(694c) Molecular Dynamics Simulations Of Vapor Bubble Growth and Detachment On A Heating Surface- Investigation Of The Contact LINE Dynamics

Based on our previous simulations of conductively-driven quasi-static vapor bubble growth in an axisymmetric, cylindrical cell comprised of solid and liquid phases of finite thicknesses with fluid motion and heat transfer obeying small Reynolds, Peclet, Capillary and Bond numbers, we found that the appearance and the motion of the three-phase contact line (CL) is a critical element, central to physics needed to explain the large heat enhancement in nucleate boiling. However, our simulations also suggested that different ad hoc models for the poorly-understood motion of the CL with phase change occurring at the CL as the vapor bubble grows do have a significant effect on vapor bubble deformation and detachment when the Bond number is no longer small. Therefore, we have used molecular dynamics (MD) to simulate a nano-scale version of our three-phase system. 

       Our MD simulation domain of interest is a cuboid, which is composed of the fluid region sandwiched between two parallel solid walls. The fluid phase is made of argon atoms interacting via a 12-6 Lennard-Jones (LJ) potential. The atoms of the two solid walls are tethered via springs to their lattice positions and also interact with both the fluid and each other via LJ potentials.  We determine the critical interaction parameters between the solid and fluid atoms in LJ form by considering the balance between the wettability of the solid surface, which is key to observe nucleation and the heat transfer rate through the solid to the fluid. Periodic boundary conditions are applied at the 4 vertical boundaries of the domain. After achieving thermal equilibrium of all the phases in the simulation domain (solid walls and liquid argon) at the uniform reduced temperature of T=0.75 (the fluid saturation temperature), we start to expend the top wall gradually at constant reduced temperature T=0.75 to some specified pressure at which no vapor bubble appears via cavitation. Next, we instantaneously increase the temperature at the bottom layer of the bottom wall while keeping the original lower temperature at the top wall, both maintained by thermostating.  This maintains a temperature gradient through the fluid phase. Meanwhile, the pressure at the top wall (which matches that in the bulk fluid) is kept fixed by allowing the top surface to freely slide up and down.

         Without applying a uniform body force (that in some way might mimic gravity) to the fluid atoms, we adjust the control pressure and the heating temperature and find the region of the pressure vs temperature plot where a vapor bubble nucleates and eventually grows to cover the entire solid surface, where a bubble nucleates but does not grow to cover the entire solid surface and where no bubble nucleates in a reasonable amount of time. By then artificially applying a uniform body force that mimics a relevant Bond number, we then successfully nucleate and grow a vapor bubble to a moderate size and observe bubble necking and detachment. We find that,, despite the existence of temperature slip between solid and liquid, the vapor bubble volume still grows as t^3/2, t being time, as in our earlier conduction-only continuum calculations. This scaling is also in agreement with numerous experimental observations. Furthermore, our temperature profiles show that the vapor bubble surface at the three-phase CL appears to be at the liquid saturation temperature, which confirms the assumptions made in our continuum calculations’ boundary condition at liquid-vapor interface and where it contacts the solid surface. By further increasing this artificial uniform body force (i.e., Bond number) and then keeping it constant, we find the body force indeed deforms the bubble and causes it to detach from the surface. This provides critical information as to how system parameters affect the size of the vapor residue left on the surface and on whether cooler fluid flowing downward to replace the rising detached bubble succeeds in quenching this residue. During this process, we reset/raise the temperature at the top wall to avoid vapor condensation at the top of vapor bubble as the bubble grows, detaches and rises into regions of lower temperature, without changing the magnitude of the uniform body force. These simulations result in, among other things, the time evolution of the CL motion. The radius of CL initially expands with the growth of the bubble at low Bond number. Then, as the bubble grows to a size where the body force becomes significant, the CL contracts sharply but continuously as the bubble deforms until detachment. Finally, we incorporate this physical CL motion into our continuum calculation to simulate the vapor bubble growth until detachment in a continuum model.