Portable Triboelectric Electrostatic Tweezer for External Manipulation of Droplets within a Closed Femtosecond Laser-Treated Superhydrophobic System.

Controllable droplet manipulation has diverse applications; however, limited methods exist for externally manipulating droplets in confined spaces. Herein, we propose a portable triboelectric electrostatic tweezer (TET) by integrating electrostatic forces with a superhydrophobic surface that can even manipulate droplets in an enclosed space. Electrostatic induction causes the droplet to be subjected to an electrostatic force in an electrostatic field so that the droplet can be moved freely with the TET on a superhydrophobic platform. Characterized by its high precision, flexibility, and robust binding strength, TET can manipulate droplets under various conditions and achieve a wide range of representative fluid applications such as droplet microreactors, precise self-cleaning, cargo transportation, the targeted delivery of chemicals, liquid sorting, soft droplet robotics, and cell labeling. Specifically, TET demonstrated the ability to manipulate internal droplets from the outside of a closed system, such as performing cell labeling experiments within a sealed Petri dish without opening the culture system.

Multifunctional Electrostatic Droplet Manipulation on the Femtosecond Laser-Prepared Slippery Surfaces.

A strategy to manipulate droplets on the lubricated slippery surfaces using tribostatic electricity is proposed. By employing femtosecond laser-induced porous microstructures, we prepared a slippery surface with ultralow adhesion to various liquids. Electrostatic induction causes the charges within the droplet to be redistributed; thus, the droplet on the as-prepared slippery surfaces can be guided by electrostatic force under the electrostatic field, with controllable sliding direction and unlimited transport distance. The combination of electrostatic interaction and slippery surfaces allows us to manipulate droplets with a wide volume range (from 100 nL to 0.5 mL), charged droplets (including electrostatic attraction and repulsion), corrosive droplets, and even organic droplets with ultralow surface tension. In addition, droplets on tilted surfaces, curved surfaces, and inverted slippery surfaces can also be manipulated. Especially, the slippery surfaces can even allow the electrostatic interaction to manipulate alcohol with surface tension as low as 22.3 mN/m and liquid droplets suspended on a downward surface, which is not possible with reported superhydrophobic substrates. The features of slippery surfaces make the electrostatic manipulation successfully applied in versatile droplet manipulation, droplet patterning, chemical microreaction, transport of solid cargo, targeted delivery of chemicals, and liquid sorting.

Unraveling the influence of biogenic volatile organic compounds and their constituents on ozone and SOA formation within the Yellow River Basin, China.

Biogenic volatile organic compounds (BVOC) assume a pivotal role during the formation stages of ozone (O3) and secondary organic aerosols (SOA), serving as their primary precursors. We used the latest MEGAN3.1 model, updated vegetation data and emission factors, combined with MODIS data analysis to simulate and estimate the integrated emissions of BVOC from nine provinces in China's Yellow River Basin in 2018. Following an extensive evaluation of the WRF-CMAQ model utilizing diverse parameters, the simulated and observed values had correlation coefficients between them that ranged from 0.94 to 0.99, implying a favorable outcome in terms of simulation efficacy. The findings from the simulation analysis reveal that the combined BVOC emissions from the nine provinces in the Yellow River Basin reached a total of 6.51 Tg in 2018. Among these provinces, Sichuan, Henan, and Shaanxi ranked highest, with emissions of 1.28 Tg, 1.04 Tg, and 0.96 Tg, respectively. BVOC emissions led to concentrations of 36.72 µg/m³ in the daily maximum 8-h ozone and 0.59 µg/m³ in the average SOA in nine provinces of the Yellow River Basin in July. Isoprene contributed the most to the O3 production with 6.31 µg/m3, and monoterpenes contributed the most to SOA production with 0.45 µg/m3. ΔSOA and ΔOzone are mainly distributed in the belts of central Sichuan Province, southern Shaanxi Province, western Henan Province, northern Qinghai Province, central Inner Mongolia, and southern Shanxi Province, and most of these areas are located 50 km around the Yellow River. O3 and SOA in Taiyuan, Xi'an, Chengdu, and Zhengzhou cities are strongly influenced by the generation of BVOCs. This study provides a reliable scientific basis for the prevention and control of air pollution in the Yellow River Basin.

