Molecular Dynamics Study of Effect of Different Wetting Conditions on Evaporation and Rapid Boiling of Ultra–thin Argon Layer Over Platinum Surface

Surface Topography Effects on Pool Boiling via Non-equilibrium Molecular Dynamics Simulations

In this work, we investigate nucleate pool boiling via non-equilibrium molecular dynamics simulations. The effect of nano-structured surface topography on nucleation and transition to a film-like boiling regime is studied at the molecular scale, by varying the cavity aspect ratio, wall superheat, and wettability through a systematic parametric analysis conducted on a Lennard-Jones (LJ) system. The interplay of the aforementioned factors is rationalized by means of a classical nucleation theory-based model. The solid surface is heated uniformly from the bottom in order to induce the nanobubble nucleation. Insight into the cavity behavior in heat transfer problems is achieved by looking at temperature and heat flux profiles inside the cavity itself, as well as at the time of nucleation, for different operating conditions. The role of the cavity size in controlling the vapor embryo formation is highlighted, and its dependence on the other investigated parameters is summarized in a phase diagram. Our results show that heterogeneity at the nanoscale plays a key role in determining pool boiling heat transfer performance, suggesting a promising approach to optimize nanostructured surfaces for energy and thermal management applications.

A Molecular Dynamics Study of Heat Transfer Enhancement during Phase Change from a Nanoengineered Solid Surface

This study investigates the enhancement of the rate of evaporation from a nanoengineered solid surface using non-equilibrium molecular dynamics simulation. Four different types of surface modifications were introduced to examine the thermal transportation behavior. The surface modification includes: (1) transformation of surface wetting condition from hydrophobic to hydrophilic, (2) implementing nanostructures on the smooth surface, (3) cutting nano slots on the smooth surface and (4) introducing nano-level surface roughness. Evaporation behavior from the same effective surface area was also studied. The simulation domain consisted of three distinct zones: solid base wall made of copper, a few layers of liquid argon, and a vapor zone made of argon. All the nano-level surface modifications were introduced on the solid base surface. The few layers of liquid argon representing the liquid zone of the domain take heat from the solid surface and get evaporated. Outside this solid and liquid zone, there is argon vapor. The simulation began at the initial time t = 0 ns and then was allowed to reach equilibrium. Immediately after equilibrium was achieved on all three-phase systems, the temperature of the solid wall was raised to a higher value. In this way, thermal transportation from the solid wall to liquid argon was established. As the temperature of the solid wall was high enough, the liquid argon tended to evaporate. From the simulation results, it is observed that during the transformation from hydrophobic to hydrophilic conditions, enhancement of evaporation takes place due to the improvement of thermal transportation behavior. At the nanostructure surface, the active nucleation sites and effective surface area increase which results in evaporation enhancement. With nano slots and nano-level surface roughness, the rate of evaporation increases due to the increase of solid-liquid contact area and effective surface area.

Solid-like Behaviors Govern Evaporative Transport in Adsorbed Water Nanofilms

The thermophysical attributes of water molecules confined in a sub-nanometer thickness significantly differ from those in bulk liquid where their molecular behaviors start governing interfacial physics at the nanoscale. In this study, we elucidate nanothin film evaporation by employing a computational approach from a molecular perspective. As the liquid thickness decreases, the solid-like characteristics of adsorbed water nanofilms make the resistance at solid-liquid interfaces or Kapitza resistance significant. Kapitza resistances not only show a strong correlation with the surface wettability but also dominate the overall thermal resistance during evaporation rather than the resistance at evaporating liquid-vapor interfaces. Once the liquid thickness reaches the critical value of 0.5-0.6 nm, the evaporation kinetics is suppressed due to the excessive forces between the liquid and solid atoms. The understanding of molecular-level behaviors explains how a hydrophilic surface plays a role in determining evaporation rates from an atomistic perspective.

