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.

Delayed Leidenfrost Effect of a Cutting Droplet on a Microgrooved Tool Surface.

Regulation over the generation of the Leidenfrost phenomenon in liquids is vitally important in a cutting fluid/tool system, with benefits ranging from optimizing the heat transfer efficiency to improving the machining performance. However, realizing the influence mechanism of liquid boiling at various temperatures still faces enormous challenges. Herein, we report a kind of microgrooved tool surface by laser ablation, which could obviously increase both the static and dynamic Leidenfrost point of cutting fluid by adjusting the surface roughness (Sa). The physical mechanism that delays the Leidenfrost effect is primarily due to the ability of the designed microgroove surface to store and release vapor during droplet boiling so that the heated surface requires higher temperatures to generate sufficient vapor to suspend the droplet. We also find six typical impact regimes of cutting fluid under various contact temperatures; it is worth noting that Sa has a great influence on the transform threshold among six impact regimes, and the likelihood that a droplet will enter the Leidenfrost regime decreases with increasing Sa. In addition, the synergistic effect of Sa and tool temperature on the droplet kinetics of cutting droplets is investigated, and the relationship between the maximum rebound height and the dynamic Leidenfrost point is correlated for the first time. Significantly, cooling experiments on the heated microgrooved surface are performed and demonstrate that it is effective to improve the heat dissipation ability of cutting fluid by delaying the Leidenfrost effect on the microgrooved heated surface.

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 Sa and higher Sku exhibits a better wear resistance, which is mainly related to the storing ability of SHB coating inside the microtextures. Moreover, the surface with appropriate Str 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.

In Situ Tuning Underwater Bubble Movement on Slippery Lubricant-Infused Anisotropic Microgrooved Surface by Unidirectional Mechanical Strain.

Numerous studies have focused on designing and fabricating functional interfaces that control movement behavior of underwater gas bubbles, which are ubiquitous in a variety of natural and industrial settings. Nevertheless, developing surfaces with in situ tunable bubble movement remain elusive because of current complicated tuning strategies on the specific materials. Inspired by natural pitcher plant and rice leaves, here we report a kind of slippery lubricant-infused anisotropic microgrooved surface (SLI-AMGS) fabricated by femtosecond laser direct writing technology and realize the in situ reversible switching between underwater bubble sliding and pinning by unidirectional mechanical tensile strain. Different experimental parameters including lubricant oil film thickness, bubble volumes and laser power have been researched to manifest the relationship with bubble sliding behaviors. The underlying mechanism of in situ reversible switching mainly lies on the decrease of the lubricant oil film thickness during the process of mechanical stretching in which the uniform and stable oil film layer becomes uneven. This uneven lubricant oil film results in an extraordinary increase of contact angle hysteresis and resistance. At last, we demonstrate a real-time dynamic modulation of the underwater bubble on the SLI-AMGS with a changing mechanical tensile strain for several repeatable times in different acid-based environments. Our work manifests great potential applications in widespread fields including underwater bubble microfluidics and microbubble robots.

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

Control over the generation of air cavities in liquids is vitally important in numerous underwater marine systems, with behaviors ranging from largely reducing the fluid resistance to minimizing the energy consumption. However, reducing the critical velocity of air cavity generation simultaneously on both original and hydrophobic materials can be mutually exclusive. In strong contrast to the view that the surface wettability is what determines the critical velocity of air cavity entrainment, we report a facile method to dramatically reduce the critical velocity of air cavity generation by etching a kind of biomimetic microstructure on the sphere surface. It is worth noting that the generation of air cavities induced by the microstructure effect is almost independent of surface wettability; even for the hydrophilic spheres, the critical velocity of air cavity generation is reduced by over 86.3%. The physical mechanism is mainly related to the pinning of the moving contact line and the regulable transition of the wetting state from the Wenzel model to the Cassie-Baxter model at the solid-liquid-air interface. We also investigated the stability of the microstructured spheres to induce underwater air cavities and found that a continuous and robust air cavity could still be produced even after 1200 repeated impacts. This research provides a simple, economical and energy-efficient method for minimizing the critical velocity for generating air cavity entrainment to reduce hydrodynamic drag.

Underwater Drag Reduction and Buoyancy Enhancement on Biomimetic Antiabrasive Superhydrophobic Coatings.

A superhydrophobic (SHB) surface with an excellent self-cleaning ability is of great significance in both human survival and industrial fields. However, it is still a challenge to achieve large-area preparation of antiabrasive SHB surfaces with great mechanical robustness for broader applications. Thus, a kind of facile SHB coating with excellent liquid repellency and antiresistance is constructed by spraying a fluorine-free suspension consisting of epoxy resin, hexadecyltrimethoxysilane (HDTMS), and silica nanoparticles on a glass sheet. The SHB coating not only shows high adhesion on various materials but also has high water repellency under various test conditions, including tape peeling after blade scraping, sandpaper abrasion, and immersing in a complex environment. Additionally, the SHB spheres coated with laser-induced microstructure armor could form a continuous gas cavity during the water entry process, which is essential to prolonging the drag reduction ability of SHB coatings in liquid. Finally, the prepared robust SHB coatings have been employed in underwater buoyancy enhancement and reducing fluid resistance, which may open new avenues for underwater drag reduction in the field of marine applications.