Entropy of strongly coupled Yukawa fluids

The entropy of strongly coupled Yukawa fluids is discussed from several perspectives. First, it is demonstrated that a vibrational paradigm of atomic dynamics in dense fluids can be used to obtain a simple and accurate estimate of the entropy without any adjustable parameters. Second, it is explained why a quasiuniversal value of the excess entropy of simple fluids at the freezing point should be expected, and it is demonstrated that a remaining very weak dependence of the freezing point entropy on the screening parameter in the Yukawa fluid can be described by a simple linear function. Third, a scaling of the excess entropy with the freezing temperature is examined, a modified form of the Rosenfeld-Tarazona scaling is put forward, and some consequences are briefly discussed. Fourth, the location of the Frenkel line on the phase diagram of Yukawa systems is discussed in terms of the excess entropy and compared with some predictions made in the literature. Fifth, the excess entropy scaling of the transport coefficients (self-diffusion, viscosity, and thermal conductivity) is reexamined using the contemporary datasets for the transport properties of Yukawa fluids. The results could be of particular interest in the context of complex (dusty) plasmas, colloidal suspensions, electrolytes, and other related systems with soft pairwise interactions.

Comment on "Microscopic kinetic theory of the mean collision force of a particle moving in rarefied gases"

In a recent paper [T. Wei et al., Phys. Rev. E 106, 034101 (2022)2470-004510.1103/PhysRevE.106.034101] a derivation of the resistance force acting on a small classical particle moving through a rarefied gas has been presented. Unfortunately, the obtained expression is inaccurate. The purpose of this Comment is to provide the accurate expression and to discuss several related aspects.

Hard-core attractive Yukawa fluid global isomorphism with the lattice gas model

In this work, we study the global isomorphism between the liquid–vapor equilibrium of the hardcore attractive Yukawa fluid (HCAYF) and that of the Lattice Gas (LG) model of the Ising-like type. The applicability of the global isomorphism transformation and the dependence of its parameters on the screening length of the Yukawa potential are discussed. These parameters determine both the slope of the rectilinear diameter of the liquid–vapor binodal and the Zeno-element, which are the core ingredients of the fluid–LG isomorphism. We compare the Zeno-element parameters with the virial Zeno-line parameters, which are commonly used in the literature for the formulation of generalized law of the correspondent states. It is demonstrated that the Zeno-element parameters appear to be sensitive to the liquid state instability when the interaction potential becomes too short-ranged, while the virial ones do not show any peculiarities connected with this specific of the HCAYF.

Sheath structure and formation of dust voids in cylindrical plasma discharges

Using a self-consistent two-dimensional fluid model the structure of the plasma sheath in a cylindrical system is investigated. The results show that there is a bumping potential in the central axis resulting in the larger outward directing ion drag force with respect to the opposite electric field force. And the process of the formation of dust voids is studied in the sheath by molecular-dynamics simulation.

Motion of plasma-dust structures and gas in a magnetic field

Numerous studies have revealed that the superposition of magnetic field causes plasma-dust structures to rotate in the plane normal to the field. The only explanation for this rotation found in the literature is that the plasma-dust structures are acted upon by the forces of ion entrainment (ion drag forces) from ions moving under the effect of the magnetic field in the azimuthal direction. However, this study demonstrates that the experimentally observed motion of plasma-dust structures cannot be explained by the forces of ion entrainment alone. We show that the observed motion of plasma-dust structures is further affected by their entrainment by gas rotating under the effect of the moment of force IxB , which exists in regions of discharge with nonuniform magnetic field in the vicinity of solenoid end faces, as well as at the narrowing of cross section of the discharge channel. An eddy electric current exists in a discharge with strata in a uniform magnetic field; this current causes the rotation of gas and is associated with the noncollinearity of the gradients of plasma density and temperature. Estimates are provided for the density of this current and for its impact on the rotation of gas in a magnetic field. Recent experimental data by Karasev [Phys. Rev. E 74, 066403 (2003)] are discussed.

Effect of trapped ions and nonequilibrium electron-energy distribution function on dust-particle charging in gas discharges

Dust-particles charging in a low-pressure glow discharge was investigated theoretically. The dust-particle charge was found on the basis of a developed self-consistent model taking into account the nonequilibrium character of electron distribution function and the formation of an ionic coat composed of bound or trapped ions around the dust particle. The dust-particle charge, the radial distributions of electron density, free and trapped ions densities, and the distribution of electrostatic potential were found. It was shown that the non-Maxwellian electron distribution function and collisional flux of trapped ions both reduce the dust-particle charge in comparison with that received with the help of the conventional orbital motion limited (OML) model. However, in rare collisional regimes in plasma when the collisional flux is negligible, the formation of ionic coat around a particle leads to a shielding of the proper charge of a dust particle. In low-pressure experiments, it is only possible to detect the effective charge of a dust particle that is equal to the difference between the proper charge of the particle and the charge of trapped ions. The calculated effective dust particle charge is in fairly good agreement with the experimental measurements of dust-particle charge dependence on gas pressure.

