How To Obtain Six Different Superwettabilities on a Same Microstructured Pattern: Relationship between Various Superwettabilities in Different Solid/Liquid/Gas Systems

Microstructured Liquid Metal-Based Embedded-Type Sensor Array for Curved Pressure Mapping

Human hands can envelop the surface of an object and recognize its shape through touch. However, existing stretchable haptic sensors exhibit limited flexibility and stability to detect pressure during deformation, while also solely achieving recognition of planar objects. Inspired by the structure of skin tissue, an embedded construction-enabled liquid metal-based e-skin composed of a liquid metal microstructured electrode (LM-ME) array is fabricated for curved pressure mapping. The embedded LM-ME-based sensor elements are fabricated by using femtosecond laser-induced micro/nanostructures and water/hydrogel assisted patterning method, which enables high sensitivity (7.42 kPa-1 in the range of 0-0.1 kPa) and high stability through an interlinked support isolation structure for the sensor units. The sensor array with a high interfacial toughness of 1328 J m-2 can maintain pressure sensation under bending and stretching conditions. Additionally, the embedded construction and laser-induced bumps effectively reduce crosstalk from 58 to 7.8% compared to conventional flexible sensors with shared surfaces. The stretchable and mechanically stable sensor arrays possess shape-adaptability that enables pressure mapping on non-flat surfaces, which has great potential for object recognition in robotic skins and human-computer interaction.

Rapid Fabrication of Wavelength-Scale Micropores on Metal by Femtosecond MHz Burst Bessel Beam Ablation

The preparation of the wavelength-scale micropores on metallic surfaces is limited by the high opacity of metal. At present, most micropores reported in the literature are more than 20 µm in diameter, which is not only large in size, but renders them inefficient for processing so that it is difficult to meet the needs of some special fields, such as aerospace, biotechnology, and so on. In this paper, the rapid laser fabrications of the wavelength-scale micropores on various metallic surfaces are achieved through femtosecond MHz burst Bessel beam ablation. Taking advantage of the long-depth focal field of the Bessel beam, high-density micropores with a diameter of 1.3 µm and a depth of 10.5 µm are prepared on metal by MHz burst accumulation; in addition, the rapid fabrication of 2000 micropores can be achieved in 1 s. The guidelines and experimental results illustrate that the formations of the wavelength-scale porous structures are the result of the co-action of the laser-induced periodic surface structure (LIPSS) effect and Bessel beam interference. Porous metal can be used to store lubricant and form a lubricating layer on the metallic surface, thus endowing the metal resistance to various liquids' adhesion. The microporous formation process on metal provides a new physical insight for the rapid preparation of wavelength-scale metallic micropores, and promotes the application of porous metal in the fields of catalysis, gas adsorption, structural templates, and bio-transportation fields.

Design of Metal-Based Slippery Liquid-Infused Porous Surfaces (SLIPSs) with Effective Liquid Repellency Achieved with a Femtosecond Laser

Slippery liquid-infused porous surfaces (SLIPSs) have become an effective method to provide materials with sliding performance and, thus, achieve liquid repellency, through the process of infusing lubricants into the microstructure of the surface. However, the construction of microstructures on high-strength metals is still a significant challenge. Herein, we used a femtosecond laser with a temporally shaped Bessel beam to process NiTi alloy, and created uniform porous structures with a microhole diameter of around 4 µm, in order to store and lock lubricant. In addition, as the lubricant is an important factor that can influence the sliding properties, five different lubricants were selected to prepare the SLIPSs, and were further compared in terms of their sliding behavior. The temperature cycle test and the hydraulic pressure test were implemented to characterize the durability of the samples, and different liquids were used to investigate the possible failure under complex fluid conditions. In general, the prepared SLIPSs exhibited superior liquid repellency. We believe that, in combination with a femtosecond laser, slippery liquid-infused porous surfaces are promising for applications in a wide range of areas.

