Stable and Efficient Nanofilm Pure Evaporation on Nanopillar Surfaces.

Molecular dynamics simulations were conducted to systematically investigate how to maintain and enhance nanofilm pure evaporation on nanopillar surfaces. First, the dynamics of the evaporation meniscus and the onset and evolution of nanobubbles on nanopillar surfaces were characterized. The meniscus can be pinned at the top surface of the nanopillars during evaporation for perfectly wetting fluid. The curvature of the meniscus close to nanopillars varies dramatically. Nanobubbles do not originate from the solid surface, where there is an ultrathin nonevaporation film due to strong solid-fluid interaction, but originate and evolve from the corner of nanopillars, where there is a quick increase in potential energy of the fluid. Second, according to a parametric study, the smaller pitch between nanopillars (P) and larger diameter of nanopillars (D) are found to enhance evaporation but also raise the possibility of boiling, whereas the smaller height of nanopillars (H) is found to enhance evaporation and suppress boiling. Finally, it is revealed that the nanofilm thickness should be maintained beyond a threshold, which is 20 Å in this work, to avoid the suppression effect of disjoining pressure on evaporation. Moreover, it is revealed that whether the evaporative heat transfer is enhanced on the nanopillar surface compared with the smooth surface is also affected by the nanofilm thickness. The value of nanofilm thickness should be determined by the competition between the suppression effect on evaporation due to the decrease in the volume of supplied fluid and the existence of capillary pressure and the enhancement effect on evaporation due to the increase in the heating area. Our work serves as the guidelines to achieve stable and efficient nanofilm pure evaporative heat transfer on nanopillar surfaces.

Dependences of Formation and Transition of the Surface Condensation Mode on Wettability and Temperature Difference.

In this work, we use molecular dynamics (MD) simulations to investigate the dependences of formation and transition of surface condensation mode on wettability (ß) and vapor-to-surface temperature difference (ΔT). We build a map of different surface condensation modes against ß and ΔT based on plenty of MD simulation results and reveal five formation mechanisms and two transition mechanisms. At low ß and ΔT, the high free energy barrier (ΔG*) prevents any surface clusters from surviving, therefore no-condensation (NC) is observed. The formation of dropwise condensation (DWC) could evolve from either nucleation or film rupture. Similarly, the formation of filmwise condensation (FWC) could evolve from either nucleation or the adsorption-induced film. The transition between NC and DWC is determined by ΔG* according to classical nucleation theory. The transition between DWC and FWC depends on the stability of condensate film; there emerges the competition between the trend that the uneven condensate film contracts and ruptures to droplets favored by lower ß and the trend that the uneven condensate film continues growing promoted by higher ΔT. We finally present a schematic overview of all of the mechanisms revealed for a better understanding of the physical phenomenon of the surface condensation mode.

Generation and Evolution of Nanobubbles on Heated Nanoparticles: A Molecular Dynamics Study.

Molecular dynamics simulations were conducted to investigate the generation and evolution of nanobubbles on heated gold-like nanoparticles (GNPs). The effects of surface wettability (ß) and heating intensity (Q) of the GNPs are studied. We found that nanobubbles are generated faster on the superhydrophobic GNP than on the superhydrophilic GNP where nanobubble formation appears after a delay. In the case of the superhydrophilic GNP, the nanobubble is observed to grow explosively because it is initially generated at a distance from the GNP surface instead of on its surface. In the case of the superhydrophobic GNP, the faster generation of the nanobubble is promoted by the larger temperature difference between the GNP and the surrounding fluid and an ultrathin low-density layer that exists before the GNP is heated. For a given ß, faster generation and growth of nanobubbles are observed with increasing Q. Furthermore, the maximum radius of the nanobubble is found to be dependent on ß and not Q. The mechanism is elaborated based on the thermal resistance analysis at the melting point of GNPs. Additionally, it was found that there exists a threshold Q for nanobubble generation and the threshold value for the case of the superhydrophobic GNP is lower than that for the case of the superhydrophilic GNP. The present results have demonstrated that the superhydrophobic GNP is favorable for fast and energy-saving nanobubble generation. Our work provides further understanding in the generation and evolution of nanobubbles and potentially offers a new insight for nanobubble manipulation.