of the dust acoustic phase velocity. The effect becomes more pronounced as the grain size increases. Finally, there is a critical grain size above which the dust acoustic waves cannot propagate, but aperiodic (nonpropagating) perturbations form instead.

Predicting freezing for some repulsive potentials.

We propose a simple method to approximately predict the freezing (fluid-solid) phase transition in systems of particles interacting via purely repulsive potentials. The method is based on the striking universality of the freezing curve for the model Yukawa and inverse-power-law interactions. This method is applied to draw an exemplary phase diagram of complex plasmas. We suggest that it can also be used to locate freezing transition in other substances with similar properties of interaction.

Electric potential around an absorbing body in plasmas: effect of ion-neutral collisions.

A simple linear kinetic model is used to investigate the combined effect of plasma absorption and ion-neutral collisions on the electric potential around a small absorbing body in weakly ionized plasmas. It is demonstrated that far from the body the potential decays considerably slower than the conventional Debye-Hückel potential. Moreover, at distances exceeding approximately the ion mean free path, the potential approaches an unscreened Coulomb-like asymptote. Some important consequences of this result are discussed in the context of complex (dusty) plasmas.

Superfluidlike motion of an absorbing body in a collisional plasma.

Motion of a small charged absorbing body (micrograin) immersed in a stationary weakly ionized high pressure plasma environment is considered. It is shown that the total frictional (drag) force acting on the grain can be directed along its motion, causing the grain acceleration. At some velocity, the forces associated with different plasma components can balance each other, allowing free undamped superfluid motion of the grain. The conditions when such behavior can be realized and the possibility of a superconductive grain current are discussed in the context of complex (dusty) plasmas.

Attraction of positively charged particles in highly collisional plasmas.

It is shown that the electrostatic interaction potential between a pair of positively charged particles embedded in a highly collisional plasma has a long-range attractive asymptote. The effect is due to continuous plasma absorption on the particles. The relevance of this result to experimental investigations of complex (dusty) plasmas is discussed.

Void Closure in Complex Plasmas under Microgravity Conditions.

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.

Critical point in complex plasmas.

The occurrence of liquid-vapor phase transition and the possible existence of a critical point in complex plasmas--systems that consist of charged micrograins in a neutralizing plasma background--is investigated theoretically. An analysis based on the consideration of the intergrain interaction potential suggests that under certain conditions systems near and at the critical point should be observable. Measurements under microgravity conditions would appear to be required. The analysis aims at determining the plasma parameter regime most suitable for planned experimental investigations.

Force field inside the void in complex plasmas under microgravity conditions.

Observations of complex plasmas under microgravity conditions onboard the International Space Station performed with the Plasma-Kristall experiment-Nefedov facility are reported. A weak instability of the boundary between the central void (region free of microparticles) and the microparticle cloud is observed at low gas pressures. The instability leads to periodic injections of a relatively small number of particles into the void region (by analogy this effect is called the "trampoline effect"). The trajectories of injected particles are analyzed providing information on the force field inside the void. The experimental results are compared with theory which assumes that the most important forces inside the void are the electric and the ion drag forces. Good agreement is found clearly indicating that under conditions investigated the void formation is caused by the ion drag force.

Particle charge in the bulk of gas discharges.

An experimental determination of particle charge in a bulk dc discharge plasma covering a wide range of neutral gas pressures, was recently reported [S. Ratynskaia, Phys. Rev. Lett. 93, 085001 (2004)]. The charges obtained were several times smaller than the predictions of collisionless orbital motion limited theory. This discrepancy was attributed to the effect of ion-neutral collisions. In the present paper a more detailed description of this experiment is provided and additional experimental results obtained with particles of different sizes are reported. The measurements are compared with molecular dynamics simulations of particle charging for conditions similar to those of the experiment, with other available experimental data on particle charge in the bulk of gas discharges, and with a simple analytical model accounting for ion-neutral collisions. All the considered evidence indicates that ion-neutral collisions represent a very important factor, which significantly affects (reduces) the particle charge under typical discharge conditions.

Kinetic approach for the ion drag force in a collisional plasma.

The linear kinetic approach to calculate the ion drag force in a collisional plasma is generalized. The model collision integral (for ion-neutral collisions) is discussed and employed to calculate the plasma response for arbitrary velocity of the plasma flow and arbitrary frequency of the collisions. The derived plasma response is used to calculate the self-consistent force on the test charged particle. The obtained results are compared to those of the traditional pair collision approach, and the importance of the self-consistent kinetic consideration is highlighted. In conclusion, the applicability of the proposed approach is discussed.

Electrostatic modes in collisional complex plasmas under microgravity conditions.

A linear dispersion relation in a highly collisional complex plasma, including ion drift, was derived in the light of recent PKE-Nefedov wave experiment performed under microgravity conditions onboard the International Space Station. Two modifications of dust density waves with wave frequencies larger than the dust-neutral collision frequency were obtained. The relevance to the space observations was analyzed and a comparison of theory and observations was made for two different complex plasma domains formed by small and large microparticles. Good qualitative agreement is found between the measurements and the theoretical dispersion relations. This allows a determination of the basic complex plasma parameters.