Molecular Dynamics Study of Bubble Nucleation on an Ideally Smooth Substrate

Questions regarding bubble nucleation on an ideally smooth surface are seemingly endless, but it can not be adequately verified yet because of the scale limitation (microscopic scale). Hence, in this study, bubble nucleation on an ideally smooth substrate is explored using the molecular dynamics simulation method. An ideally smooth hydrophilic platinum substrate at 145 K is conducted to heat the simple L-J liquid argon. Results show that a visible bubble nucleus successfully forms on the ideally smooth substrate without any additional disturbance, which is common in boiling studies using the traditional numerical simulation methods. However, the nucleation position is unpredictable. At the atomic level, the thermal energy transfer from an ideally smooth substrate to liquid atoms is inhomogeneous due to atomic inhomogeneous distribution and irregular movement, which are the key influencing factors for achieving bubble nucleation. The inhomogeneity will be highlighted with the heating process. As a result, some local liquid atoms near the ideally smooth surface absorb more thermal energy to overcome their potential barrier at a specific moment, causing the emergence of a distinct nucleus there. Furthermore, nanostructure substrates are introduced to make a comparison with the smooth substrate in bubble nucleation. There is no significant difference in the inception temperature of nucleation between the ideally smooth and nanostructure substrates, but the latter has better performance in improving the bubble nucleation rate.

Molecular Dynamics Study of Bubble Nucleation on a Substrate with Nonuniform Wettability

In the present study, the molecular dynamics simulation method is adopted to study bubble nucleation on a platinum substrate with nonuniform wettability. The central region of the substrate has strong hydrophilicity and both sides have weak hydrophobicity. It is interesting that the bubble nucleation happens in the hydrophobic region when the substrate temperature is low, and the nucleation position moves to the hydrophilic region with the increase of the substrate temperature. The intrinsic regime for the change of nucleation position with the substrate temperature is fully illustrated based on the competition between the suffered potential restriction and the absorbed thermal energy of liquid atoms. When the liquid atoms on one region obtain enough thermal energy to break their potential barrier, they convert into a bubble nucleus. Both the potential barrier for liquid atoms clinging to the substrate surface and the solid-liquid heat transfer efficiency improve with the enhancement of substrate hydrophilicity. The potential barrier is decided only by the atomic distribution and interatomic interaction. However, the substrate temperature changes the absorbed thermal energy of the liquid atoms within a specific time, causing the movement of the nucleation position. Furthermore, a hydrophilic nanostructure is introduced to replace the central smooth hydrophilic region and promote lateral heat transfer to the liquid on the hydrophobic region, leading to the improvement of the bubble nucleation efficiency.

Nanoscale Study of Bubble Nucleation on a Cavity Substrate Using Molecular Dynamics Simulation

In this paper, the molecular dynamics simulation method is utilized to investigate the phase transition behavior of an argon film placed on cavity substrates with different wettability conditions. A simple Lennard-Jones liquid is heated by a metal platinum substrate at different temperatures, and a complete process of bubble nucleation is successfully visualized on the cavity substrate at temperatures of 150 and 160 K. Moreover, the bubble nucleation behavior shows dependence on cavity wettability. A layer of liquid atom is attracted to the strongly hydrophilic cavity and obtains more energy to nucleate first. In contrast, the liquid atom suffers a large repulsive force from the metal atom in the hydrophobic cavity, thus an original small bubble nucleus stably stays inside before the incipient boiling time. With an increase in the heating time, the original bubble nucleus grows up from the hydrophobic cavity. This bubble nucleation behavior on a hydrophobic cavity is in agreement with macro theory, which states that a cavity provides an original nucleus for bubble formation and growth. Besides, cavity wettability plays a crucial role in the incipient boiling temperature of an argon film. The incipient boiling temperature increases with the weakening of the cavity hydrophobicity, and this trend is in accordance with macro experiments, which show that liquid is easier to boil on a more hydrophobic substrate.