Multimode Multidrop Serial Coalescence Effects during Condensation on Hierarchical Superhydrophobic Surfaces

Abstract

The prospect of enhancing the condensation rate by decreasing the maximum drop departure diameter significantly below the capillary length through spontaneous drop motion has generated significant interest in condensation on superhydrophobic surfaces (SHS). The mobile coalescence leading to spontaneous drop motion was initially reported to occur only on hierarchical SHS, consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on nanostructured surfaces rather than on hierarchical SHS. In this work, we investigate the impact of microscale topography of hierarchical SHS on the droplet coalescence dynamics and wetting states during the condensation process. We show that isolated mobile and immobile coalescence between two drops, almost exclusively focused on in previous studies, are rare. We identify several new droplet shedding modes, which are aided by tangential propulsion of mobile drops. These droplet shedding modes comprise of multiple droplets merging during serial coalescence events, which culminate in formation of a drop that either departs or remains anchored to the surface. We directly relate postmerging drop adhesion to formation of drops in nanoscale as well as microscale Wenzel and Cassie−Baxter wetting states. We identify the optimal microscale feature spacing of the hierarchical SHS, which promotes departure of the highest number of microdroplets. This optimal surface architecture consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state yet, at the same time, provides sufficient spacing in-between the features for occurrence of mobile coalescence.

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

“…In principle, any sub-microscale structures with a small feature size (tip size and interspace) and a certain height (or depth) can become effective candidates for creating CMDSP surfaces. In fact, except for arrays of closely packed nanotips, including nanocones, [22,27,50,69,70] nanoneedles, [21,28,31,66,71] nanopencils, [23] and tip-like nanotubes, [72,73] other architectures such as nano wires, [24,74] nanosheet arrays, [20,29,62,75] nanorod-capped nano pores, [68,76] the porous structure of nanoparticles, [67] nanoparticle aggregates, [73,[77][78][79][80] and two-tier structures [25,57,63,[81][82][83][84][85][86][87][88][89] have all been verified to be effective in endowing material surfaces with the desired CMDSP functionality as long as they follow these basic construction rules.…”

“…Benefiting from the wellestablished top-down micro-and nanofabrication technologies, various silicon nanostructures (e.g., nanoneedles [71,90] and nanocones [69] ) and their combination with regular microstructures (e.g., micropillars [25,85] and micropyramids [84] ) have been successively made for creating bionic CMDSP surfaces, and the transport behaviors of condensate microdrops at the microscale showing the typical coalescence-induced self-propelling details of condensed microdrops on the nanoneedle surface. f,g) Reproduced with permission.…”

“…[25,85,[110][111][112][113][114] The latest nanobionic research shows that the inclined setae on the legs of the water strider [4] (Figure 9c,d) and ratchet-like microstructures on butterfly wings [9] (Figure 9e,f) have the remarkable ability to directionally transport condensed microdrops. The next issue is to develop metal-based CMDSP surfaces with rationally designed wettability-contrast patterns and asymmetrical structures at the microscale, in order to achieve more efficient condensation heat-transfer performance.…”

Exploiting Microscale Roughness on Hierarchical Superhydrophobic Copper Surfaces for Enhanced Dropwise Condensation

“…[13][14][15] Previous studies [14,15] have shown that on superhydrophobic surfaces with only microscale roughness, condensate droplets tend to nucleate and grow in the cavities between microstructures, forming sticky droplets in the Wenzel state, [16] which are pinned strongly to the surface at the three-phase contact line. To overcome this limitation, superhydrophobic surfaces with nanoscale [17,18] or hybrid micro/nanoscale [19][20][21][22][23][24] roughness have been developed that enable the formation of condensate droplets in the Cassie state.…”

“…[18][19][20][21][22][23] These studies are limited to silicon substrates, however, and are not suitable for scaled-up industrial applications. Therefore, it is highly desirable to develop hybrid surfaces that are compatible with materials commonly employed for heat transfer applications, such as copper.…”