Fabrication of Superhydrophobic Gully-Structured Surfaces by Femtosecond Laser and Imprinting for High-Efficiency Self-Cleaning Rain Collection

Freshwater is considered an essential need for humanity. Moreover, it is important to collect and make full use of rainwater. This work utilizes a femtosecond laser to fabricate micro-nanostructures on aluminum alloy substrates as molds. Then, the structures are imprinted on cheap and wildly used polypropylene (PP) materials. The just-imprinted PP surfaces with instinctive surface energy and replicated micro-nanostructures have an excellent superhydrophobic property with contact angles greater than 160° and anisotropic sliding angles smaller than 5° in parallel directions and smaller than 10° in the vertical directions. A small-scale rain collection device formed by a combination of the superhydrophobic PP surfaces is used to investigate the effects of the rain collection efficiency and total surface area relating to manufacturing cost. The rain collection device formed by the imprinted PP surfaces has high rain collection efficiency in terms of the volume of the collected water per square centimeter. For the light rain, the rain collection efficiency can reach an approximated maximum of 90%, more than 100% efficiency improvement of the device formed by flat PP surfaces in some cases. Therefore, the rain collection device is helpful in collecting water from rains in arid areas.

Emerging Separation Applications of Surface Superwettability

Human beings are facing severe global environmental problems and sustainable development problems. Effective separation technology plays an essential role in solving these challenges. In the past decades, superwettability (e.g., superhydrophobicity and underwater superoleophobicity) has succeeded in achieving oil/water separation. The mixture of oil and water is just the tip of the iceberg of the mixtures that need to be separated, so the wettability-based separation strategy should be extended to treat other kinds of liquid/liquid or liquid/gas mixtures. This review aims at generalizing the approach of the well-developed oil/water separation to separate various multiphase mixtures based on the surface superwettability. Superhydrophobic and even superoleophobic surface microstructures have liquid-repellent properties, making different liquids keep away from them. Inspired by the process of oil/water separation, liquid polymers can be separated from water by using underwater superpolymphobic materials. Meanwhile, the underwater superaerophobic and superaerophilic porous materials are successfully used to collect or remove gas bubbles in a liquid, thus achieving liquid/gas separation. We believe that the diversified wettability-based separation methods can be potentially applied in industrial manufacture, energy use, environmental protection, agricultural production, and so on.

Water/gas separation based on the selective bubble-passage effect of underwater superaerophobic and superaerophilic meshes processed by a femtosecond laser

To solve the problems caused by tiny bubbles in liquids and the difficulties involved in collecting useful gas underwater, this paper proposes a method to separate bubbles from water by integrating underwater superaerophobic and superaerophilic porous membranes, including bubble removal and collection methods. Inspired by fish scales and lotus leaves, underwater superaerophobic microstructures and underwater superaerophilic microstructures are prepared on a stainless steel (SS) mesh by femtosecond laser processing, respectively. The as-prepared underwater superaerophobic mesh has an anti-bubble ability, while the underwater superaerophilic mesh has a bubble-absorption ability in water. Based on the different dynamic behavior of bubbles on these two kinds of superwetting meshes, efficient water/bubble separation is achieved by using laser-induced superwetting meshes. Tiny bubbles can be completely removed from the water flow in a pipe or easily collected. Such water/gas separation methods based on underwater superaerophobic and superaerophilic porous membranes provide an effective way to prevent the damage caused by bubbles and to collect the available gas in liquids, which has great potential applications in energy utilization, environmental protection, medical and health care, microfluidic chips, chemical manufacturing, agricultural breeding, and so on.

