Magnetically Responsive Superhydrophobic Surface with Switchable Adhesivity Based on Electrostatic Air Spray Deposition

Recent Advances in Superhydrophobic Materials Development for Maritime Applications

Underwater superhydrophobic surfaces stand as a promising frontier in materials science, holding immense potential for applications in underwater infrastructure, vehicles, pipelines, robots, and sensors. Despite this potential, widespread commercial adoption of these surfaces faces limitations, primarily rooted in challenges related to material durability and the stability of the air plastron during prolonged submersion. Factors such as pressure, flow, and temperature further complicate the operational viability of underwater superhydrophobic technology. This comprehensive review navigates the evolving landscape of underwater superhydrophobic technology, providing a deep dive into the introduction, advancements, and innovations in design, fabrication, and testing techniques. Recent breakthroughs in nanotechnology, magnetic-responsive coatings, additive manufacturing, and machine learning are highlighted, showcasing the diverse avenues of progress. Notable research endeavors concentrate on enhancing the longevity of plastrons, the fundamental element governing superhydrophobic behavior. The review explores the multifaceted applications of superhydrophobic coatings in the underwater environment, encompassing areas such as drag reduction, anti-biofouling, and corrosion resistance. A critical examination of commercial offerings in the superhydrophobic coating landscape offers a current perspective on available solutions. In conclusion, the review provides valuable insights and forward-looking recommendations to propel the field of underwater superhydrophobicity toward new dimensions of innovation and practical utility.

Recent progress on the development of bioinspired surfaces with high aspect ratio microarray structures: From fabrication to applications

Surfaces with high aspect ratio microarray structures can implement sophisticated assignment in typical fields including microfluidics, sensor, biomedicine, et al. via regulating their deformation or the material properties. Inspired by natural materials and systems, for example sea cockroaches, water spiders, cacti, lotus leaves, rice leaves, and cedar leaves, many researchers have focused on microneedle functional surface studies. When the surface with high aspect ratio microarray structures is stimulated by the external fields, such as optical, electric, thermal, magnetic, the high aspect ratio microarray structures can undergo hydrophilic and hydrophobic switching or shape change, which may be gifted the surfaces with the ability to perform complex task, including directional liquid/air transport, targeted drug delivery, microfluidic chip sensing. In this review, the fabrication principles of various surfaces with high aspect ratio microarray structures are classified and summarized. Mechanisms of liquid manipulation on hydrophilic/hydrophobic surfaces with high aspect ratio microarray structures are clarified based on Wenzel model, Cassie model, Laplace pressure theories and so on. Then the intelligent control strategies have been demonstrated. The applications in microfluidic, drug delivery, patch sensors have been discussed. Finally, current challenges and new insights of future prospects for dynamic manipulation of liquid/air based on biomimetic surface with high aspect ratio microarray structures are also addressed.

Adhesion performance of magnetically responsive surfaces under wet conditions

Adhesion is the key functionality to pick-and-place objects in wet environments. Recently, various micropillars and external stimuli have been proposed to achieve reversible wet adhesion. However, their underlying mechanisms of liquid/solid regulations have not been sufficiently revealed. Herein, two kinds of magnetically responsive micropillar arrays with different terminals (pointed and flat) are developed using a spray self-assembly method. The coupling effect of geometric structures and external stimuli on the wet adhesion performance between a solid substrate and the developed surface is discussed. In situ observation and analysis of theoretical models demonstrate that changes in adhesive forces are mainly caused by the length of the liquid bridge and the apparent contact angle of the developed surface. The adhesion conversion efficiency in the presence of an on/off magnetic field can achieve a highest value of 72% for the micropillar arrays with flat terminals, which exceeds 3 times that of the micropillar arrays with pointed terminals. In addition, wet adhesion measurements during the process of repeatedly switching the magnetic field demonstrate the durability and cyclic reversibility of the magnetically responsive surface. Furthermore, the transportation of microcomponents verifies the application potential of the magnetically responsive surface, which may provide inspiration for transfer printing systems and wet climbing robots.