Durable Janus membrane with on-demand mode switching fabricated by femtosecond laser.

Despite their notable unidirectional water transport capabilities, Janus membranes are commonly challenged by the fragility of their chemical coatings and the clogging of open microchannels. Here, an on-demand mode-switching strategy is presented to consider the Janus functionality and mechanical durability separately and implement them by simply stretching and releasing the membrane. The stretching Janus mode facilitates unidirectional liquid flow through the hydrophilic micropores-microgrooves channels (PG channels) fabricated by femtosecond laser. The releasing protection mode is designed for the in-situ closure of the PG channels upon encountering external abrasion and impact. The protection mode imparts the Janus membrane robustness to reserve water unidirectional penetration under harsh conditions, such as 2000 cycles mechanical abrasion, 10 days exposure in air and other rigorous tests (sandpaper abrasion, finger rubbing, sand impact and tape peeling). The underlying mechanism of gridded grooves in protecting and enhancing water flow is unveiled. The Janus membrane serves as a fog collector to demonstrate its unwavering mechanical durability in harsh real-world conditions. The presented design strategy could open up new possibilities of Janus membrane in a multitude of applications ranging from multiphase separation devices to fog harvesting and wearable health-monitoring patches.

Self-Driving Underwater "Aerofluidics".

Here, the concept of "aerofluidics," in which a system uses microchannels to transport and manipulate trace gases at the microscopic scale to build a highly versatile integrated system based on gas-gas or gas-liquid microinteractions is proposed. A kind of underwater aerofluidic architecture is designed using superhydrophobic surface microgrooves written by a femtosecond laser. In the aqueous medium, a hollow microchannel is formed between the superhydrophobic microgrooves and the water environment, which allows gas to flow freely underwater for aerofluidic devices. Driven by Laplace pressure, gas can be self-transported along various complex patterned paths, curved surfaces, and even across different aerofluidic devices, with an ultralong transportation distance of more than 1 m. The width of the superhydrophobic microchannels of the designed aerofluidic devices is only ≈42.1 µm, enabling the aerofluidic system to achieve accurate gas transportation and control. With the advantages of flexible self-driving gas transportation and ultralong transportation distance, the underwater aerofluidic devices can realize a series of gas control functions, such as gas merging, gas aggregation, gas splitting, gas arrays, gas-gas microreactions, and gas-liquid microreactions. It is believed that underwater aerofluidic technology can have significant applications in gas-involved microanalysis, microdetection, biomedical engineering, sensors, and environmental protection.

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.

Filtration and removal of liquid polymers from water (polymer/water separation) by use of the underwater superpolymphobic mesh produced with a femtosecond laser.

The widespread use of liquid polymers may pollute water, causing grave environmental problems and even various human diseases. The separation of a mixture of a liquid polymer and water is extremely important in research, but the high viscosity, low fluidity, and high adhesion performance of liquid polymers make this task highly challenging. In this paper, we propose a novel strategy for separating a polymer/water mixture wherein porous underwater superpolymphobic micro/nanostructures are used for the first time. Femtosecond laser (fsL) processing is used to form micro/nanoscale surface structures on a stainless steel mesh (SSM), resulting in excellent repellence (underwater superpolymphobicity) to various liquid polymer droplets in water. The laser-induced underwater superpolymphobicity is very stable even though the SSM suffers from different damage treatments (e.g., sandpaper abrasion, acid or alkali solutions corrosion, UV light irradiation, and tape peeling). The underwater superpolymphobicity is ascribed to an underwater Cassie contact state between the underwater liquid polymer and the surface microstructure of the laser-treated SSM. We demonstrate that the underwater superpolymphobic SSM can be effectively and repeatedly used to separate liquid polymer/water mixtures with a high separation efficiency of 99.0% and a high separation flux of 4.45 × 105 L m-2h-1. The mixtures of water and different polymers are successfully separated. Such a separation strategy can potentially alleviate pollution from liquid polymer discharge, recycle waste polymer resources, and be applied in polymer production and manufacturing.

Magnetically Controllable Isotropic/Anisotropic Slippery Surface for Flexible Droplet Manipulation.

