281794 Molecular Dynamics Simulations of Vapor Bubble Growth and Detachment On A Heating Surface
Nucleate boiling is an attractive alternative for systems that require large heat transfer from a hot solid due to the large latent heats that are accessible at relatively low wall superheat above the fluid's saturation temperature. 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 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 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 nanoscale version of our three-phase system. Molecular interaction physics is the only input, to this calculation and we then observe the resulting CL motion. Because MD not only includes heat transfer, but also fluid flow, this simulation removes many of the restrictions of our earlier continuum calculation for this nano-sized system. Since terrestrial gravity is utterly negligible at the nanoscale, we introduce a fictitious body force to mimic relevant Bond numbers. In our MD simulations, we study not only the heterogeneous birth of a vapor nucleus on the solid-fluid interface at constant pressure by virtue of heat transfer through the solid and its achievement of stability, but also its subsequent growth, deformation and detachment.
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 two solid walls are tethered via springs to their lattice positions and also interact with both the fluid and each other via LJ potentials. By considering the effects such as the wettability of the solid surface, we find the appropriate interaction parameters between the solid and fluid atoms in LJ form, which are critical parameters for being able to observe nucleation and heat transfer. Periodic boundary conditions are applied in 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. Next, we instantaneously increase the temperature at the bottom layer of the bottom wall while keep 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 is kept fixed by allowing the top surface to freely slide up and down. Our simulations show that after some vapor patches appear and disappear on the solid surface randomly in space and time, one of these patches successfully grows into a stable vapor bubble, whose growth and CL motion we then trace.
As noted, by artificially applying a uniform body force that creates a relevant Bond number, we have successfully nucleated and grown a vapor bubble to a moderate size and observed bubble necking and detachment. We found that, despite the existence of temperature slip between solid and liquid, the vapor bubble volume still grows at t3/2, 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 vapor bubble surface and the three-phase CL appear to be at the liquid saturation temperature, which confirms the assumptions made in our continuum calculations' boundary condition at liquid-vapor interface. By further increasing this artificial uniform body force (i.e., Bond number) and keeping it constant, we find the body force indeed deforms the bubble and causes it to detach from the surface. 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 rise 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. In addition, we also consider the effects of different parameters, such as those that influence the temperature slip between solid and liquid, the magnitude of the body force, the temperature of the bottom wall and the thickness of the bottom wall on vapor bubble growth and detachment.