How nanorough is rough enough to make a surface superhydrophobic during water condensation?

Abstract

Nanostructured surfaces which manifest superhydrophobic properties during water condensation have a potential to dramatically enhance energy efficiency in power generation and desalination systems. Although various such surfaces have been reported, their development has been fortuitous, not driven by an understanding of the underlying physical processes. In this work, we perform a comprehensive study of microscale water condensation dynamics on nanostructured superhydrophobic surfaces made using a variety of synthetic methods. We demonstrate that the growth mechanism of individual water microdroplets on these surfaces is universal and independent of the surface architecture. The key role of the nanoscale topography is confinement of the base area of forming droplets, which allows droplets to grow only through contact angle increase. The nearly spherical droplets formed in this fashion become highly mobile after coalescence. By comparing experimentally observed drop growth with interface free energy calculations, we show that the minimum observed confined microdroplet base diameter depends directly on the nanoscale surface roughness and degree of interfacial wetting. Specifically, we show that the microscale condensation mechanism depends on the height of a liquid film with volume equal to the fill volume between the nanostructures. This introduced roughness length scale is a universal metric that allows for facile comparison of arbitrarily complex surface architectures. We use this new fundamental insight to develop quantitative design guidelines for superhydrophobic surfaces intended for condensation applications.

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

“…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.…”

Condensation heat transfer on superhydrophobic surfaces

“…Their study showed that these droplets can either grow above the structure forming a 'balloon' like PW droplet (Figure 3b), or laterally spread into the structure forming a highly adhered W droplet (Figure 3c) 54,55 . While the droplet morphology is dictated by the intricate liquid/structure interaction dynamics, it can be approximately predicted by comparing the energies of the non-equilibrium advancing Cassie and Wenzel states with a dimensionless energy ratio: (4) where r = 1 + πdh/l 2 is the surface roughness, and θ a is the advancing contact angle on a flat surface.…”

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

“…[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.…”

“…Combined with a faster growth rate for individual droplets, this ordered droplet nucleation process increases the probability for coalescence of multiple droplets in the interstices between micro-posts (in comparison to the random coalescence events that occur on the more homogeneous nano-structured surface). Second, the presence of nanoscale roughness on the hierarchical surface increases the local condensate droplet contact angle , [18,42,43] which reduces the work of adhesion ( ad W ) of the droplet to the surface…”

“…[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.…”