Investigation on molecular cluster behavior and initiation of capillary condensation within nanoarrays

Abstract

To examine the cluster transitions during adsorption and capillary condensation on heterogeneous surfaces, a sequence of molecular simulations is carried out in this work. Results show that compared with the smooth surface, more statistical adsorption sites are obtained on the nanostructured surface, and the corresponding potential energy is lower. The coalescence of clusters on the adsorption site induces the onset of capillary condensation when relative pressure of bulk vapor is less than unity. Once the capillary condensation is initiated, two distinct paths for film growth are observed. The first path is presented as gaseous atoms preferentially fill the adsorption sites to form various types of clusters. Then, multi-molecular clusters between adjacent nanopillars coalesce to form a liquid film and cover the nanostructure. As the surface wettability decreases, a thin liquid film is observed at the bottom of the nanostructure, then the liquid film continues to grow upwards until it completely covers the nanostructure, which is donated as the second path. After the liquid film is formed, single-molecular clusters are still detected near the surface. The number of residual single-molecular clusters does not change with the solid surface wettability but gradually approaches the lowest point with the increase of the relative pressure. The adsorption and condensation map shows that the relative pressure required for the initiation of capillary condensation increases from 0.05 to 0.90 as the decrease of surface wettability. Finally, the adsorption isotherms on the nanostructured surface are obtained and three different types of isotherms are characterized and distinguished with the variation of surface wettability.