A Universal Strategy for the Preparation of Dual Superlyophobic Surfaces in Oil-Water Systems

There are some methods to prepare superwetting surfaces with underwater superoleophobicity (UWSOB) or underoil superhydrophobicity (UOSHB), but it is still thorny to put forward a universal strategy for constructing dual superlyophobic surfaces in oil-water systems due to a thermodynamic contradiction. Herein, a universal strategy was proposed to prepare the dual superlyophobic surfaces in oil-water systems only via delicately controlling surface chemistry, that is, adjusting the ratios of superhydrophilic and superhydrophobic counterparts in the spray solution. Three types of materials, attapulgite (APT), TiO₂, and loess, were chosen to prepare a diverse series of mixed coatings (mass gradient of superhydrophobic counterparts from 0 to 100 wt %). With the proportion of each superhydrophobic counterpart increasing, the underwater oil contact angle (θ_o/w^*) of each mixed coating slightly decreased but still was more than 150°, that is, UWSOB. In contrast, the underoil water contact angle (θ_w/o^*) was significantly improved, realizing the transformation from UOHL (or UOHB) to UOSHB. More importantly, the respective mass ratios of superhydrophobic counterparts in the resulting mixed coatings of APT, TiO₂, and loess were finally determined to be 0.3, 0.4, and 0.2, respectively. Taking APT as a model, a train of mixed APT coatings with different superhydrophobic components were systematically characterized and analyzed. Finally, the prepared superlyophobic separation mesh in oil-water systems was applied to the separation of various surfactant-stabilized oil-water emulsions. We envision that this universal strategy we proposed will show a significant application potential in addressing scientific and technological challenges in the field of interfacial chemistry such as oil-water separation, microfluidics, microdroplet manipulation, antifogging/icing, cell engineering, drag reduction, and so forth.

Towards controlled bubble nucleation in microreactors for enhanced mass transport

The exact location of bubbles with respect to the catalytic layer impacts the microreactor performance. In this work, we propose to control bubble nucleation using micropits to maximize the microreactor efficiency.

New insights into unusual droplets: from mediating the wettability to manipulating the locomotion modes

The ability to manipulate droplets can be utilized to develop various smart sensors or actuators, endowing them with fascinating applications for drug delivery, detection of target analytes, environmental monitoring, intelligent control, and so on. However, the stimuli-responsive superhydrophobic/superhydrophilic materials for normal water droplets cannot satisfy the requirements from some certain circumstances, i.e., liquid lenses and biosensors (detection of various additives in water/blood droplets). Stimuli-responsive wetting/dewetting behaviors of exceptional droplets are open issues and are attracting much attention from across the world. In this perspective article, the unconventional droplets are divided into three categories: ionic or surfactant additives in water droplets, oil droplets, and bubble droplets. We first introduce several classical wettability models of droplets and some methods to achieve wettability transition. The unusual droplet motion is also introduced in detail. There are four main types of locomotion modes, which are vertical rebound motion, lateral motion, self-propulsion motion, and anisotropic wettability controlled sliding behavior. The driving mechanism for the droplet motion is briefly introduced as well. Some approaches to achieve this manipulation goal, such as light irradiation, electronic, magnetic, acid-base, thermal, and mechanical ways will be taken into consideration. Finally, the current researches on unconventional droplets extending to polymer droplets and liquid metal droplets on the surface of special wettability materials are summarized and the prospect of unconventional droplet research directions in the field of on-demand transport application will be proposed.

Relationship and Interconversion Between Superhydrophilicity, Underwater Superoleophilicity, Underwater Superaerophilicity, Superhydrophobicity, Underwater Superoleophobicity, and Underwater Superaerophobicity: A Mini-Review

Superwetting surfaces have received increasing attention because of their rich practical applications. Although various superwettabilities are independently achieved, the relationship between those superwettabilities is still not well-clarified. In this mini-review, we show that superhydrophilicity, underwater superoleophilicity, underwater superaerophilicity, superhydrophobicity, underwater superoleophobicity, and underwater superaerophobicity can be obtained on a same structured surface by the combination of hierarchical surface microstructures and proper chemistry. The relationship and interconversion between the above-mentioned different superwettabilities are also well-discussed. We believe that the current discussion and clarification of the relationship and interconversion between different superwettabilities has important significance in the design, fabrication, and applications of various superwetting materials.

