Biomimetic Water-Repelling Surfaces with Robustly Flexible Structures

Effect of Photolithographic Biomimetic Surface Microstructure on Wettability and Droplet Evaporation Process

In nature, engineering technology and daily life, wetting phenomena are widespread and have essential roles and significance. Bionics is becoming increasingly important nowadays and exploring the mechanism that influences biomimetic surface microstructure on droplet wetting process and heat and mass transfer characteristics is becoming more meaningful. In this paper, based on photolithography technology, SU-8 photoresist was used as raw material to prepare biomimetic surfaces with microstructures in various arrangements. The research results show that the wettability of biomimetic functional surfaces can be regulated by regulating the shape and arrangement of photoresist micro-pillars. At the same time, the effects of surface microstructure configuration and roughness on the heat and mass transfer processes within the droplets were also comprehensively studied. The results show that a biomimetic surface with cylindrical micro-pillars can effectively inhibit the evaporative cooling effect of the liquid-vapour interface. This effect becomes more evident with the increase in roughness, and the interface temperature difference can be reduced by up to 18%. Similarly, the biomimetic surface with cylindrical micro-pillars can also effectively promote the evaporation rate of sessile droplets, which can be increased by about 13%. In addition, the research also shows that regardless of the structure, substrate temperature changes will significantly impact the wetting phenomenon of the biomimetic surface. This study aims to guide the optimal design of biomimetic surfaces prepared based on photoresistance.

Water Impalement Resistance and Drag Reduction of the Superhydrophobic Surface with Hydrophilic Strips

Superhydrophobic surfaces (SHS) offer versatile applications by trapping an air layer within microstructures, while water jet impact can destabilize this air layer and deactivate the functions of the SHS. The current work presents for the first time that introducing parallel hydrophilic strips to SHS (SHS-s) can simultaneously improve both water impalement resistance and drag reduction (DR). Compared with SHS, SHS-s demonstrates a 125% increase in the enduring time against the impact of water jet with velocity of 11.9 m/s and a 97% improvement in DR at a Reynolds number of 1.4 × 104. The key mechanism lies in the enhanced stability of the air layer due to air confinement by the adjacent three-phase contact lines. These lines not only impede air drainage through the surface microstructures during water jet impact, entrapping the air layer to resist water impalement, but also prevent air floating up due to buoyancy in Taylor-Couette flow, ensuring an even spread of the air layer all over the rotor, boosting DR. Moreover, failure modes of SHS under water jet impact are revealed to be related to air layer decay and surface structure destruction. This mass-producible structured surface holds the potential for widespread use in DR for hulls, autonomous underwater vehicles, and submarines.

3D-Printed Micro/Nano-Scaled Mechanical Metamaterials: Fundamentals, Technologies, Progress, Applications, and Challenges

Micro/nano-scaled mechanical metamaterials have attracted extensive attention in various fields attributed to their superior properties benefiting from their rationally designed micro/nano-structures. As one of the most advanced technologies in the 21st century, additive manufacturing (3D printing) opens an easier and faster path for fabricating micro/nano-scaled mechanical metamaterials with complex structures. Here, the size effect of metamaterials at micro/nano scales is introduced first. Then, the additive manufacturing technologies to fabricate mechanical metamaterials at micro/nano scales are introduced. The latest research progress on micro/nano-scaled mechanical metamaterials is also reviewed according to the type of materials. In addition, the structural and functional applications of micro/nano-scaled mechanical metamaterials are further summarized. Finally, the challenges, including advanced 3D printing technologies, novel material development, and innovative structural design, for micro/nano-scaled mechanical metamaterials are discussed, and future perspectives are provided. The review aims to provide insight into the research and development of 3D-printed micro/nano-scaled mechanical metamaterials.

