The liquid-vapor interface has been subject to extensive studies for more than one century because of its critical importance in many industrial applications, such as phase-change heat transfer, spread wetting, and material processing. The thickness of an interfacial region is in the nanometer range, making experimental studies of such a thin region extremely difficult. Although there exist a few experimental works on the liquid-vapor interface, physical understanding of interfacial phenomena still relies heavily on theoretical analysis and numerical simulations. Molecular Dynamics (MD) simulation is one of the most effective tools to study interfacial phenomena since it can yield detailed information on the molecular structure of an interface if the appropriate intermolecular potential is given.

This work applies the MD simulation method to study both planar and curved liquid-vapor interfaces. In the case of the planar interface, a liquid film sandwiched by its vapor is constructed to investigate the film thickness effect on its surface tension and film stability. The liquid film thickness L_s varies from 4.28s to 8.55s. Here, s is the length constant in Lennard-Jones 12-6 potential. For the curved interface, a liquid droplet and a vapor bubble are simulated to study the curvature effect on the surface tension.

Simulation results of the planar interface indicate that before L_s becomes so small that film rupture occurs, surface tension values are constant with the change of film thickness. However, the distribution of local stress (in the direction parallel to film surface) appears quite different. When the film gets thinner, two liquid-vapor interfaces move closer and start to interact (overlapping of two interfacial regions), as shown in Fig. 1 for two cases of L_s = 8.55s and L_s = 4.70s. As a result, the stress at the film center increases. Further decrease of film thickness destroys the film. Figure 2 depicts the initial and final position of molecules in the computational domain for the film with L_s = 4.28s. Results also show that if the system temperature increases or the cross sectional area of the computation domain increases, the film will be less stable.

Fig. 1 Local stress distribution at T ^* =0.818 (Z^* corresponds to the film center)

Initial Condition	Final Condition
X-Z View Y-Z View	X-Z View Y-Z View

Fig. 2 Initial and final position of molecules in computational domain

Figure 3 illustrates the curvature effect on surface tension for droplets and bubbles where g is the surface tension for a curved interface, g₀ the surface tension for a planar interface, R_s the radius of the surface of tension, and d the Tolman's length. Classically, the curvature effect can be express by Tolman's equation, in which the surface tension of droplets (positive curvature) increases with the radii while that of bubbles (negative curvature) decreases. It can be observed in Fig. 3 that the surface tension of droplets does follow Tolman's prediction very well while the surface tension of bubbles does not. The physical mechanism in this difference is still under investigation.

Fig. 3 Curvature effect on surface tension for droplets and bubbles at T*=0.818

Future work includes further explore on interface stability for planar and curved interfaces and simulation of other interesting phenomena in the solid-liquid interface and three phase equilibrium, such as disjoin pressure, the contact angle, and surface wettability.

References

Park, S. H., J. G. Weng, and C. L. Tien, "A Molecular Dynamics Study of Surface Tension of Microbubbles," to appear in International Journal of Heat and Mass Transfer (2000).

Weng, J. G., S. H. Park, J. R. Lukes, and C. L. Tien, "Molecular Dynamics Investigation of Thickness Effect on Liquid Films," Journal of Chemical Physics, 113, 5917-5923 (2000).

Park, S. H., J. G. Weng, and C. L. Tien, "Cavitation and Bubble Nucleation Using Molecular Dynamics Simulation," Microscale Thermophysical Engineering, 4, 161-175 (2000).

Last updated October 28, 2000.
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