Simple and Low-Cost Oil/Water Separation Based on the Underwater Superoleophobicity of the Existing Materials in Our Life or Nature

The achievement of high-efficiency oil/water separation has huge implications for protecting environment and reducing economic losses, but there is still a great challenge. Currently, most artificial oil/water separating materials are fabricated through complex preparation process, resulting in the very high cost of separation. In this paper, we present a simple and low-cost method to achieve oil/water separation by using the underwater superoleophobic materials that already exist in our life or nature. Taking filter paper and zeolite layer as examples, we show the inherent porous microstructures of these materials. Such porous microstructures endow filter paper and zeolite layer with strong ability to absorb water and the underwater superoleophobicity. Based on the porous feature and underwater superoleophobicity, the pre-wetted filter paper and zeolite layer can be used to effectively separate the mixture of water and oil, with great separation capacity. The existing materials (e.g., filter paper and zeolite layer) with both porous microstructure and underwater superoleophobicity in our life or nature are green and low-cost, and can be easily obtained. Such advantages allow those materials to potentially solve the pollution problems caused by the discharge of industrial oily wastewater and the oil-spill accidents.

Highly Floatable Superhydrophobic Metallic Assembly for Aquatic Applications

Water-repellent superhydrophobic (SH) surfaces promise a wide range of applications, from increased buoyancy to drag reduction, but their practical use is limited. This comes from the fact that an SH surface will start to lose its efficiency once it is forced into water or damaged by mechanical abrasion. Here, we circumvent these two most challenging obstacles and demonstrate a highly floatable multifaced SH metallic assembly inspired by the diving bell spiders and fire ant assemblies. We study and optimize, both theoretically and experimentally, the floating properties of the design. The assembly shows an unprecedented floating ability; it can float back to the surface even after being forced to submerge under water for months. More strikingly, the assembly maintains its floating ability even after severe damage and piercing in stark contrast to conventional watercrafts and aquatic devices. The potential use of the SH floating metallic assembly ranges from floating devices and electronic equipment protection to highly floatable ships and vessels.

Femtosecond-Laser-Produced Underwater "Superpolymphobic" Nanorippled Surfaces: Repelling Liquid Polymers in Water for Applications of Controlling Polymer Shape and Adhesion

A femtosecond (fs)-laser-processed surface that repels liquid polymer in water is reported in this paper. We define this phenomenon as the "superpolymphobicity". Three-level microstructures (including microgrooves, micromountains/microholes between the microgrooves, and nanoripples on the whole surface) were directly created on the stainless steel surface via fs laser processing. A liquid polydimethylsiloxane (PDMS) droplet on the textured surface had the contact angle of 156 ± 3° and contact angle hysteresis less than 4° in water, indicating excellent underwater superpolymphobicity of the fs-laser-induced hierarchical microstructures. The contact between the resultant superhydrophilic hierarchical microstructures and the submerged liquid PDMS droplet is verified at the underwater Cassie state. The underwater superpolymphobicity enables to design the shape of cured PDMS and selectively avoid the adhesion at the PDMS/substrate interface, different from the previously reported superwettabilities. As the examples, the microlens array and microfluidics system were prepared based on the laser-induced underwater superpolymphobic microstructures.

Reversible Tuning between Isotropic and Anisotropic Sliding by One-Direction Mechanical Stretching on Microgrooved Slippery Surfaces

Dynamically responsive liquid-infused interfacial materials have broad technological implications in manipulating droplet motions. However, present works are mainly about reversible tuning of the isotropic slippery surface; the reversible switching between isotropic and anisotropic sliding has not been deeply explored. Here, we report a kind of liquid-infused elastic-grooved surface (LIEGS) by femtosecond laser ablation and realize reversible switching between isotropic and anisotropic sliding by one-direction mechanical stretching. Under mechanical stretching and strain release, droplet motion can be reversibly switched between the sliding and pinned states along the perpendicular direction to the grooves, whereas the droplet keeps sliding along the parallel direction to the grooves. The mechanism of reversible switching mainly contributes to the decrease of film thickness during the stretching process in which the film thickness decreases from 13 to 4 μm with the increase of the strain from 0 to 60%. Finally, we demonstrate the real-time flexible control over a droplet sliding/pinned on the strain-changing LIEGS.

