Molecular Dynamics Simulation of the Influence of Nanoscale Structure on Water Wetting and Condensation

Abstract

Recent advances in the microfabrication technology have made it possible to control surface properties at micro- and nanoscale levels. Functional surfaces drastically change wettability and condensation processes that are essential for controlling of heat transfer. However, the direct observation of condensation on micro- and nanostructure surfaces is difficult, and further understanding of the effects of the microstructure on the phase change is required. In this research, the contact angle of droplets with a wall surface and the initial condensation process were analyzed using a molecular dynamics simulation to investigate the impact of nanoscale structures and their adhesion force on condensation. The results demonstrated the dependence of the contact angle of the droplets and condensation dynamics on the wall structure and attractive force of the wall surface. Condensed water droplets were adsorbed into the nanostructures and formed a water film in case of a hydrophilic surface.

Keywords: condensation; functional surface; molecular dynamics; nanoscale structure; wettability.

Figure 1

Schematic diagrams of Wenzel state and Cassie-Baxter state.

Figure 2

Prepared surface structure. (flat, asperity height 0.724 nm, asperity height 1.448 nm, and asperity height 2.896 nm).

Figure 3

Schematic diagram of contact angle θ measurement on nanostructures by half angle method.

Figure 4

Snapshot of water vapor and nanostructure prepared as the initial condition of the condensation simulation.

Figure 5

Relation between the water–wall interaction and the contact angle. The surface with asperity resulted in the increase of the contact angle.

Figure 6

Snapshot of the water film on the flat and nanostructured surfaces with the Lennard-Jones parameter ε = 19.74 kJ/mol (the left upper: 2259 water molecules, others: 3787 water molecules).

Figure 7

Droplets on nanostructures with ε = 3.948 kJ/mol.

Figure 8

Droplets on nanostructures with ε = 0.9869 kJ/mol.

Figure 9

Snapshot of water molecules during the condensation on the ε = 19.74. kJ/mol surface from the different viewpoints. The water molecules are adsorbed into the pillar and formed water film.

Figure 10

Snapshot of water molecules during the condensation on the ε = 1.974. kJ/mol surface from the different viewpoints. The water molecules formed small clusters in the pillar.

Figure 11

Snapshot of water molecules during the condensation on the ε = 0.06168. kJ/mol surface from the different viewpoints. The water droplet did not enter the nanostructure.

Figure 12

Absorption behavior of water droplets intruding into the inside of asperities with ε = 1.976 kJ/mol. (0 s, 35 ps, 90 ps, 350 ps).

Figure 13

Snapshot of the two-dimensional structure of water observed in a nanostructured surface with ε = 19.74. kJ/mol.

Figure 14

Number of water molecules in the nanostructure on surfaces.

Figure 15

Mean square displacement (MSD) of water molecules in the nanostructure on surfaces.