Non‐linear screening and phase states of a complex plasma

Abstract

The applicability limit of the well‐known phase diagram of complex plasmas in the κ–Γ plane (κ is the structural parameter, Γ is the coupling parameter) is under discussion. The present work is devoted to the analysis of the range of applicability of a basic assumption in the initial phase diagram, that is, linearized (Debye) screening of macro‐ions by micro‐ions, which leads to the Yukawa form of effective interactions between macro‐ions. Parameters of non‐linear screening for macro‐ions were calculated within the direct Poisson–Boltzmann approximation in an averaged Wigner–Seitz cell. Two effects were revealed as a result of such calculations: (a) decomposition of all micro‐ions into two subclasses, free and bound ones, and (b) significant reduction of the effective charge Z* of initial bare macro‐ion Z under non‐linear screening by the small high‐density envelope of bound micro‐ions. This effect leads to a re‐normalization of initial Γ and κ into Γ* and κ* (Γ* < Γ, κ* < κ). It is assumed that the phase states of a complex plasma under non‐linear screening are still the same as in the initial phase diagram, but in κ*–Γ* plane instead of the κ–Γ one. The corresponding calculated shifts of the phase states are discussed and illustrated.

Macroion effective charge in complex plasmas with regard to microion correlations

“…The main effect of Coulomb macroion-microion correlations (including the non-linearity of these correlations) is taken into account in the framework of the PB approximation [9,12,[23][24][25] in the present paper. Here, the presence of the microions excluded volume is explicitly taken into account when formulating the boundary conditions for the microions density at the macroion surface in calculating microions distributions in the WS cell.…”

“…Until now, in our previous works [9,12,[23][24][25] , the effects of screening non-linearity were taken into account, however, the effects of microions-microions correlations (due to both microions Coulomb repulsion and their additional short-ranged repulsion due to the presence of macroions excluded volume) were not taken into account. Below, both of these effects will be considered within the framework of the averaged spherical WS cells.…”

Self‐similarity of nonlinear screening in asymmetric complex plasmas

“…Then, the effective charge is determined as in the electroneutral WS cell, where R and R Z are the WS cell and the macroion radii, respectively. This way of determining the effective charge was used in the study by Martynova et al [ 10 ] to take the nonlinear screening effect into account in the two‐component system of the finite‐sized macroions and the point‐like microions.…”

“…in the electroneutral WS cell, where R and R Z are the WS cell and the macroion radii, respectively. This way of determining the effective charge was used in the study by Martynova et al [10] to take the nonlinear screening effect into account in the two-component system of the finite-sized macroions and the point-like microions. One of the main results of this work is revealing the self-similarity of the Z * (Z) dependence in the PB approximation.…”

“…Neglecting of nonlinear screening effect can change finding results about the existence or absence of some phase transitions in the studied plasma. For example, it is shown in the previous studies [ 10,11 ] that the attainable zone of two‐component complex plasmas characteristic parameters is much smaller than the whole phase diagram of one‐component complex plasmas [ 1 ] because of taking the effect of nonlinear screening into account and the resulting definition of a macroion effective charge. Also, it was shown in the study by Diehl et al [ 12 ] that the neglecting of the nonlinear screening in a two‐component system can lead to a significant overestimation of the zone of thermodynamic parameters where the loss of thermodynamic stability and the appearance of a fluid–fluid phase transition are possible.…”