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

Controlling vapor nucleation on micro-/nanostructured surfaces is critical to achieving exciting droplet dynamics and condensation enhancement. However, the underlying mechanism of nucleation phenomena remains unclear because of its nature of nanoscale and transience, especially for the complexstructured surfaces. Manipulating vapor nucleation via the rational surface design of micro-/nanostructures is extremely challenging. Here, we fabricate hierarchical surfaces comprising tapered nanowire bunches and crisscross microgrooves. Nanosteps are formed around the top of the nanowire bunches, where the nanowires all around agglomerate densely because of surface tension. The theoretical analysis and molecular dynamics simulation show that nanostep morphologies that are around the top of the nanowire bunches can enable a lower energy barrier and a higher nucleation capability than those of the sparsely packed nanowires at the center and bottom of the nanowire bunches. Vapor condensation experiments demonstrate that the nucleation preferentially occurs around the top of the nanowire bunches. The results provide guidelines to design micro-/nanostructures for promoting vapor nucleation and droplet removal in condensation.

Section: Introductionmentioning

confidence: 99%

Preferential Vapor Nucleation on Hierarchical Tapered Nanowire Bunches

Cheng

Yang

et al. 2020

“…For the interaction between fluid atoms, ε ff = 0.0104 eV and σ ff = 3.40 Å. For the interaction between fluid–solid atoms, ε fs = 2ε ff and σ fs = 0.91σ ff , which guarantees a strong surface wettability , and benefits mass and heat transfer.…”

Section: Computational Methodsmentioning

confidence: 99%

Stable and Efficient Nanofilm Pure Evaporation on Nanopillar Surfaces

Wang

Sun

et al. 2021

Self Cite

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.

“…Plenty of studies have revealed the condensation mechanisms and characteristics on nanosurfaces. − The droplet condensation on the surface can be classified into dropwise condensation and filmwise condensation. Due to the smaller thermal resistance and disturbance by the dropping of droplets, dropwise condensation has a superior heat transfer capability than filmwise condensation .…”

Section: Introductionmentioning

confidence: 99%

“…Using the molecular dynamics (MD) method, droplet trajectories during condensation were monitored, which revealed that dropwise condensation processes contained droplet nucleation, growth, coalescence, and departure . The surface wettability, heat flux, and nanostructures can exert a significant impact on droplet dynamics behaviors on nanosurfaces. , Moreover, it is preferable to avoid the transition from dropwise condensation to filmwise condensation, which could be controlled by tuning both the surface wettability and vapor-to-surface temperature difference . In short, current condensation studies mainly concentrate on heat transfer and transition of condensation on nanosurfaces, but they do not consider flow behaviors of condensation droplets or films.…”

Section: Introductionmentioning

confidence: 99%

Flow Condensation Heat Transfer Characteristics of Nanochannels with Nanopillars: A Molecular Dynamics Study

Wang

Sun

Cheng³

2021

Flow condensation in nanochannels is a high-efficiency method to deal with increasingly higher heat flux from micro/nanoelectronic devices. Here, we study the flow condensation heat transfer characteristics of nanochannels with different nanopillar crosssectional areas and heights using molecular dynamics simulation. Results show that two phases containing vapor in the middle of the channel and liquid near walls can be distinguished by obvious interfaces when the fluid is at a stable state. The condensation performance can be promoted by adding nanopillars. With the increase in nanopillar crosssectional areas or heights, the time that the fluid spends to reach stability will be put off, while the condensation performance enhances. Different from the small enhancement of nanopillar cross-sectional areas, the condensation heat transfer performance improves significantly at a higher nanopillar height, which increases the heat transfer rates by 11.6 and 35.8% when heights are 6a and 8a, respectively. The preeminent condensation heat transfer performance is ascribed to the fact that nanopillars with a higher height disturb the vapor−liquid interface and vapor region, which not only allows vapor atoms with strong Brownian motion to collide with nanopillar atoms directly but also increases deviations of vapor−liquid potential energy to facilitate condensation heat transfer in nanochannels.