Directional Manipulation of Drops and Solids on a Magneto-Responsive Slippery Surface

The cloaking of the droplet and solid spheres by a thin ferrofluid layer forms a ferrofluid-wetting ridge, enabling the magnet-assisted directional manipulation of droplets and solid spheres on the magneto-responsive slippery surface. Understanding the interplay of various forces governing motion unravels the manipulation mechanism. The transportation characteristics of droplets and solid spheres on such surfaces enable their controlled manipulation in multiple applications. Here, we prepare magneto-responsive slippery surfaces by using superhydrophobic coatings on glass slides, creating a porous network and impregnating them with ferrofluid. Using a permanent magnet (and its translation) in the proximity of these surfaces, we manipulate the motion of liquid drops and solid spheres. Upon dispensing the droplet on the magneto-responsive slippery surface, the droplet is cloaked by a thin ferrofluid layer and forms a ferrofluid wetting ridge. Incorporating the magnetic field creates a magnetic-nanoparticle-rich zone surrounding the ferrofluid ridge. Thereafter, the motion of the magnet gives rise to the movement of the droplet. We found that the interplay of the magnetic force and viscous drag leads to the magnetic manipulation of droplets in a controlled fashion up to a certain magnet speed. Increasing the magnet speed further results in the uncontrolled motion of the droplet, where the droplet cannot follow the magnet trajectory. Moreover, we delineate multifunctional droplet manipulations, such as trapping, pendant droplet manipulation, coalescence, and microchemical reactions, which have wide engineering applications.

Free-Boundary Microfluidic Platform for Advanced Materials Manufacturing and Applications

Microfluidics, with its remarkable capacity to manipulate fluids and droplets at the microscale, has emerged as a powerful platform in numerous fields. In contrast to conventional closed microchannel microfluidic systems, free-boundary microfluidic manufacturing (FBMM) processes continuous precursor fluids into jets or droplets in a relatively spacious environment. FBMM is highly regarded for its superior flexibility, stability, economy, usability, and versatility in the manufacturing of advanced materials and architectures. In this review, a comprehensive overview of recent advancements in FBMM is provided, encompassing technical principles, advanced material manufacturing, and their applications. FBMM is categorized based on the foundational mechanisms, primarily comprising hydrodynamics, interface effects, acoustics, and electrohydrodynamic. The processes and mechanisms of fluid manipulation are thoroughly discussed. Additionally, the manufacturing of advanced materials in various dimensions ranging from zero-dimensional to three-dimensional, as well as their diverse applications in material science, biomedical engineering, and engineering are presented. Finally, current progress is summarized and future challenges are prospected. Overall, this review highlights the significant potential of FBMM as a powerful tool for advanced materials manufacturing and its wide-ranging applications.

Self-assembled micropillar arrays via near-field electrospinning

Self-assembly in near-field electrospinning is reported for the first time in this paper, which realized the conversion from two-dimensional planar printing to three-dimensional (3D) structures. Repeatedly stacked fibres formed a micropillar array structure (MPAS) with intervals on the deposition paths by adding carbonyl iron powder particles to a polyethylene oxide (PEO) solution. The growth process of the self-assembled MPAS is documented, and the mechanism of the self-assembled MPAS is proposed. In addition, the effects of substrate speed and injection speed on self-assembly were investigated. Electric field distribution simulations show that the electric field strength around the MPAS is enhanced by nearly ten times so that the micropillar can attract the jet for further deposition. Self-assembly can obtain MPASs with arbitrary paths on different substrates, and the interval of the MPAS can be controlled by using bulging substrates. Furthermore, a self-assembled MPAS has been successfully used to prepare mold cavities, which can be used to prepare MPASs of other materials. Due to their small feature size, large surface area and structural periodicity, micropillar arrays will have promising applications, such as hydrophobicity of surfaces and electrochemical detection. Self-assembly in near-field electrospinning can significantly reduce the preparation cost of an MPAS and provide new processes and ideas.

Self-healing system of superhydrophobic surfaces inspired from and beyond nature

Superhydrophobic surfaces show wide prospects in a variety of applications requiring self-cleaning, anti-fog, anti-ice, anti-corrosion and anti-fouling properties, which have attracted the attention of many researchers. However, superhydrophobic surfaces are inevitably affected by chemical corrosion, scratches and wear in practical applications, resulting in the loss of superhydrophobicity. To solve this problem, researchers have developed superhydrophobic surfaces with self-healing properties. In this paper, the research achievements of self-healing superhydrophobic materials in recent years are summarized, and the preparation and repair principle of self-healing superhydrophobic surfaces are introduced from three aspects: surface chemical composition repair, surface roughness repair and double repair. In addition, some multifunctional self-healing superhydrophobic surfaces are introduced, such as conductive, stretchable, antibacterial, etc. Finally, in order to provide a reference for the preparation of widely used long-acting superhydrophobic materials, some existing problems and future development prospects are described in order to attract more researchers' attention and promote the development of this field.

