Asymmetric structure of gas-liquid interface

The density profiles of the gas-liquid interface of the three-dimensional Lennard-Jones (12-6) particle system are studied by nonequilibrium molecular dynamics simulation. The system is a rectangular parallelepiped box of the size of L_x ≫ L_y x L_z. Two Nosé-Hoover heat baths with different temperature are attached to the regions near both ends in the x-direction. Heat flows in the system in the x-direction and it maintains the system in a gas-liquid coexisting state. A gas-liquid interface exists steadily in the system box and an asymmetric density profile is observed. We consider the characteristic lengths ξ_g and ξ_l in the gas and liquid phase sides defined using density in each phase ρ_g,l = aξ_g,l^-3, respectively. The sum of the characteristic lengths ξ = ξ_g + ξ_l corresponds to the thickness of the interface. Using ξ_g,l, the density profile in the interface is well described by the tanh form in each phase side ρ_g,l(x) = ρ_g,l + β _g,l tanh |(x - x₀)/ξ_g,l|. The critical behavior of this model is consistent with the three-dimensional Ising universality.