A molecular dynamics study of water vapor nucleation in the presence of ions

Enrichment Effects Induced by Non-uniform Wettability Surfaces in the Presence of Non-condensable Gas: A Molecular Dynamics Simulation

For vapor condensation, the control of heterogeneous nucleation and spatial distribution of nuclei are crucial for regulating droplet dynamics and improving condensation efficiency. However, due to the complex characteristics of multicomponent, multiphase, and multiscale, the underlying mechanism of mixed vapor condensation remains unclear, especially at the nucleation stage. In this paper, we focus on the enrichment effects of non-uniform wettability surfaces by molecular dynamics simulation, which could intensify the droplet nucleation and growth processes in a water-air mixed system. The results clarify the inhibitory effect of non-condensable gas on droplet nucleation and prove that only 1% of non-condensable gas could reduce one half of the condensation performance from a molecular perspective. Furthermore, non-uniform surfaces are designed to promote the efficient enrichment of vapor molecules on nucleation sites, and the synergistic effect of hydrophilic and hydrophobic regions is proposed. In addition, the non-uniform wettability surfaces are characterized by varying the proportion and dispersion of hydrophilic regions. The results reveal that an optimal proportion of hydrophilic region (R = 5/6) could furnish the non-uniform surface with the best transfer performance. Moreover, the enhancement of condensation performance can also be achieved through the dispersed arrangement of hydrophilic regions. The results provide guidance for the optimized design of functionalized surfaces with enhanced mixed vapor condensation.

Nucleation parameters of SPC/E and TIP4P/2005 water vapor measured in NPT molecular dynamics simulations

Nucleation rates for droplet formation in water vapor are measured in molecular dynamics (MD) simulations of SPC/E and TIP4P/2005 water by monitoring individual nucleation events. The nucleation process is simulated in the NPT ensemble to evaluate the steady-state nucleation rate in accordance with the assumptions of classical nucleation theory (CNT). Nucleation rates measured between 300 and 425 K for the SPC/E model, and between 325 and 475 K for the TIP4P/2005 model, agree with the CNT predictions roughly within the standard deviation of the MD measurements of the nucleation rates.