Single dust-particle rotation in glow-discharge plasma

Rotation of a single dust granule (spin) is investigated experimentally in a stratified glow discharge. We employ the technique of measurement of the angular velocity, which is based on coordinate tracing of the light scattered by a hollow transparent particle. The angular velocity measured in the experiment is about 1-2 orders of magnitude higher than observed in previous experiments. We found that the angular velocity depends linearly on the discharge current. The mechanism of rotation of the granule is also described.

Nonlinear Poisson-Boltzmann equation in spherical symmetry

The Poisson-Boltzmann problem in spherical symmetry has been considered using the distribution of a self-consistent potential around a charged grain in a thermal collisional plasma as an example. The qualitative patterns of all possible solutions have been presented and a study of their asymptotics has been carried out. It has been demonstrated that for large potentials it is possible to neglect the curvature of the grain surface and to use the solution of the plane problem. It has also been demonstrated that the electrical interaction of the grains is possible only at distances smaller than eight screening lengths.

Rotational motion of dusty structures in glow discharge in longitudinal magnetic field

The investigation of dust structure formed in glow discharge in an external longitudinal magnetic field with induction up to 400 G applied is presented in this work. The dust structure starts to rotate in the magnetic field. The angular-velocity magnitude is one to two orders larger than one in other discharge types. Its dependence on the magnetic field is nonmonotonic. The rotation direction inverses with an increase of the magnetic induction value up to a certain magnitude B0. In close range of induction around B0 and under certain conditions the rotation of the upper and lower parts of the structure in the opposite direction is observed. Rotation is caused by the ion-drag force. The inversion of rotation direction relates with the change of plasma flows in the area of their formation in stratum with the magnetic field applied. The effect of ion flows was investigated in two additional experiments on the observation of structure rotation onset and on gravity-driven probing of stratum. The angular-velocity unhomogeniety allowed us to investigate shearing and to observe melting of the dust crystal. The correlation functions approach showed the occurrence of structure transformation and its phase transition of the meltinglike type in the magnetic field.

Ordered spatial structures of dust grains in the thermal plasma

The thermal complex plasma of atmospheric pressure, containing the grains of condensed phase, has been studied. It has been demonstrated that the existence of the space charge areas around the dust grains leads to the inhomogeneous ionization of the plasma and the occurrence of the fluxes of nonequilibrium charge carriers. These fluxes change the pressure of the gas on the grain surfaces and define the forces that force the grains to move towards the zone of maximum ionization perturbation of the plasma. It has been shown that the combined operation of the electrical forces and the forces of the interface pressure leads to the formation of the ordered structures, corresponding to the balance of forces. The results of computer simulation, corresponding to the experimental data, are given.

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.

Vertical wave packets observed in a crystallized hexagonal monolayer complex plasma

Propagation of vertical wave packets was observed experimentally in a crystallized hexagonal monolayer complex plasma. It was found that the phase velocity exceeded the group velocity by a factor 65 and was directed into the opposite direction as expected for an inverse optical-like dispersion relation. The wave packets propagated keeping their width constant. The explanation of this behavior is based on three-dimensional equations of motion and uses a long-wavelength weak dispersion weak inhomogeneity approximation. While the wave dispersion causes the wave packet to spread, lattice inhomogeneity and neutral gas drag counteract spreading. A plasma diagnostic method was developed that is based on the ratio between vertical and dust-lattice wave speeds. This ratio is very sensitive to the lattice parameter kappa (ratio of the particle separation to the screening length) in a very useful range of kappa < or = 2 . It was found that only a two-dimensional lattice model can provide a quantitative description of the vertical waves, while a linear chain model gives only a qualitative agreement.

Electrostatic interaction of charged planes in the thermal collision plasma: detailed investigation and comparison with experiment

The spatial distribution of electrostatic potential between the metal planes in a thermal collision plasma at atmospheric pressure has been investigated. It has been shown that the potential requirement must be calculated with respect to the bulk plasma potential, which depends on the ionization equilibrium in the plasma. It has also been shown that the electrostatic perturbation in the plasma is detected only at distances of less than four screening lengths. Long-range perturbation is described by the bulk plasma potential. The electrostatic pressure on the plane as a function of boundary conditions has been found. The experimental results prove the existence of interaction between the planes, located in the low-temperature plasma at a distance that considerably exceeds the screening length, caused by changing the bulk plasma potential. The application of the results to a complex dusty plasma has shown the diminution of-the dust component dissipation in strongly coupled plasmas.

Momentum transfer in complex plasmas

Momentum transfer in complex plasmas (systems consisting of ions, electrons, neutrals, and charged macroscopic grains) is investigated assuming an interaction potential between the charged species of the screened Coulomb (Yukawa) type. Momentum transfer cross sections and rates are derived. Applications of the results are discussed; in particular, we classify the possible states of complex plasmas in terms of the momentum transfer due to grain-grain collisions and its competition with that due to interaction with the surrounding medium. The resulting phase diagrams are presented.