Femtosecond Laser-Structured Underwater "Superpolymphobic" Surfaces

In this work, the surfaces that repel liquid polydimethylsiloxane (PDMS) droplets in water were created by femtosecond laser treatment. We define this superwetting phenomenon as underwater "superpolymphobicity". The resultant underwater superpolymphobic silicon surface shows a contact angle of 159 ± 1° and a sliding angle of 1.5 ± 0.5° to liquid PDMS droplets in water. This underwater superpolymphobicity can be achieved on a wide range of hydrophilic materials, including semiconductors, glass, and metals. The adhesion between the liquid polymer and a solid substrate is effectively prevented by the underwater superpolymphobic microstructures. The underwater superpolymphobicity will have a great significance in designing the adhesion between the polymer and a solid substrate, controlling the shape of the cured polymer materials, as well as nearly all the applications based on the polymer materials.

Development of Durable, Fluorine-free, and Transparent Superhydrophobic Surfaces for Oil/Water Separation

Although artificial superhydrophobic materials have extensive and significant applications in antifouling, self-cleaning, anti-icing, fluid transport, oil/water separation, and so forth, the poor robustness of these surfaces has always been a bottleneck for their development in practical industrial applications. Here, we report a facile, economical, efficient, and versatile strategy to prepare environmentally friendly, mechanically robust, and transparent superhydrophobic surfaces by combining adhesive and hydrophobic paint, which is applicable for both hard and soft substrates. The coated substrates exhibit excellent superhydrophobic property and ultralow adhesion with water (contact angle ≈ 160° and sliding angle <2°). Additionally, the coated surface maintained its superhydrophobicity even after 325 sandpaper abrasion cycles, showing remarkable mechanical robustness. Furthermore, the coated surfaces were applied to separate oil/water mixtures because of their unique characteristics of being simultaneously superhydrophobic and superoleophilic. In addition, it is believed that this fabrication method is significant, promising, and feasible for mass production of superhydrophobic surfaces for industrial applications.

A review of femtosecond laser-structured superhydrophobic or underwater superoleophobic porous surfaces/materials for efficient oil/water separation

Oil/water separation (OWS) technology has become an increasingly crucial tool to protect the environment and reduce the economic losses caused by the discharge of oily wastewater and oil spills. Recently, porous materials with superwettability have been applied in effective OWS and have achieved tremendous success. Herein, we review recent advancements of OWS utilizing femtosecond (fs) laser-structured superhydrophobic or underwater superoleophobic porous materials. We will review the enabling materials processing and treatment methods, their surface wettability, the separating methods and processes, and the separation mechanisms. Inspired by lotus leaves and fish scales, superhydrophobic and underwater superoleophobic properties are artificially achieved on substrate surfaces by fs laser processing. By using fs laser-structured superwetting porous materials, various oil/water mixtures (OWMs) are successfully separated through different separation methods. Presently, the research of fs laser-based OWS is still in its infancy. We will also discuss the current challenges and future prospects in this emerging field. It is expected that the advanced features of fs laser microfabrication will lead to exciting applications for OWS.

Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil–Water Separation

Femtosecond (FS) laser-induced surface structuring is a robust, maskless, non-contact, and single-step process for producing micro- and nanoscale structures on a material’s surface, which remarkably alters the optical, chemical, wetting, and tribological properties of that material. Wettability control, in particular, is of high significance in various applications, including self-cleaning, anti-fouling, anti-icing, anti-corrosion, and, recently, oil–water separation. Due to growing energy demands and rapid industrialization, oil spill accidents and organic industrial discharges frequently take place. This poses an imminent threat to the environment and has adverse effects on the economy and the ecosystem. Oil–water separation and oil waste management require mechanically robust, durable, low-cost, and highly efficient oil–water manipulation systems. To address this challenge superhydrophobic–superoleophilic and superhydrophilic–underwater superoleophobic membrane filters have shown promising results. However, the recyclability and durability issues of such filters are limiting factors in their industrial application, as well as in their use in oil spill accidents. In this article, we review and discuss the recent progress in the application of FS laser surface structuring in producing durable and robust oil–water separation membrane filters. The wide variety of surface structures produced by FS laser nano- and micromachining are initially presented here, while the excellent wetting characteristics shown by specific femtosecond-induced structures are demonstrated. Subsequently, the working principles of oil–water separation membranes are elaborated, and the most recent advances in the topic are analyzed and discussed.

