Dramatically reducing the critical velocity of air cavity generation via biomimetic microstructure effects

Suppressed Droplet Splashing on Positively Skewed Surfaces for High-Efficiency Evaporation Cooling

Two types of functional surfaces with the same roughness but completely different surface topographies are prepared, namely positively skewed surfaces filled with micropillar arrays (Sa ≈4.4 µm, Ssk >0) and negatively skewed surfaces filled with microcavity arrays (Sa ≈4.4 µm, Ssk <0), demonstrating promoting droplet splashing. Remarkably, the critical Weber number for generating satellite droplets on the negatively skewed surfaces is significantly lower than that on the positively skewed surfaces, indicating that the negatively skewed surface with microcavity arrays is more likely to promote droplet splashing. It is mainly attributed to the fact that air on the negatively skewed surface can make the liquid film take on a Cassie-Baxter state on the surface so that the stabilizing capillary force of the liquid film exceeds the destabilizing stress of the air film. Moreover, the surface topography promoting droplet spreading and the mechanical properties of three-phase moving contact lines are analyzed from the perspective of microscopic interface mechanics. Finally, it is demonstrated the designed positively skewed surfaces can be employed for large-area heat dissipation by means of high-efficiency evaporation.

Armored Superhydrophobic Surfaces with Excellent Drag Reduction in Complex Environmental Conditions

Superhydrophobic surfaces (SHSs) have possibilities for achieving significantly reduced solid-liquid frictional drag in the marine sector due to their excellent water-repelling properties. Although the stability of SHSs plays a key role in drag reduction, little consideration was given to the effect of extreme environments on the ability of SHSs to achieve drag reduction underwater, particularly when subjected to acidic conditions. Here, we propose interconnected microstructures to protect superhydrophobic coatings with the aim of enhancing the stability of SHSs in extreme environments. The stability of armored SHSs (ASHSs) was demonstrated by the contact angle and bounce time of droplets on superhydrophobic surfaces treated by various methods, resulting in an ASHS surface with excellent stability under extreme environmental conditions. Additionally, inspired by microstructures protecting superhydrophobic nanomaterials from frictional wear, the armored superhydrophobic spheres (ASSPs) were designed to explain from theoretical and experimental perspectives why ASSPs can achieve sustainable drag reduction and demonstrate that the ASSPs can achieve drag reduction of over 90.4% at a Reynolds number of 6.25 × 104 by conducting water entry experiments on spheres treated in various solutions. These studies promote a fundamental understanding of what drives the application of SHSs under extreme environmental conditions and provide practical strategies to maximize frictional drag reduction.

In Situ Activation of Superhydrophobic Surfaces with Triple Icephobicity at Low Temperatures

Superhydrophobic surfaces have been widely studied due to their potential applications in aerospace fields. However, superhydrophobic surfaces with excellent water-repellent, anti-icing, and icephobic performances at low temperatures have rarely been reported. Herein, superhydrophobic surfaces with heating capability were prepared by etching square micropillar arrays on the surface of multiwalled carbon nanotube (MWCNT)/poly(dimethylsiloxane) (PDMS) films. The fabricated superhydrophobic surface has triple icephobicity, which can be activated even at low temperatures. The triple icephobicity is triggered by an applied voltage to achieve excellent water-repellent and icephobic capabilities, even at -40 °C. Additionally, theoretical calculations reveal that a droplet on a superhydrophobic surface loses heat at a rate of 8.91 × 10^-5 J/s, which is 2 orders of magnitude slower than a flat surface (2.15 × 10^-3 J/s). Also, at -40 °C, the mechanical interlocking force formed between the superhydrophobic surface and ice can be released by the heating property of the superhydrophobic surface. This low-energy, multifunctional superhydrophobic surface opens up new possibilities for bionic smart multifunctional materials in icephobic applications.

Functional Microtextured Superhydrophobic Surface with Excellent Anti-Wear Resistance and Friction Reduction Properties

The wear-resistant superhydrophobic (SHB) surfaces with excellent water-repellency ability were prepared by constructing a microtextured armor on an aluminum surface. With the assistance of laser-induced microtextures, the SHB surface could keep a longer water-repellency ability and a lower friction coefficient even after repeated friction tests under different loads and at different speeds. The mechanism of microtexture-protecting SHB coating is revealed based on both theoretical and elemental analysis. Additionally, we explore the relationship between the three-dimensional topography parameters (ISO 25178) and variation of water contact angles under different test recycles. The results show that the rough surface with appropriate S_a and higher S_ku exhibits a better wear resistance, which is mainly related to the storing ability of SHB coating inside the microtextures. Moreover, the surface with appropriate S_tr exhibits excellent wear resistance, which is mainly associated with better chip-removal ability. Finally, the tribological properties of the microtextured SHB surface are researched. It is worth noting that compared with the microtextured surface without SHB coating and the SHB-coated surface without microtextures, the microtextured SHB surface has the lowest friction coefficient under dry friction because the SHB coating would largely decrease the surface energy of the interface, so the adhesion friction decreases. The microtexture armor on the surfaces would protect the wear of SHB coating, so the SHB coating inside the microtexture could continuously play the role of a particle lubricant at the sliding interface and decrease the friction force of the sliding interface. We believe that the present study would contribute to the further understanding of the constructing mechanism of anti-wear SHB surfaces and provide a new strategy for topography design of engineering surfaces with friction reduction properties.