Hydrophobic nanoscale ribs alter the condensation mode of water vapor: A molecular dynamics simulation

This study employs molecular dynamics simulations to investigate the condensation behavior of water vapor on nano-ribbed copper substrates featuring three distinct rib heights and tunable surface wettability, with the primary objective of elucidating the microscopic mechanisms governing the critical potential well depth at which the transition from dropletwise to filmwise condensation occurs as a function of increasing nanoscale ribs height. By modulating the surface wettability via systematic variation of the potential well depth between copper and oxygen atoms, the study constructed three sets of nanorib models—low-rib (LC, 10 Å), medium-rib (MC, 20 Å), and high-rib (HC, 30 Å)—and conducted quantitative analyses of condensation morphology, intrinsic contact angles, and the evolution of the system's potential energy. The results demonstrate that the wettability of the nanoscale rib is the key factor governing the condensation morphology: hydrophobic nanoscale ribs can transform a flat substrate—which would otherwise exhibit filmwise condensation—into dropwise condensation, whereas hydrophilic nanoscale ribs sustain continuous liquid film growth. Moderate increases in nanoscale ribs height can reduce the dependence of condensation morphology on wettability while simultaneously elevating the critical potential well depth required for the condensation mode transition. On low-rib (LC) substrates, an abrupt morphological transition occurs at a potential well depth of 0.007 eV, with a complete shift to filmwise condensation at 0.010 eV. On medium-rib (MC) substrates, a continuous liquid film forms only when the potential well depth reaches 0.011 eV. For high-rib (HC) substrates, the critical potential well depth for the condensation transition is 0.008 eV. When the characteristic height of the nanoscale rib in the Z-direction is comparable to the thickness of the condensed water layer, the modulating effect of wettability on the condensation contact angle is most pronounced. This study elucidates the synergistic regulatory mechanism by which nanostructure height and surface wettability govern the phase transition of water vapor, thereby offering a novel theoretical foundation for the structural optimization of atmospheric water-harvesting substrates.