Controllable wetting surfaces play a significant role in numerous applications such as smart liquid manipulation, lab-on-a-chip, drug delivery, liquid robot, and so on. A novel type of magnetically controllable isotropic/anisotropic slippery surface was prepared by femtosecond laser ablation. The slippery liquid-infused porous surface (SLIPS) can be switched between an isotropic smooth state and an anisotropic groove state by the magnetic field. The relationship between the sliding property of the SLIPS and the magnetic flux density, water droplet volume, microgroove width, and microgroove height are systematically studied. Passively flexible movement on the isotropic SLIPS and actively directional movement on the anisotropic SLIPS of water droplets were realized. This work provides a fresh understanding of the controllable isotropic/anisotropic SLIPS and reveals great potential in versatile applications which are related to magnetically controllable smart liquid manipulation.

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.

Mini-Review on Bioinspired Superwetting Microlens Array and Compound Eye.

Microlens arrays (MLAs) and MLA-based artificial compound eyes (ACEs) are the important miniaturized optical components in modern micro-optical systems. However, their optical performance will seriously decline once they are wetted by water droplets (such as fog, dew, and rain droplets) or are polluted by contaminations in a humid environment. In this mini-review, we summarize the research works related to the fabrication of superwetting MLAs and ACEs and show how to integrate superhydrophobic and superoleophobic microstructures with an MLA. The fabrication strategy can be split into two categories. One is the hybrid pattern composed of the MLA domain and the superwetting domain. Another is the direct formation of superwetting nanostructures on the surface of the microlenses. The superhydrophobicity or superoleophobicity endows the MLAs and ACEs with liquid repellence and self-cleaning function besides excellent optical performance. We believe that the superwetting MLAs and ACEs will have significant applications in various optical systems that are often used in the humid or liquid environment.

A femtosecond Bessel laser for preparing a nontoxic slippery liquid-infused porous surface (SLIPS) for improving the hemocompatibility of NiTi alloys.

A slippery liquid-infused porous surface (SLIPS) is able to improve the hemocompatibility of implantable medical materials, which have saved countless lives. However, the preparation of a SLIPS on an implantable metal substrate (especially NiTi alloys) is still a substantial challenge because of the great difficulty of forming abundant porous microstructures on hard metals. In this paper, a novel strategy to prepare a SLIPS on a NiTi alloy substrate is reported. We used the laser pulse train of a femtosecond Bessel laser rather than the common Gaussian beam to directly create deep porous microstructures on the surface of the implantable NiTi alloy. Based on the laser-induced porous microstructure, the SLIPS was obtained by lowering the surface energy and infusing the lubricant liquid into the pores. The as-prepared SLIPS very effectively repelled water and blood. The hemocompatibility of the NiTi alloy was greatly improved after the formation of the SLIPS by the femtosecond Bessel laser processing. It was demonstrated that the SLIPS gives the NiTi alloy a remarkable anticoagulation property, very low hemolysis rate, and antibacterial property. We believe that the laser-induced SLIPS will accelerate the broad application of metal implants in the medical field in a healthier and safer way.

Femtosecond Laser Microfabrication of Porous Superwetting Materials for Oil/Water Separation: A Mini-Review.

Frequent oil-leakage accidents and large quantities of oil-bearing wastewater discharge cause severe environmental pollution and huge economic losses. Recently, superwetting porous materials are successfully utilized to separate oil/water mixture (OWM) based on the different interfacial behavior of water and oil. Here, we summarize the recent development of efficient oil/water separation (OWS) based on the femtosecond laser-induced superwetting materials. The typical wettability-based separation manners (including "oil-removing" and "water-removing") and the characteristic of the femtosecond laser are introduced as background. Various laser-structured porous sheets with either superhydrophobicity or underwater superoleophobicity are successfully used to separate different OWMs. The laser processing methods, surface wettability, separation process, and separation mechanism of these laser-structured separation materials are reviewed. Finally, the current challenges and prospects in achieving OWS by femtosecond laser microfabrication are discussed.

Femtosecond laser hybrid fabrication of a 3D microfluidic chip for PCR application.

Microfluidic chips have gradually become a focus of scientific research. However, the fabrication of key functional components in microfluidic chips is always limited by the existing processing methods. The microfluidic chip is difficult to be three-dimensional (3D) and integrated. In response to the key problems of 3D integrated microfluidic chip fabrication, this paper presents a hybrid method for fabricating a microfluidic chip integrated 3D microchannels and metal microstructures by femtosecond laser wet etch technology and liquid metal injection. The integrated microfluidic chip fabricated by this method is expected to be applied to the core reaction unit of integrated PCR devices.

