Recent progress in bio-inspired macrostructure array materials with special wettability—from surface engineering to functional applications

Bio-inspired macrostructure array (MAA, size: submillimeter to millimeter scale) materials with special wettability (MAAMs-SW) have attracted significant research attention due to their outstanding performance in many applications, including oil repellency, liquid/droplet manipulation, anti-icing, heat transfer, water collection, and oil–water separation. In this review, we focus on recent developments in the theory, design, fabrication, and application of bio-inspired MAAMs-SW. We first review the history of the basic theory of special wettability and discuss representative structures and corresponding functions of some biological surfaces, thus setting the stage for the design and fabrication of bio-inspired MAAMs-SW. We then summarize the fabrication methods of special wetting MAAs in terms of three categories: additive manufacturing, subtractive manufacturing, and formative manufacturing, as well as their diverse functional applications, providing insights into the development of these MAAMs-SW. Finally, the challenges and directions of future research on bio-inspired MAAMs-SW are briefly addressed. Worldwide efforts, progress, and breakthroughs from surface engineering to functional applications elaborated herein will promote the practical application of bio-inspired MAAMs-SW

Modeling and Experimental Study of the Localized Electrochemical Micro Additive Manufacturing Technology Based on the FluidFM

In this work, the localized electrochemical micro additive manufacturing technology based on the FluidFM (fluidic force microscope) has been introduced to fabricate micro three-dimensional overhang metal structures at sub-micron resolution. It breaks through the localized deposition previously achieved by micro-anode precision movement, and the micro-injection of the electrolyte is achieved in a stable electric field distribution. The structure of electrochemical facilities has been designed and optimized. More importantly, the local electrochemical deposition process has been analyzed with positive source diffusion, and the mathematical modeling has been revealed in the particle conversion process. A mathematical model is proposed for the species flux under the action of pulsed pressure in an innovatively localized liquid feeding process. Besides, the linear structure, bulk structure, complex structure, and large-area structure of the additive manufacturing are analyzed separately. The experimental diameter of the deposited cylinder structure is linearly fitted. The aspect ratio of the structure is greater than 20, the surface roughness value is between 0.1–0.2 μm at the surface of bulk structures, and the abilities are verified for deposition of overhang, hollow complex structures. Moreover, this work verifies the feasibility of 3D overhang array submicron structure additive manufacturing, with the application of pulsed pressure. Furthermore, this technology opens new avenues for the direct fabrication of nano circuit interconnection, tiny sensors, and micro antennas

Bouncing Dynamics of Impact Droplets on the Biomimetic Plane and Convex Superhydrophobic Surfaces with Dual-Level and Three-Level Structures

Reducing the contact time of a water droplet on non-wetting surfaces has great potential in the areas of self-cleaning and anti-icing, and gradually develops into a hot issue in the field of wettability surfaces. However, the existing literature on dynamic behavior of water drops impacting on superhydrophobic surfaces with various structural shapes is insufficient. Inspired by the microstructure of lotus leaf and rice leaf, dual-level and three-level structures on plane and convex surfaces were successfully fabricated by wire electrical discharge machining on aluminum alloy. After spraying hydrophobic nanoparticles on the surfaces, the plane and convex surfaces with dual-level and three-level structures showed good superhydrophobic property. Bouncing dynamics of impact droplets on the superhydrophobic surfaces wereinvestigated, and the results indicated that the contact time of plane superhydrophobic surface with a three-level structure was minimal, which is 60.4% less than the plane superhydrophobic surface with dual-level structure. The effect of the interval S, width D, and height H of the structure on the plane superhydrophobic surface with three-level structure on contact time was evaluated to obtain the best structural parameters for reducing contact time. This research is believed to guide the direction of the structural design of the droplet impinging on solid surfaces

Programmable droplet transport on multi-bioinspired slippery surface with tridirectionally anisotropic wettability

Directional droplet transport on functional surfaces with anisotropic wettability has shown great potential applications in various fields such as water harvesting, chemical micro-reaction, and biomedical analysis. However, the in-plane manipulation of the anisotropic droplet motion in more than two directions is still a challenge. Herein, through the fusion of inspirations from rice leaves, butterfly wings and Pitcher plants, we report a tridirectionally anisotropic slippery surface (TASS) with periodic step-like micro grooves for programmable droplet transport. TASS possesses a tridirectional droplet sliding behavior, i.e., the ultra-slipperiness along the grooves with a sliding angle of ∼ 2°, and the bidirectionally anisotropic sliding perpendicular to the grooves with sliding angle difference up to ∼ 50°, which is caused by the pinning effect of the step edge. Under the assistance of periodic vertical vibration, groove-features and droplet-volume dependent unidirectional droplets transports are realized on horizontally placed TASS, based on which two micro-reactors are designed to control the sequence of droplets merging and subsequent chemical reactions. Additionally, by utilizing the slipperiness (i.e., ultra-low sliding angle for liquid droplet) along the grooves simultaneously, programmable droplet transport under vertical vibration is further demonstrated on a tilted TASS. This work will provide a new avenue for the understanding of anisotropic wettability on asymmetric slippery surface, and thus offer a great opportunity to develop advanced interface for multidirectional droplet transport, chemical micro-reactor, etc

A scalable method toward robust underwater superoleophobic surfaces with microstructure arrays on 304 stainless steel substrates

