(250c) Molecular Dynamics of Heterogeneous Vapor Bubble Growth: Exploring the Physics of the 3-Phase Contact Line
Dramatic heat transfer enhancements in nucleate boiling have attracted intensive interest since the 1930's due to the stability and efficiency of such heat transfer at the relatively low thermal driving force associated with nucleate boiling incipience. Our previous continuum work modeled transport process by the quasi-steady heat conduction equation for the growth of a single bubble at the interface of solid and liquid layers, each of finite extent under the assumptions of small Reynolds, thermal Peclet, Capillary and Bond numbers. Its results argued that the presence and motion of the 3-phase contact line when a vapor bubble is present on the solid surface plays a central role in the large observed heat transfer enhancement in nucleate boiling. However, a direct continuum description based on quasi-steady heat conduction equation and boundary conditions does not explain the physics of the dynamics of contact line, but rather needs the contact line’s evolution as an input to the fluid dynamics. In this work we have therefore employed molecular dynamics to simulate a nanoscale version of our system and examine heterogeneous nucleation and growth of a vapor bubble on the surface of a solid that is heated from below. We then apply a uniform body force to mimic an exaggerated version of gravity to elongate, deform and detach the growing vapor bubble from the surface. We track the motion of the three-phase contact line during this process and, among other things, examine its effect on our continuum calculation.
The simulation domain we set for the program of interest is a cuboid, which is comprised of the two solid walls and a fluid region sandwiched between two walls. The simulations consider a fluid made of argon atoms interacting via a Lennard-Jones (LJ 12-6) potential function with energy scale parameter 1. Solid atoms are tethered to their lattice positions. We also apply L-J potential function form between argon and the solid’s atoms, but this potential has a energy scale parameter that is different from the fluid-fluid potential and delicately chosen so as to maintain heat transfer from the solid to the adjacent to the liquid while still permitting vapor bubble nucleation. We apply periodic boundary conditions at other 4 vertical boundaries of the domain. Initially we achieve thermal equilibrium of all the phases in the simulation domain (solid walls and liquid argon) at the uniform reduced temperature of T=0.75. We then expend the top wall gradually at constant reduced temperature T=0.75 to some specified pressure (at this pressure, no vapor bubble appears). We then fix the pressure by allowing the top surface to freely slide up and down and instantaneously increase the temperature at the bottom of the solid to a higher temperature while maintaining the original lower temperature at the top wall. Heat transfers through solid to the liquid and evaporates liquid adjacent to the solid. Initially small vapor patches appear and disappear on the solid surface randomly in space and time until, at some later time, one of these patches successfully grows to a stable vapor bubble. With little or no body force, this nucleate grows until it covers nearly the whole solid-liquid interface and heat transfer essentially stops. An appropriately tuned uniform body force appears to deform and detach the vapor bubble as gravity would do in a terrestrial, macroscopic situation.
We initially discuss the effect of different solid-fluid energy parameters and their relation to surface wettablity and examine their effects on solid-fluid heat transfer and on vapor bubble nucleation and its growth. At zero body force, we find that bubble volume V grows nearly linearly in time t, independent of solid bottom temperature, solid thickness and random seeds in simulations. We examine how this dependence depends on the presence and strength of the body force. Note that our previous continuum model calculations yielded V ~ t1.5 and molecular dynamics exhibit a temperature slip at the solid-fluid interface that is absent in the continuum calculation. We track the variation of contact angle and the contact line’s motion as the bubble grows, deforms and detaches as the appropriately-defined Bond number approaches unity and input these dynamics into our earlier continuum model to see, among other things, how this motion affects that model’s predicted V(t).