[en] A dust void appears as a dust-free region where the plasma ionization rate (comparatively higher than that in a dust region due to strong plasma dissipation on dust particles) supports an outward ion flow exerting an outward ion drag force on the dust particles. The sharp dust-void boundary presents a new type of discontinuities in a dusty plasma where the dust density can change rapidly while the ion and electron densities as well as the dust charge are continuous. The observed boundaries are often stationary, with the established balance of forces acting on dust particles. Here, we investigate the dust void structures in the case when the ion diffusion on the neutral gas atoms plays an important role. We demonstrate that this dissipative process not only allows the existence of the dust voids (i. e., the absence of dust grains in the void region) but can create new types of discontinuities inside the dust structure. The results are obtained for a one-dimensional model, with planar geometry that is symmetric about the center of the void, by numerical investigation of the stationary force balance equations for the void structure. In the considered model, we numerically investigate the non-linear balance equations and obtain new results. Namely, we show that if the ion diffusion on neutral gas atoms is taken into account, the dust density has no discontinuity (according to the dairy conditions used) at the surface of the dust boundary; nevertheless, the ion diffusion process still creates sharp dust boundaries with complete absence of dust in the void region. It appears that not the dust density but the derivatives of various plasma parameters have discontinuities at the dust boundary. Note that in the presence of the ion diffusion the condition that the dust density is zero at the void boundary is automatically fulfilled when the boundary conditions are satisfied. We therefore loose the previous important condition that the dust density jump at the boundary is positive which was a crucial for the selection of those surfaces that can describe the dust voids from other non-physical surfaces satisfying the boundary conditions. Thus in order to construct the theory of dust voids in this case we also have to revise the main concepts and procedures of the theoretical study of the dust void formation. (Author)

[en] Dust lattice (DL) modes arising in strongly coupled dusty plasmas have been extensively studies in many experiments using radio-frequency discharge plasmas in the laboratory and under microgravity conditions. In the laboratory experiments, the highly negatively charged particles levitate in the sheath region of the horizontal negatively biased electrode where there is a balance between the gravitational and electrostatic forces acting in the vertical direction. If the particles are magnetized by an external field, the magnetic force can also contribute to the force balance and support particle levitation. There are a few characteristic features of the particle trapping in discharge plasma. On the one hand, the complex plasma structures reveal the anisotropy due to vertical ion flows that lead to the formation of an ion wake underneath the suspended microparticles. On the other hand, an equilibrium particle charge strongly depends on the vertical particle position in the sheath region. This introduces anisotropy of the system related to a vertical profile of the equilibrium particle charge. Finally, if the particles are magnetized, there exists a vertical non-uniformity of the magnetic moments induced by the external in homogenous magnetic field. We study the influence of all these factors on coupling of the transverse and longitudinal DL modes in a one-dimensional horizontal particle string. It is found two different ways of the reconnection of the dispersion curves for the longitudinal and transverse modes in the vicinity of their intersection point. When the particles levitate at that part of the complex plasmas, which provides a coupling coefficient to be positive, the transverse and longitudinal branches become confluent in such a way, that there is a considerable frequency gap in the vicinity of the intersection point, in which both DL modes are evanescent. This type of reconnection can be considered as a kind of the linear mode conversion. Conversely if the particle levitate at that part of the discharge where the coupling coefficient is negative, a confluence of the transverse and longitudinal branches occurs separately in the long-wavelength and short wavelength ranges, thus leading to the gap in the wavenumber domain, and resultant instability of the hybrid mode. Using the typical complex plasma parameters, it is found that this instability could be of importance at gas pressures below of a few tens Pa. the experimentally observed spontaneous excitations of DL waves that occur in plasma crystals when the gas pressure is decreased below a critical value can be a manifestation of the discovered instability. (Author)

[en] The effect of ion-neutral collisions on the potential of interaction between a pair of negatively charged grains in isotropic weakly collisional plasmas is investigated. Two interaction mechanisms are considered: Electric repulsion between like-charged grains and attraction due to the so-called ''ion shadow'' effect. It is demonstrated that in the presence of collisions, both interaction potentials exhibit Coulomb-like ∝r^-1 decay at large distances. A necessary condition for the existence of long-range attraction is derived. The obtained results are then used to reconsider the possibility of liquid-vapor critical point occurrence in complex plasmas

[en] The influence of an inhomogeneous magnetic field, ion focusing effect and equilibrium charge gradient on the propagation of dust-lattice (DL) modes in a one-dimensional string formed by paramagnetic particles is considered. In typical discharge conditions, all three anisotropic factors can compete with each other even at moderate magnetic fields (∼0.1-0.2 T), modifying the DL waves and leading to mode coupling. The characteristics of the mode coupling can be controlled externally by varying the magnetic-field regime, thus opening new possibilities for studies of collective effects in strongly coupled systems

[en] A model of grain surface temperature in a low-pressure weakly ionized complex plasma is refined by taking into account the effect of rare ion-neutral collisions in the vicinity of the grain. These collisions enhance plasma collection by the grain and can result in a considerable increase of the grain temperature. It is shown that the collision enhanced plasma collection model yields generally better agreement with the results of existing experiments. The effect of the temperature difference between the grain surface and the surrounding gas on the grain-grain interaction potential is also briefly discussed

[en] 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.

No abstract available

[en] The interaction of a focused powerful laser beam with micron-sized melamine formaldehyde (MF) particles was studied experimentally. The microspheres had a thin palladium coating on their surface and were suspended in a radio frequency argon plasma as a single layer (plasma crystal). A particle hit by the laser beam usually accelerated in the direction of the laser beam, consistent with the radiation pressure force mechanism. However, random-direction acceleration up to the speeds on the order 1 m/s was sometimes observed. Rocket-force mechanism is proposed to account for the random-direction acceleration. Similar, but much less pronounced, effect was also observed for MF particles without palladium coating.

[en] Ion collisionality is known to be a major factor which determines the magnitude of the surface (floating) potential of an individual particle immersed in a plasma. In this paper a simple interpolation formula for the ion flux collected by such a particle in the entire range of ion collisionality is proposed. The dependency of the floating potential on ion collisionality calculated using this formula as well as using other analytic approximations developed recently are compared. The reliability of different approaches is discussed.

[en] The onset of free thermal convection in complex fluids with background friction is theoretically investigated. It is shown that in the limit when friction prevails--opposite to the classic Rayleigh-Benard case--the onset is determined by a renormalized Rayleigh number and also depends on the Prandtl number. Such convection should be observable in experiments with complex plasmas