Features of Phase Transitions in Models of Complex Plasma

Abstract

Some features of melting curves and fluid-crystal phase transitions in complex plasmas are under discussion. The base for a consideration is the well-known phase diagram of dusty plasma (Hamaguchi, 1997) for an equilibrium charged system with the Yukawa potential in Γ − κ plane (Γ is the Coulomb non-ideality parameter,κ is a screening parameter). This phase diagram is converted for a one-component Yukawa system in ordinary density -temperature plane. A melting curve is converted for this system in temperature -pressure plane. There are some density gap estimations based on a hypothesis of similar melting properties in Yukawa systems and Soft Spheres systems. The initial phase diagram is also converted for two one-temperature models of complex plasmas in density -temperature plane. Here simplified variants of complex plasmas models are considered as a thermodynamically equilibrium ensemble of classical Coulomb particles: a 2-component electroneutral system of macro-and microions (+Z, -1) and a 3-component electroneutral mixture of macro-ions and two kinds of microions (+Z, -1, +1). The resulting phase diagram for (+Z,-1) or (-Z,+1) in ln n − ln T plane has a form of a linear combination of crystalline and fluid zones separated by the boundaries Γ = const. Parameters and locations of these zones are analyzed in dependence on macroion charge number Z. There are huge negative pressure and negative compressibility areas in the initial phase diagram if one uses equations of state (Hamaguchi,1997) and (Khrapak, 2014). Thus, questions of thermodynamic stability and an existence of an additional phase transition gas-liquid are discussed.

Non‐linear screening and phase states of a complex plasma

“…Actually, it is one line. It is rather easy to prove it if one uses (1) and the equation for r D for an electro‐neutral system which consists of macro‐ and micro‐ions . The right symbols on each of the lines (3) and (4)–(7) correspond to n Z = 10 10 cm −3 .…”

“…It was stressed in Martynova and Iosilevskiy that large regions of negative total pressure and negative total compressibility exist at not too large values of Γ and κ on the phase diagram as one uses non‐ideal corrections P ex ( T Z , T i , Z , n Z ) from Hamaguchi et al and Khrapak et al in the total equations of state of a complex plasma: …”

“…However, as T i = 0.03 eV and R Z = 1 μm, Ze 2 R Z T ≤ 1 is valid as Z ≤ 22. So, we should not use the linearization condition because the representative charge of complex plasmas is usually much greater than 22 (see Martynova and Iosilevskiy [9,12] for details). Thus, a potential of the Yukawa form is not adequate to describe the thermodynamics of the considered systems.…”

Effect of the non‐linear screening on a modification of the Debye–Hückel plus hole approximation in complex plasma

“…It was shown in, for example, that the non‐linear screening stabilizes charged colloidal suspensions preventing fluid–fluid phase separation. Moreover, we suppose that the neglect of the non‐linear screening is one of the causes of the existence of excessive total negative pressure and total negative compressibility areas in the phase diagram of the complex plasma. These areas were obtained in by means of the equations of state).…”

“…Moreover, we suppose that the neglect of the non‐linear screening is one of the causes of the existence of excessive total negative pressure and total negative compressibility areas in the phase diagram of the complex plasma. These areas were obtained in by means of the equations of state). We expect that if the non‐linear screening effect would be taken into account, then the areas of negative compressibility will be very small in the very least.…”