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.

Femtosecond laser preparing patternable liquid-metal-repellent surface for flexible electronics.

HYPOTHESIS: Controlling the wetting behaviour of gallium-based liquid metal is highly desired for soft electronics applications. Currently, achieving durable and patternable liquid-metal-repellent surfaces by a simple and flexible method is challenging. The femtosecond laser has a remarkable ability to modify the morphology and wettability of a solid surface. It can also potentially be applied to control the wettability of liquid metal and achieve complete liquid-metal patterns. EXPERIMENTS: Femtosecond laser processing was used to form a microstructure on a polydimethylsiloxane (PDMS) surface. With regard to the laser-ablated surface, its morphology was observed by a scanning electron microscope, and its wettability to liquid metal was characterized by measuring the contact angle, sliding angle, and adhesive force. Finally, its potential applications in soft electronics were demonstrated. FINDINGS: A layer of micro/nanostructures was directly prepared on the PDMS surface by laser ablation, presenting excellent liquid-metal repellence. Without expensive masks and complex operation processes, programmable liquid-metal-repellent patterns were easily obtained by femtosecond laser selectively treating the PDMS surface, enabling EGaIn to be patterned on the textured surface. The as-prepared liquid-metal patterns can be used as a flexible microheater and a microstrip patch antenna. It is believed that laser-patterned liquid-metal-repellent surfaces will have significant applications in soft electronics, such as antennas, microcircuits, lab on chips, and wearable electronic devices.

Tuning a surface super-repellent to liquid metal by a femtosecond laser.

Eutectic gallium-indium (EGaIn) liquid metal (LM) attracts increasing interest because of its broad applications in flexible circuits and soft devices. However, LM can easily adhere to a solid substrate due to the existence of a high-adhesive oxide outside layer, which greatly limits the real application of LM materials. Current methods to reduce the LM adhesion are mostly based on chemical treatment rather than surface microstructure, which are not suitable for most practical applications. In this paper, microstructure was simply created on various substrates by femtosecond laser ablation. The resultant surfaces show excellent repellence to EGaIn LM and the LM is difficult to adhere to the structured surface. Such a surface is defined as a "super-metal-phobic" surface. For example, the laser-ablated silica glass surface shows a contact angle of 157° ± 3°, sliding angle of 10°, and adhesive force of 1.2 µN to a LM droplet. The LM repellence is very stable even if the LM droplet is heavily pressed towards the sample surfaces. It is demonstrated that the adhesion of LM can be significantly reduced by the laser-induced surface microstructures. The method of controlling the wettability of LM has important potential applications in manipulating LM and preparing flexible circuits.

Underwater superoleophobic and anti-oil microlens array prepared by combing femtosecond laser wet etching and direct writing techniques.

As an important micro-optical device, microlens array (MLA) also has broad applications in aqueous environment apart from atmosphere, such as bioscience research, ocean exploration, and microfluidic systems. However, the surface of the normal MLA is easily polluted by oil contaminations when the MLA is practically applied in a water medium, leading to the loss of its optical imaging ability. Herein, we fabricated a functional MLA with underwater anti-oil and self-cleaning abilities by combining the femtosecond laser wet etching (FLWE) and the femtosecond laser direct writing (FLDW) techniques. The as-prepared close-packed MLA is composed of 10000 single microlenses with the aperture diameter of 50 µm. The surface of each microlens is further textured with micro/nanoparticles. Clear and uniform images could be captured by using the resultant MLA in water, demonstrating great underwater imaging ability. The modulation transfer function value is larger than 0.6 at 55 lp/mm. In addition, the micro/nanostructures endow the as-fabricated MLA surface with underwater superoleophobicity and oil-repellent performance. Various oils can be repelled by the resultant MLA in water. Underwater 1,2-dichloroethane oil droplet on the textured MLA has a contact angle of 158.0 ± 0.5° and a sliding angle of 2.0 ± 0.2°. The underwater superoleophobic MLA also has good mechanical durability. The anti-oil and self-cleaning functions will broaden the applications of the MLA in ocean exploration, bioscience research, microfluidic system, and many underwater MLA-based systems.

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.