Coalescence-induced droplet jumping has received more attention recently, because of its potential applications in condensation heat transfer enhancement, anti-icing and self-cleaning, etc. In this paper, the molecular dynamics simulation method is applied to study the coalescence-induced jumping of two nanodroplets with equal size on the surfaces of periodic strip-like wettability patterns. The results show that the strip width, contact angle and relative position of the center of two droplets are all related to the jumping velocity, and the jumping velocity on the mixed-wettability superhydrophobic surfaces can exceed the one on the perfect surface with a 180° contact angle on appropriately designed surfaces. Moreover, the larger both the strip width and the difference of wettability are, the higher the jumping velocity is, and when the width of the hydrophilic strip is fixed, the jumping velocity becomes larger with the increase of the width of the hydrophobic strip, which is contrary to the trend of fixing the width of the hydrophobic strip and altering the other strip width.
When changing surface wettability and nanostructure size, condensation behavior displays distinct features. In this work, we investigated evaporation on a flat hydrophilic surface and condensation on both hydrophilic and hydrophobic nanostructured surfaces at the nanoscale using molecular dynamics simulations. The simulation results on hydrophilic surfaces indicated that larger groove widths and heights produced more liquid argon atoms, a quicker temperature response, and slower potential energy decline. These three characteristics closely relate to condensation areas or rates, which are determined by groove width and height. For condensation heat transfer, when the groove width was small, the change of groove height had little effect, while change of groove height caused a significant variation in the heat flux with a large groove width. When the cold wall was hydrophobic, the groove height became a significant impact factor, which caused no vapor atoms to condense in the groove with a larger height. The potential energy decreased with the increase of the groove height, which demonstrates a completely opposing trend when compared with hydrophilic surfaces.
The influence of different wettability on explosive boiling exhibits a significant distinction, where the hydrophobic surface is beneficial for bubble nucleation and the hydrophilic surface enhances the critical heat flux. Therefore, to receive a more suitable surface for the explosive boiling, in this paper a hybrid hydrophobic–hydrophilic nanostructured surface was built by the method of molecular dynamics simulation. The onset temperatures of explosive boiling with various coating thickness, pillar width, and film thicknesses were investigated. The simulation results show that the hybrid nanostructure can decrease the onset temperature compared to the pure hydrophilic surface. It is attributed to the effect of hydrophobic coating, which promotes the formation of bubbles and causes a quicker liquid film break. Furthermore, with the increase of the hydrophobic coating thickness, the onset temperature of explosive boiling decreases. This is because the process of heat transfer between the liquid film and the hybrid nanostructured surface is inevitably enhanced. In addition, the onset temperature of explosive boiling on the hybrid wetting surface decreases with the increase of pillar width and liquid film thickness.
The phenomenon of droplet coalescence and jumping has received increasing attention due to its potential applications in the fields of condensation heat transfer and surface self-cleaning. Basic research on the process and mechanism of coalescence-induced droplet jumping has been carried out, and some universal laws have been established. However, it is found that the focus of these studies is based on two identical droplets, and the coalescence-induced jumping with different radii is rarely investigated, which is commonly encountered in nature. Therefore, it is essential to proceed with the research of coalescence and jumping of droplets with unequal radii. In this paper, molecular dynamics (MD) simulations are performed to reveal the effects of radius ratio and radius of small droplets on jumping velocity. The results show that as the increasing of radius ratio with an unchanged small droplet radius of 8.1 nm, the jumping velocity increases then decreases, which indicates there is an optimal radius ratio to maximize the jumping velocity. Additionally, it is found that if the small droplet radius is changed, the critical radius ratio for characterizing whether the coalesced droplet jumping increases with increasing the small droplet radius. Furthermore, according to energy conservation, the conversion efficiency of energy is discussed. The results show that when the radius ratio is greater than 1.3 with three different small droplet radii, the energy conversion efficiency rapidly decreases to below 1.0%; and the critical radius ratios are consistent with the result obtained from the velocity analysis. This work broadens the understanding of the more general phenomenon of coalescence-induced droplet jumping and can better guide industrial applications.
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