Molecular dynamics simulation of nanoscale condensation on hybrid wetting surface

Abstract

In this study, molecular dynamics simulations have been conducted in order to investigate the condensation process over hybrid wetting surfaces at nanoscale. With a view of pursing this objective, an enclosed three-phase atomistic system has been modeled, consisting of two solid surfaces and two different phases of fluid atoms. The temperature of the upper and lower solid substrates is kept at 130 K and 90 K, respectively, to make them serve as evaporating and condensing surfaces. In the simulation process, during the non-equilibrium period, the vaporized fluid atoms move from the lower surface to the upper surface and start to get condensed at the upper surface. The wetting characteristics of the lower surface are kept unchanged in all of the cases to corroborate uniform evaporating conditions. The upper condensing surface's wetting configuration was altered to assess how different wetting profiles can affect nanoscale condensation phenomena at different philic-phobic proportions and wettability contrasts. On the upper surface, two types of wetting profiles have been modeled, namely, gradient and patterned wetting profiles. Functional wettability gradient (FWG) surfaces have been modeled using the power-law function from the concept of functionally graded material (FGM), whereas patterned wetting surfaces have been developed by juxtaposing different wettability zones. For determining the performance of hybrid wetting surfaces at different wettability contrasts, four different cases have been modeled by modifying the wettability of the philic-phobic atoms. Nucleation, coalescence, and growth of the condensate, solid-liquid interfacial thermal resistance, condensed atoms, condensation mass flux, condensing wall heat flux, and surface tension profile are the important parameters evaluated for assessing the performance of the condensing surfaces. The simulation results have revealed that an increased proportion of hydrophilic atoms along with lower strip size of philic-phobic pattern enhances the condensation heat transfer. In addition, FWG surfaces have been found to be preferred to patterned ones in terms of condensation heat transfer enhancement, specifically on surfaces with larger hydrophobic fraction. The effect of wettability contrast is prominent on the surfaces with larger hydrophobic fraction and better performance has been observed in case of higher wettability of solid-fluid atoms in terms of condensation heat transfer rate.