Dynamic wetting of component surfaces can be investigated by finite element phase field simulations. Often these models use a double-well potential or the van der Waals equation to define the local part of the free energy density at a point of the computational domain. In order to give the present model a stronger physical background the molecular dynamics based perturbed Lennard-Jones truncated and shifted (PeTS) equation of state is used instead. This results in phase field liquid-vapor interfaces that agree with the physical density gradient between the two phases. In order to investigate dynamic scenarios, the phase field description is coupled to the compressible Navier-Stokes equations. This coupling requires a constitutive equation that complies with the surface tension of the liquid-vapor interface resulting from the PeTS equation of state and is comparable to the so-called Korteweg tensor.
Phase Field ModelThe Phase Field (PF) model used in this work couples the Navier-Stokes equations with the static PF model described in [1]. The coupling is done via the capillary stress tensor described by Korteweg in 1901, see e.g. [2]. Therefore, the model can be classified as a Navier-Stokes-Korteweg model. A central component of Navier-Stokes-Korteweg models is the equation of state that formulates the free energy density of the bulk phases as well as the transition zone between them. Instead of applying the commonly used van der Waals equation of state, the present model utilizes the PeTS equation of state [3] which provides an accurate energy density formulation for the Lennard-Jones truncated and shifted fluid.For a domain B that is bounded by ∂B the mass and momentum balanceṡare solved for particle density ρ( x, t) and velocity v( x, t). Position in space and time and gravitational acceleration are denoted by x, t and g. The material time derivative is denote by( ·) = d(·) dt . The boundary conditions read v = 0 on ∂B ,The outer normal to the boundary and the part of ∂B that is given by a solid surface are given by n and ∂B s . With ϕ = ρ−ρ ′′ ρ ′ −ρ ′′ , where ρ ′ and ρ ′′ are the liquid and vapor bulk densities, (4) ensures a specified contact angle Θ for a droplet that is in contact with ∂B s , see also [4,5]. The solid-liquid and solid-vapor surface tensions are denoted by γ sl and γ sv . The temperature T is assumed to be constant (T = const.). The stress tensor σ readsand ∇ s (·) = 1 2 (∇(·) + (∇(·)) T ), trace tr, dyadic product ⊗, and viscosity η. The free energy per particle a = a(ρ, T ) is taken from the PeTS equation of state [3] and the constant κ is set to κ = 2.7334 [1]. The Lennard-Jones dimensions are used for all physical quantities. They are nondimensionalized by using the convention σ LJ = ε LJ = M LJ = 1 for the size parameter, the energy parameter, and the mass per particle. Second derivatives in the weak form of the momentum balance (2) are avoided by using the chemical potential µ as an additional degree of freedom. For the finite element implementation, v is discretized with quadratic s...