Phase distribution in nanochannels of supercritical fluid with different fluid–wall interactions

Abstract

Supercritical fluids (SFs) are classically regarded as single-phase fluids without bubbles or interfaces, but a recent study shows nanobubbles in SFs under unconfined conditions. The objective of this paper is to explore the phase distribution under confined conditions. Molecular dynamics simulations are performed for supercritical argon. Two walls containing the SF have equal fluid–wall interactions with equal and unequal wall temperatures. An external force is applied on the top wall to control the pressure at 1.5Pc, in which Pc is the critical pressure. Periodic boundary conditions are applied on the four side surfaces of the simulation box. The study indicates that the bulk fluid density is not only dependent on pressure and temperature, but also on fluid–wall interactions, this result deviates from the classical theory, where density depends on only pressure and temperature. For strong fluid–wall interactions, three- or five-layer structures are found, including liquid-like (LL) layers on the walls and two-phase-like (TPL) and gas-like (GL) layers (depending on bulk density) in the channel core. For weak fluid–wall interactions, the phase distribution becomes GL on the wall, and TPL and LL (depending on bulk density) in the channel core, which is inverse to those of strong fluid–wall interactions. Correspondingly, the phase distributions for strong and weak fluid–wall interactions can be analogous to annular or Leidenfrost patterns at subcritical pressures, respectively. The density profile is symmetric against the channel centerline at equal wall temperatures, but symmetry-breaking may exist when applying different wall temperatures. This work provides a phase-distribution link between subcritical and supercritical pressures, which is useful for the design and analysis of SF systems.