Abstract

As electronic devices continue to reduce in size, effective thermal management becomes increasingly critical. Thin-film evaporation offers a promising solution due to its high heat flux capacity and passive nature. However, the applicability of continuum theory to thin-film evaporation becomes uncertain at the nanoscale. This work presents a two-phase Molecular Dynamics(MD) study of liquid argon confined within two parallel platinum walls, focusing on both equilibrium and non-equilibrium phase change behavior under nanoscale confinement.

In the first phase, Equilibrium Molecular Dynamics (EMD) simulations investigate the influence of channel height (4, 8, and 16 nm) and wall-fluid interaction strength on bulk and interfacial properties. Smaller channels exhibit pronounced liquid layering and stress oscillations, with negative liquid pressures observed under strong wall attraction. Surface tension is evaluated via both atomic-scale mechanical stress and the continuum-based Young–Laplace relation. Surface tension found using mechanical stress shows convergence to bulk value with increase in channel height, while surface tension computed from the Young-Laplace equation fluctuates according to the wetting conditions.

In the second phase, Non-Equilibrium Molecular Dynamics (NEMD) simulations are conducted in an 8 nm channel to model steady-state evaporation and condensation under three different sets of applied wall temperature (hot walls: 130–150 K, cold walls: 90–70 K), which induces thermal gradient inside the channel. The evaporating interface is divided into three regions: adsorbed layer, thin film, and meniscus. The thin film region dominates the overall mass flow rate, whereas the meniscus shows localized condensation and even severe flow reversal. Notably, the adsorbed layer, which is traditionally considered non-evaporating, contributes approximately 20% of total mass flux, emphasizing its role in nanoscale evaporation.

Surface tension increases with greater temperature gradient inside the channel, resulting in larger pressure drops but a reduction in mass flow rate, revealing an inverse relationship unique to superheated meniscus. On the condensing front, the temperature jumps and Knudsen layers with thicknesses up to five times the mean free path are demonstrated. These non-equilibrium phenomena further emphasize the limitations of classical models. The findings in this study provide new insight into the coupling of confinement, interface curvature, and thermodynamic conditions, informing future nanoscale phase change and heat transport modeling.

Degree Date

Spring 5-17-2025

Document Type

Thesis

Degree Name

M.S.M.E.

Department

Mechanical Engineering

Advisor

Ali Beskok

Format

.pdf

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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