Underwater superoleophobic surfaces have huge application prospects due to their multiple functions such as antifouling, self-cleaning, manipulation of oil microdroplets, and oil − water separation. However, efficient and low-damage methods for the scalable fabrication of robust underwater superoleophobic surfaces on stainless steel substrates are still lacking. Here, a maskless electrochemical machining technology was developed to fabricate robust underwater superoleophobic surfaces on 304 stainless steel substrates. The square micro pit array was obtained on the surface, and the surface showed excellent superoleophobicity with a contact angle of 154.93 ± 1.42° when submerged in water. In addition, the fabricated underwater superoleophobic surface with the square micro pit array showed good chemical stability, mechanical stability, and anti-friction performance. The friction coefficient of the fabricated underwater superoleophobic surface was 0.0748, which was 60.7% lower than the friction coefficient of a smooth surface. Compared with traditional methods, the proposed technology will promote the practical applications of underwater superoleophobic surfaces in complex underwater environments

Green fabrication of anti-friction slippery liquid-infused metallic surface with sub-millimeter-scale asymmetric bump arrays and its application

In this work, we present a simple technique for green fabrication of slippery liquid-infused surface (SLIS) with anti-friction property on various metallic substrates using wire electrical discharge machining. Micro-crater structures were successfully obtained, and the surface had excellent liquid-repellent property after modification and infusion of silicone oil. A wide range of liquids including water, juice, coffee, tea, vinegar, albumin, glycerol, and ketchup could easily slid down the surface tilted at an angle of 10° without leaving any trace. The influences of the number of cutting step on the morphology and wettability of the surface were studied comprehensively. Further, the tribological properties of the surface were analyzed and the results showed that the SLIS had a decrease of 73.2% in friction coefficient as compared to that of the smooth surface. By studying the morphology of the worn surfaces, it is found that the SLIS had slight abrasive wear behavior. To demonstrate the precision processing ability of this technology, we fabricated slippery sub-millimeter-scale asymmetric bump arrays, and the experiment results showed that the asymmetric bump arrays had excellent water harvesting ability at low temperatures. This kind of environment-friendly precision machining technology will promote the practical applications of metallic functional materials

Microstructural evolution, mechanical properties and surface quality of TC11 titanium alloy subjected to waterjet-assisted laser direct inscription

Waterjet-assisted laser direct inscription (WJALDI) technology has presented a positive effect to alleviate the thermal damage in workpieces caused by laser ablation. This research pays attention to the effect of aqueous media on the microstructure evolution and phase composition of materials during laser ablation, and few relevant researches have been reported. In this work, titanium alloy (TC11) was applied to fabricate microgrooves utilizing WJALDI and laser direct inscription in the air (LDI). The interaction effects between the laser, water, and the material within a complete ablation cycle were analyzed in depth. The geometry and thermal damage region were observed and discussed to prove the machining performance. The microstructure evolution that happened on and underneath the microgroove surface was also studied. The results revealed that the effect of cooling and impacting significantly reduced the accumulation of debris and molten metal, and better processing quality was obtained in WJALDI. According to the analysis of electron backscattered diffraction (EBSD), the grain refinement of the inner material was alleviated, low-angle grain boundaries with the features of uniform distribution were generated, and the adjacent grains of α phase have small kernel average misorientation. In addition, compared with LDI, WJALDI offered a wide microgroove with a small size of heat affected zone (HAZ) and thin thickness of the recast layer, while less depth. Therefore, the developed technology in this work could be a promising approach for the micromachining of titanium alloy or even other difficult-to-cut materials with high quality

Nanosecond Laser-Induced Underwater Superoleophobic and Underoil Superhydrophobic Mesh for Oil/Water Separation

Materials with special wettability have drawn considerable attention especially in the practical application for the separation and recovery of the oily wastewater, whereas there still remain challenges of the high-cost materials, significant time, and complicated production equipment. Here, a simple method to fabricate the underwater superoleophobic and underoil superhydrophobic brass mesh via the nanosecond laser ablation is reported for the first time, which provided the micro-/nanoscale hierarchical structures. This mesh is superhydrophilic and superoleophilic in air but superoleophobic under water and superhydrophobic under oil. On the basis of the special wettability of the as-fabricated mesh, we demonstrate a proof of the light or heavy oil/water separation, and the excellent separation efficiencies (>96%) and the superior water/oil breakthrough pressure coupled with the high water/oil flux are achieved. Moreover, the nanosecond laser technique is simple and economical, and it is advisable for the large-area and mass fabrication of the underwater superoleophobic and underoil superhydrophobic mesh in the large-scale oil/water separation

Nanosecond Laser-Induced Underwater Superoleophobic and Underoil Superhydrophobic Mesh for Oil/Water Separation

Materials with special wettability have drawn considerable attention especially in the practical application for the separation and recovery of the oily wastewater, whereas there still remain challenges of the high-cost materials, significant time, and complicated production equipment. Here, a simple method to fabricate the underwater superoleophobic and underoil superhydrophobic brass mesh via the nanosecond laser ablation is reported for the first time, which provided the micro-/nanoscale hierarchical structures. This mesh is superhydrophilic and superoleophilic in air but superoleophobic under water and superhydrophobic under oil. On the basis of the special wettability of the as-fabricated mesh, we demonstrate a proof of the light or heavy oil/water separation, and the excellent separation efficiencies (>96%) and the superior water/oil breakthrough pressure coupled with the high water/oil flux are achieved. Moreover, the nanosecond laser technique is simple and economical, and it is advisable for the large-area and mass fabrication of the underwater superoleophobic and underoil superhydrophobic mesh in the large-scale oil/water separation