Disjoining Pressure Effects in Ultra-Thin Liquid Films in Micropassages—Comparison of Thermodynamic Theory With Predictions of Molecular Dynamics Simulations

[+] Author and Article Information
V. P. Carey

Mechanical Engineering Department,  University of California, Berkeley, CA 94720-1740vcarey@me.berkeley.edu

A. P. Wemhoff

Mechanical Engineering Department,  University of California, Berkeley, CA 94720-1740wemhoff2@llnl.gov

J. Heat Transfer 128(12), 1276-1284 (Mar 01, 2006) (9 pages) doi:10.1115/1.2349504 History: Received August 26, 2005; Revised March 01, 2006

The concept of disjoining pressure, developed from thermodynamic and hydrodynamic analysis, has been widely used as a means of modeling the liquid-solid molecular force interactions in an ultra-thin liquid film on a solid surface. In particular, this approach has been extensively used in models of thin film transport in passages in micro evaporators and micro heat pipes. In this investigation, hybrid μPT molecular dynamics (MD) simulations were used to predict the pressure field and film thermophysics for an argon film on a metal surface. The results of the simulations are compared with predictions of the classic thermodynamic disjoining pressure model and the Born-Green-Yvon (BGY) equation. The thermodynamic model provides only a prediction of the relation between vapor pressure and film thickness for a specified temperature. The MD simulations provide a detailed prediction of the density and pressure variation in the liquid film, as well as a prediction of the variation of the equilibrium vapor pressure variation with temperature and film thickness. Comparisons indicate that the predicted variations of vapor pressure with thickness for the three models are in close agreement. In addition, the density profile layering predicted by the MD simulations is in qualitative agreement with BGY results, however the exact density profile is dependent upon simulation parameters. Furthermore, the disjoining pressure effect predicted by MD simulations is strongly influenced by the allowable propagation time of injected molecules through the vapor region in the simulation domain. A modified thermodynamic model is developed that suggests that presence of a wall-affected layer tends to enhance the reduction of the equilibrium vapor pressure. However, the MD simulation results imply that presence of a wall layer has little effect on the vapor pressure. Implications of the MD simulation predictions for thin film transport in micro evaporators and heat pipes are also discussed.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 5

Simulated argon vapor pressure values from the hybrid MD simulation of the wall-affected film compared to results for a simulated thick liquid film, the NPT plus test particle method, and ASHRAE recommended values. Simulations were run for 400,000 time steps for this study.

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Figure 6

Calculated local pressure profile for argon on a metallic solid surface using MD and hydrostatic analyses; external conditions match saturation data for argon at 1atm. Simulation featured 300,000 time steps, 50 collection bins, and approximately 1980 molecules.

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Figure 1

Cross section of a micropassage containing thin liquid films

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Figure 2

Schematic used for derivation of disjoining pressure

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Figure 3

MD simulation domain

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Figure 4

Argon liquid film mass density profile: Tr=0.57, 100 collection bins, and 400,000 time steps

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Figure 7

Mass density profile for argon film on solid surface, Tr=0.6. Film thickness was calculated as 2.6nm. Simulation was run with lateral dimensions of 10.0σLJ×10.0σLJ and 500,000 time steps.

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Figure 8

Comparison of MD results with conventional theory

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Figure 9

Wall layer model

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Figure 10

Comparison of MD simulation results with predictions of conventional theory and the wall layer model



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