Tribological Behavior of Bioinspired Surfaces

Energy losses due to various tribological phenomena pose a significant challenge to sustainable development. These energy losses also contribute toward increased emissions of greenhouse gases. Various attempts have been made to reduce energy consumption through the use of various surface engineering solutions. The bioinspired surfaces can provide a sustainable solution to address these tribological challenges by minimizing friction and wear. The current study majorly focuses on the recent advancements in the tribological behavior of bioinspired surfaces and bio-inspired materials. The miniaturization of technological devices has increased the need to understand micro- and nano-scale tribological behavior, which could significantly reduce energy wastage and material degradation. Integrating advanced research methods is crucial in developing new aspects of structures and characteristics of biological materials. Depending upon the interaction of the species with the surrounding, the present study is divided into segments depicting the tribological behavior of the biological surfaces inspired by animals and plants. The mimicking of bio-inspired surfaces resulted in significant noise, friction, and drag reduction, promoting the development of anti-wear and anti-adhesion surfaces. Along with the reduction in friction through the bioinspired surface, a few studies providing evidence for the enhancement in the frictional properties were also depicted.

Second-Level Microgroove Convexity is Critical for Air Plastron Restoration on Immersed Hierarchical Superhydrophobic Surfaces

Air plastrons trapped on the surfaces of underwater superhydrophobic surfaces are critical for their function. Fibrillar morphologies offer a natural pathway, yet they are limited to a narrow range of liquid-surface systems and are vulnerable to pressure fluctuations that irreversibly destroy the air layer plastron. Inspired by the convexly grooved bases of water fern (Salvinia) leaves that support their fibrous outgrowths, we focus on the effect of such second-level grooved structures or microgrooves on the plastron restoration on immersed three-dimensional (3D)-printed hierarchical surfaces. Elliptical, interconnected microgrooves are fabricated with varying surface curvatures to study the effect of their morphology. Immersion experiments reveal that the convex groove curvature stabilizes a seed gas layer (SGL) that facilitates plastron restoration for all immersed hydrophobic surfaces. Theoretical calculations and atomic-scale computations reveal that the SGL storage capacity that sets the SGL robustness follows from the liquid menisci adaption to the groove geometry and pressure, from micro- to nanoscales, and it can be further tuned using separated grooves. Our study highlights groove convexity as a key morphological feature for the design of second-level architectures for underwater air plastron restoration on hierarchical superhydrophobic surfaces.

Recent advances of bio-inspired anti-icing surfaces

The need for improved anti-icing surfaces is the demand of the time and closely related to many important aspects of our lives as surface icing threatens not only industrial production but also human safety. Freezing on a cold surface is usually a heterogeneous nucleation process induced by the substrate. Creating an anti-icing surface is mainly achieved by changing surface morphology and chemistry to regulate the interaction between the surface and the water/ice to inhibit freezing on the surface. In this paper, recent research progress in the creation of biomimetic anti-icing surfaces is reviewed. Firstly, basic strategies of bionic anti-icing are introduced, and then bionic anti-icing surface strategies are reviewed according to four aspects: the process of ice formation, including condensate self-removing, inhibiting ice nucleation, reducing ice adhesion, and melting accumulated ice on the surface. The remaining challenges and the direction of future development of biomimetic anti-icing surfaces are also discussed.

Liquid-Pressure-Guided Superhydrophobic Surfaces with Adaptive Adhesion and Stability

The dynamic application environments of superhydrophobic surfaces, such as in the manufacturing, chemical, and garment industries, require the fast adaptiveness of the surfaces to their surroundings. Despite the progress in materials and structural design of superhydrophobic surfaces, simultaneously achieving high superhydrophobic stability and low adhesion by traditional design is still challenging. Here, a liquid-pressure-guided superhydrophobic surface with self-adjustable solid-liquid stability, and adhesion is demonstrated when reacting to the dynamic environmental requirements. To understand the underlying adaptive processes, the liquid impalement dynamics is imaged in three dimensions by confocal microscopy and the stability and superhydrophobicity are measured in varied systems. It is envisioned that the design strategy of liquid-pressure-guided superhydrophobic surfaces with dynamic anti-infiltration capability can stimulate the development of stable superhydrophobicity under complex wetting conditions.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.