Femtosecond Laser-Induced Underwater Superoleophobic Surfaces with Reversible pH-Responsive Wettability

Wettability-switchable surfaces have become a research hotspot because they can exhibit different superwetting states. In this paper, the copper surfaces with pH-responsive underwater-oil wettability were prepared by femtosecond laser treatment and subsequent chemical modification. The resultant surfaces showed underwater superoleophobicity in alkaline solutions but quasi-superoleophilicity in acidic solutions. The contact angles of an underwater-oil droplet on the resultant surfaces could be reversibly tuned between 157° and 12° by changing the pH of aqueous solutions. Such switchable wettability is ascribed to the modification of the alkyl and carboxylic acids groups on the laser-structured surfaces. The as-prepared surfaces have both oil-resistance and oil-collection abilities by selectively showing underwater superoleophobicity and superoleophilicity. The smart surfaces with pH-responsive oil wettability will have important applications in controlling the oil behavior in water.

A femtosecond laser-induced superhygrophobic surface: beyond superhydrophobicity and repelling various complex liquids

Surfaces that can strongly repel various complex liquids, not just pure water, are highly desirable and the fabrication of such surfaces still remains a huge challenge because the liquids one wants to repel usually have a complex chemical composition, viscosity, and concentration. Here, a superhygrophobic surface microstructure was created on a polytetrafluoroethylene (PTFE) surface by femtosecond laser treatment. The laser-ablated surface was composed of a micro/nanoscale hierarchical structure and micropores with a certain degree of re-entrant curvature. After femtosecond laser ablation, the sample surface is directly endowed with superhygrophobicity and has great ability to repel various pure and complex liquids, such as water, 10 000 ppm bovine serum albumin, cola, 10 000 ppm glucose, juice, and saline. It is because the combined effect of the ultralow surface energy of the PTFE material, the laser-induced hierarchical rough microstructure, and the partly re-entrant surface curvature of the porous structure allows the complex liquid droplets to be at the robust Cassie state on the laser-induced surface microstructure. Such superhygrophobic surfaces can be potentially applied in cell engineering, medical instruments, food packaging, microfluidics technology, chemical engineering, and so on.

Microfluidic Channels Fabrication Based on Underwater Superpolymphobic Microgrooves Produced by Femtosecond Laser Direct Writing

A strategy is proposed here to fabricate microfluidic channels based on underwater superpolymphobic microgrooves with nanoscale rough surface structure on glass surface produced by femtosecond (fs) laser processing. The fs laser-induced micro/nanostructure on glass surface can repel liquid polydimethylsiloxane (PDMS) underwater, with the contact angle (CA) of 155.5 ± 2.5° and CA hysteresis of 2.7 ± 1.5° to a liquid PDMS droplet. Such a phenomenon is defined as the underwater "superpolymphobicity". Microchannels as well as microfluidic systems are easily prepared and formed between the underwater superpolymphobic microgroove-textured glass substrate and the cured PDMS layer. Because the tracks of the laser scanning lines are programmable, arbitrary-shaped microchannels and complex microfluidic systems can be potentially designed and prepared through fs laser direct writing technology. The concept of "underwater superpolymphobicity" presented here offers us a new strategy for selectively avoiding the adhesion at the polymer/substrate interface and controlling the shape of cured polymers; none of these applications can find analogues in previously reported superwetting materials.