Self-confined particle pairs in complex plasmas

The liquid-crystal type of phase transition in complex plasmas has been observed repeatedly. However, more studies need to be done on the liquid-vapor transition in complex plasmas. In this paper, the phenomenon of coupling (condensation) of particles into self-confined particle pairs in an anisotropic plasma medium with ion flow is considered analytically and numerically using the Langevin molecular dynamics method. We obtain the stability conditions of the pair (bound) state depending on the interaction parameters and particle kinetic energy. It was shown that the breakup of the particle pair is very sensitive to the ratio of particle charges; for example, it is determined by the influence of the upper particle on the ion flow around the lower one. We also show that a self-confined pair of particles exists even if their total kinetic energy is much greater than the potential well depth for the pair state. This phenomenon occurs due to velocity correlation of particles, which arises with the nonreciprocity of interparticle interaction.

Figure 1

Regions of stable existence of self-confined vertical configuration of a particle pair (filled areas) with point-wake interaction: Bold curve indicates the condition (3); dashed curve, condition (4); and dash-dot curve, condition (5). (a) Two particles with screening parameter $\kappa =2$ and different ratios of their charges $Q_1/\phantom{Q_1{Q}_2}{Q}_2=Q^{*}$ given in the filled areas: Between black curves $Q^{*}=100\%$; between red curves, 99%; green curves, 90%; and blue, 70%. (b) Two particles with $Q^{*}=100\%$ and different values of screening parameters $\kappa =1-4$ given in the colored areas.

Figure 2

Evolution of the region of stable existence of vertically arranged particle pair interacting via point-wake model $(\kappa =2)$ with the rise in the particle kinetic temperature (LMD simulations for $\omega /\phantom{\omega \nu }\nu \approx 15$). The boundaries of each colored region correspond to the fixed values of full kinetic energy of two particles (with $T<0.2\mathrm{eV}$) given in the legend.

Figure 3

For a point marked by the star in Fig. 2: (a) Contour plot near the equilibrium point of particle separation denotes the area where conditions (3, 4, 5) are true. (b) Spatial distribution of the total potential energy $U_{\mathrm{\Sigma }}$ inside the contour area. (c) The surface of a potential well, where the bottom corresponds to the equilibrium pair state.

Figure 4

The normalized kinetic energy of two particles $K_{x,z}/\phantom{K_{x,z}T}T$ (for a point marked by the star in Fig. 2 and $T\approx 0.2$ eV) depending on the ratio of the main system frequencies $\omega /\phantom{\omega \nu }\nu$: measured in simulations (symbols); calculated by (6) for the equilibrium particle separation ${\mathbf{r}}_{\mathrm{p}}$ (dashed curves); and calculated by (6) for the most probable separation ${\mathbf{r}}_{\mathrm{mp}}$ (bold curves).