Janus Fabric with Asymmetric Wettability for Switchable Emulsion Separation and Controllable Droplets with Low Friction

Superwetting surfaces have recently attracted extensive attention in oil-water emulsion separation and droplet manipulations, which are widely used in various situations ranging from wastewater treatment, to flexible electronics, to biochemical diagnosis. However, it still remains challenging to obtain asymmetric materials with high efficiency during oil-water separation. Meanwhile, excellent robustness of the superhydrophobic surface is of significance but retards the mobility of droplets due to increased lateral adhesion of small spacing between solid protrusions. Herein, a facile approach is demonstrated to obtain the excellent robustness of Janus fabrics with asymmetric wettability. As for one side of water-in-oil emulsion separation, mimicking the soft earthworm with periodically wrinkled skin, an adaptive superhydrophobic fabric was fabricated by wrapping soft wrinkled poly(dimethylsiloxane) (PDMS) polymer with a cross-linking structure on woven fabric fibers induced by Ar plasma treatment. In addition, inspired by the desert beetle's structure but with reversed wettability, the other side of the Janus fabric was constructed for treating emulsion of oil-in-water. In addition, the underwater superoleophobic surface consisting of magnetically responsive PDMS microcilia with slippery heads, which shows robustness against pH, improved water drop mobility and lowered the resistance of fluid friction similar to the intrinsic hydrophobic Salvinia molesta with additional slippery performance. Hence, we propose a novel and easy approach that optimizes enhanced emulsion separation and reduced fluid drag properties simultaneously, which actively broadens their widespread applications.

Gecko-Inspired Adhesive Mechanisms and Adhesives for Robots—A Review

Small living organisms such as lizards possess naturally built functional surface textures that enable them to walk or climb on versatile surface topographies. Bio-mimicking the surface characteristics of these geckos has enormous potential to improve the accessibility of modern robotics. Therefore, gecko-inspired adhesives have significant industrial applications, including robotic endoscopy, bio-medical cleaning, medical bandage tapes, rock climbing adhesives, tissue adhesives, etc. As a result, synthetic adhesives have been developed by researchers, in addition to dry fibrillary adhesives, elastomeric adhesives, electrostatic adhesives, and thermoplastic adhesives. All these adhesives represent significant contributions towards robotic grippers and gloves, depending on the nature of the application. However, these adhesives often exhibit limitations in the form of fouling, wear, and tear, which restrict their functionalities and load-carrying capabilities in the natural environment. Therefore, it is essential to summarize the state of the art attributes of contemporary studies to extend the ongoing work in this field. This review summarizes different adhesion mechanisms involving gecko-inspired adhesives and attempts to explain the parameters and limitations which have impacts on adhesion. Additionally, different novel adhesive fabrication techniques such as replica molding, 3D direct laser writing, dip transfer processing, fused deposition modeling, and digital light processing are encapsulated.

Anisotropic Wettability of Bioinspired Surface Characterized by Friction Force

Bioinspired surfaces with special wettabilities attract increasing attention due to their extensive applications in many fields. However, the characterizations of surface wettability by contact angle (CA) and sliding angle (SA) have clear drawbacks. Here, by using an array of triangular micropillars (ATM) prepared by soft lithography, the merits of measuring the friction force of a water droplet on ATM over measurements of CA and SA in characterizing the surface wettability are demonstrated. The CA and SA measurements show ignorable differences in the wettabilities of ATM in opposite directions (1.13%) and that with different periodic parameters under the elongation ranging from 0 to 70%. In contrast, the friction measurement reveals a difference of > 10% in opposite directions. Moreover, the friction force shows a strong dependence on the periodic parameters which is regulated by mechanical stretching. Increasing the elongation from 0 to 50% increases the static and kinetic friction force up to 23.0% and 22.9%, respectively. Moreover, the stick-slip pattern during kinetic friction can reveal the periodic features of the measured surface. The friction force measurement is a sensitive technique that could find applications in the characterization of surface wettabilities.

Adhesion behaviors of water droplets on bioinspired superhydrophobic surfaces

The adhesion behaviors of droplets on surfaces are attracting increasing attention due to their various applications. Many bioinspired superhydrophobic surfaces with different adhesion states have been constructed in order to mimic the functions of natural surfaces such as a lotus leaf, a rose petal, butterfly wings, etc. In this review, we first present a brief introduction to the fundamental theories of the adhesion behaviors of droplets on various surfaces, including low adhesion, high adhesion and anisotropic adhesion states. Then, different techniques to characterize droplet adhesion on these surfaces, including the rotating disk technique, the atomic force microscope cantilever technique, and capillary sensor-based techniques, are described. Wetting behaviors, and the switching between different adhesion states on bioinspired surfaces, are also summarized and discussed. Subsequently, the diverse applications of bioinspired surfaces, including water collection, liquid transport, drag reduction, and oil/water separation, are discussed. Finally, the challenges of using liquid adhesion behaviors on various surfaces, and future applications of these surfaces, are discussed.