Enhancing Recurrent Droplet Jumping Phenomena on Heterogeneous Surface Designs

Abstract

transfer by facilitating droplet removal through coalescence-induced droplet jumping behaviors. [15,17,[21][22][23][24][25][26][27][28] On superhydrophobic surfaces, when two or more adjacent droplets grow and coalesce into a larger droplet, droplet jumping occurs by transferring excessive surface energy of the droplets into kinetic energy in the normal direction of the surface by overcoming surface adhesion between the surface and droplets. [24,[29][30][31] However, the enhancement of condensation heat transfer using homogeneous surface characteristics remains a challenge due to several competing factors. More specifically, despite their great capability to remove droplets from the surfaces, superhydrophobic surfaces are energetically unfavorable for new droplet nucleation. Their high energy barrier for water nucleation requires high supersaturation levels. [32][33][34] Conversely, undesired droplet pinning into the microstructures eventually results in irreversible flooding, which degrades heat transfer performance. [18,19] To overcome the challenge mentioned above, various engineering surfaces inspired by natural organisms living in the arid regions, such as cactus leaves or desert beetles, have been suggested by integrating hierarchical or heterogeneous morphologies. [35][36][37][38][39][40][41][42] In many cases, heterogeneous surfaces address the drawbacks of homogeneous superhydrophobic surfaces. For example, biphilic surfaces inspired by desert beetles have enhanced condensation heat transfer by employing both hydrophobic and hydrophilic wettability. [29,43,44] Specifically, while hydrophilic areas continuously provide new condensation nuclei, hydrophobic areas improve heat transfer performance by efficient droplet removal. [8,18,45] Another past study showed that nanostructured hydrophilic surfaces allow stable dropwise condensation by promoting new condensation nuclei on surface patterns and delaying the transition from dropwise to filmwise condensation. [19] The droplets on biphilic surfaces have much lower interfacial thermal resistances compared to suspended droplets on hydrophobic surfaces, [21] contributing to better heat transfer. [44,46] Recently, hierarchical structures, such as micro-patterned nanowires, two-tier micropyramids, and microposts decorated with flowerlike nanofeatures, [1,2,[47][48][49][50] have been suggested to enable droplet jumping-enhanced condensation. With those surfaces, it is observed that condensation can be much improved with the non-homogeneous or heterogeneous wetting characteristics that can be responsible for two different Coalescence-induced droplet jumping phenomena on superhydrophobic surfaces can significantly enhance their heat transfer performances by effectively removing droplets from the surfaces. However, understanding the ideal design for condensing surfaces is still challenging due to the complex nature of droplet dynamics associated with their nucleation, coalescing, and jumping mechanisms. The intrinsic dynamic nature of droplet behaviors suggests the us...

Enhanced Capillary Wicking through Hierarchically Porous Constructs Derived from Bijel Templates

“…Liquid transport through porous media continues to draw great interest in the scientific community due to its technological relevance in applications, ranging from oil separation , to battery technologies, − catalysis, , and thermal management. − Porous materials offer a number of important properties in this context, including high specific surface area, sample-spanning thermal conductivity, mechanical flexibility, and liquid capillarity. − Among these properties, liquid capillarity, which mediates the ability to sustain continuous liquid flow through porous media via capillary action or capillary wicking, is a particularly attractive feature as it can help to enhance a two-phase heat transfer by preventing local or global dry-out regions. − To this end, various types of porous structures, such as microposts, copper inverse opals, and vertically aligned nanowires, have been suggested and evaluated for use in phase change applications. − However, to improve liquid capillarity, one must navigate an intricate balance to maximize capillary forces, which favors smaller pores, and minimize friction forces, which necessitates a reduction in surface area (or increase in pore size). As such, porous materials with a uniform pore morphology fall short in meeting the demands of the rapidly evolving thermal management technologies that rely on liquid capillarity and phase change heat transfer.…”