Superhydrophobic nanostructured surfaces have demonstrated outstanding capability in energy and water applications by promoting dropwise condensation, where fast droplet growth and efficient condensate removal are two key parameters. However, these parameters remain contradictory. Although efficient droplet removal is easily obtained through coalescence jumping on uniform superhydrophobic surfaces, simultaneously achieving fast droplet growth is still challenging. Also, on such surfaces droplets can grow to larger sizes without restriction if there is no coalescence. In this work, we show that superhydrophobic nanostructured microporous surfaces can manipulate the droplet growth and jumping. Microporous surface morphology effectively enhances the growth of droplets in pores owing to large solid-liquid contact area. At low supersaturations, the upward growth rate (1-1.5 μm/s) of these droplets in pores is observed to be around 15-25 times that of the droplets outside the pores. Meanwhile, their top curvature radius increases relatively slowly (∼0.25 μm/s) due to pore confinement, which results in a highly stretched droplet surface. We also observed forced jumping of stretched droplets in pores either through coalescence with spherical droplets outside pores or through self-pulling without coalescence. Both experimental observation and theoretical modeling reveal that excess surface free energy stored in the stretched droplet surface and micropore confinement are responsible for this pore-scale-forced jumping. These findings reveal the insightful physics of stretched droplet dynamics and offer guidelines for the design and fabrication of novel super-repellent surfaces with microporous morphology.

Nanostructured materials with desired wettability and optical property can play an important role in reducing the energy consumption of oily water treatment technologies. For effective oily water treatment, membrane materials with high strength, sunlight-sensitive anti-fouling, relative low fabrication cost, and controllable wettability are being explored. In the proposed oily water treatment approach, nanostructured TiO2-coated copper (TNS-Cu) meshes are used. These TNS-Cu meshes exhibit robust superhydrophilicity and underwater oleophobicity (high oil intrusion pressure) as well as excellent chemical and thermal stability (≈250 °C). They have demonstrated high separation efficiency (oil residue in the filtrate ≤21.3 ppm), remarkable filtration flux (≥400 kL h−1 m−2), and sunlight-sensitive anti-fouling properties. Both our theoretical analysis and experimental characterization have confirmed the enhanced light absorption property of TNS-Cu meshes in the visible region (40% of the solar spectrum) and consequently strong anti-fouling capability upon direct solar light illumination. With these features, the proposed approach promises great potential in treating produced oily wastewater from industry and daily life.

Droplet coalescence jumping on superhydrophobic surfaces attracts much research attention owing to its capability in enhancing condensation for energy and water applications. In this work, we reveal the impact of the finite surface adhesion to explain velocity discrepancies observed in recent droplet jumping studies, particularly when droplet sizes are a few micrometers (1-10 μm). Surface adhesion, which is usually neglected, can significantly affect both droplet coalescence and departure dynamics. It causes oscillations on velocity and contact area in the droplet coalescence process, as observed numerically and experimentally. Comparing the increasing rate of jumping velocity with contact angle for three different droplet sizes, we show that smaller droplets exhibit higher sensitivity to the change of surface hydrophobicity. We also specify the range of surface superhydrophobicity where the jumping velocity monotonically decreases (θ ≳ 170°), increases (θ ≲ 160°), or changes non-monotonically in transition (160° ≲ θ ≲170°) with droplet size. As a result, there exists a broad jumping velocity range for micrometer-sized droplets on a superhydrophobic surface with a slight contact angle variation. This work offers an extended understanding of the droplet coalescence and jumping dynamics to resolve the discrepancies in recent experimental observations.

Nucleation is the first stage of phase change phenomena, including condensation on nanostructured superhydrophobic surfaces. Despite plenty of theoretical studies on the effect of nanostructure density and shape on water droplet nucleation, not many experimental investigations have been reported. Here, we show both experimentally and theoretically that a moderate increase in the nanostructure density can lead to an increase in the nucleation density of water droplets because of the decreased energy barrier of nucleation in cavities formed between the nanostructures. Specifically, we observed droplets aligned in regions with denser nanostructures. The number and average volume of the aligned droplets in these regions were larger than that of the droplets in the surrounding areas. However, nucleation in cavities subsequently caused initial pinning of the droplet base within the nanostructures, forming a balloonlike, slightly elongated droplet shape. The dewetting transition of the pinned droplets from the Wenzel state to the unpinned Cassie state was predicted by quantifying the aspect ratio of droplets ranging from 3 to 30 μm. Moreover, the coalescence-jumping of droplets was followed by a new cycle of droplet condensation in an aligned pattern in an emptied area. These findings offer guidelines for designing enhanced superhydrophobic surfaces for water and energy applications.

Jumping-droplet enhanced condensation has recently attracted huge interest due to its remarkable potential of heat transfer performance enhancement, and studies have been done to design superhydrophobic surfaces with various surface morphologies. We fabricated a superhydrophobic micromesh-covered surface using a facile and scalable method. ESEM condensation experiment results show that droplets in pores formed by the mesh wires had faster growth rate in the upward direction than droplets on wires. This is mainly because of the confining role of the wires and higher heat transfer rate due to larger solid-liquid contact area. Also, these droplets always jumped at the size of pores (∼35 μm) when they coalesced with other droplets on wires. Moreover, droplets in pores were distorted by mesh wires, resulting in larger surface area. Theoretical predictions show, for a specific droplet radius, coalescence jumping of distorted droplets on the mesh-covered surface releases more excess surface free energy, and has larger jumping velocity than that of spherical droplets on the plate surface without mesh. This better performance was further validated by constant exposure of those two surfaces to electron beam during which work of adhesion was gradually increased. As expected, droplets on the mesh-covered surface coalesced and jumped while coalescing droplets on the plate surface could not as the exposure time increased. Our results offer new insights for designing hierarchical structured superhydrophobic surfaces to further enhance the performance of condensation heat transfer processes.