Critical point in complex plasmas.

The occurrence of liquid-vapor phase transition and the possible existence of a critical point in complex plasmas--systems that consist of charged micrograins in a neutralizing plasma background--is investigated theoretically. An analysis based on the consideration of the intergrain interaction potential suggests that under certain conditions systems near and at the critical point should be observable. Measurements under microgravity conditions would appear to be required. The analysis aims at determining the plasma parameter regime most suitable for planned experimental investigations.

Hydrodynamic theory of density relaxation in near-critical fluids

This paper gives a complete hydrodynamic theory of density relaxation after a temperature step at the boundary of a cell filled with a nearly supercritical pure fluid in microgravity conditions. It uses the matched asymptotic expansion technique to solve the one-dimensional Navier-Stokes equations written for a viscous, low-heat-diffusing, near-critical van der Waals gas. The continuous description obtained for density relaxation in space and time confirms that it is governed by two fundamental mechanisms, the piston effect and heat diffusion. It gives a space-resolved description of density inside the cell during the divergently long heat diffusion time, which is shown to be the ultimate one to achieve complete thermodynamic equilibrium. On that very long time scale, the still measurable density inhomogeneities are shown to follow the diffusion of the vanishingly small temperature perturbations left by the piston effect. Temperature, which relaxes first to nonmeasurable values, and density, which relaxes on a much longer time scale, may thus appear to be uncoupled. The relaxation of density on the diffusion time scale is shown to be driven by a bulk expansion-compression process slowly moving at the heat diffusion speed, which is generated by heat diffusion coupled with the large compressibility of the near-critical fluid. The process is shown to be the signature of the thermoacoustic events that occur during the very short piston effect time period. The generalization of the theory to real critical behavior opens the present results to future experimental investigation.

Rapid thermal relaxation in near-critical fluids and critical speeding up: discrepancies caused by boundary effects

We present one-dimensional numerical simulations reporting the temperature evolution of a pure fluid subjected to heating near its liquid-vapor critical point under weightlessness. In this model, thermal boundary conditions are imposed at the outer edges of the solids in contact with the fluid. Our investigations concern the thermal conditions at the edges of the fluid and their consequences on the fluid's global response. The results for piston effect heating are shown to be significantly affected by the simulation of the solid boundaries. Concerning critical speeding up, it is even found that taking conductive solids into account can make the bulk fluid temperature change in a way opposed to that predicted in their absence.