Stable and Efficient Nanofilm Pure Evaporation on Nanopillar Surfaces

Abstract

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.

Nanoscale Thin-Film Boiling Processes on Heterogeneous Surfaces

“…Unlike traditional computational fluid dynamics simulations, the MD method can directly explore the nanoscopic origins, elucidate phenomena from a molecular perspective, and provide a fundamental understanding that is not accessible by experiments; thus, it has been widely used to investigate various interfacial phenomena, such as droplet wetting, , impact, − coalescence, , and evaporation, , as well as liquid boiling − and vapor condensation. ,− In this paper, nonequilibrium molecular dynamics simulations were performed to investigate the nanoscale thin-film boiling process. Aiming at understanding the initial bubble behaviors and the heat and mass transfer performance, this work was conducted and illustrated around the following objectives: (1) capturing the triple-phase interface to record the lifetime of nanobubbles, (2) visualizing the internal fluid flow and thermal characteristics, (3) studying the effects of surface physicochemical properties on boiling performance, and (4) revealing the essential regulation mechanism.…”

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

“…The miniaturization and integration of micro/nanoelectronic devices give rise to the high heat flux density of these devices and irresistible outcome of overheating. The failure rate of the device caused by overheating increases exponentially when its temperature is over 75 °C, which appeals to the development of efficient thermal management techniques to solve these thermal issues. , Due to the full utilization of both sensible heat and latent heat of coolants, the phase-change cooling technology possesses a larger capacity of heat removal and has attracted extreme attention currently. − As a high-efficiency method of phase-change cooling, flow condensation in microchannels, which serves to cool the boiling vapor heated by the heat sink, is crucial to reduce the energy and coolant input and thus improve the whole efficiency of the cooling system. Lots of studies have investigated flow condensation characteristics in microchannels, which reveal that heat transfer coefficients of condensation in microchannels could be enhanced with an increase in mass flux and a decrease in diameter. − On the other hand, previous investigations have found that condensation flow patterns in microchannels are mainly annular flow, slug flow, and bubbly flow, rather than the traditional stratified flow, which is due to the dominance of shear and surface tension at interfaces in microchannels. − Due to the flourishing development of manufacturing technology, the size of electronic devices is minimized from the microscale to the nanoscale, at which the system spatial scale is comparable to the molecular mean free path .…”