The design of underwater superoleophobic Ni/NiO microstructures with tunable oil adhesion

Strategies to modulate underwater oil wettability and adhesion

Inspired by the extreme underwater oil repellence found in fish scales, formally defined as underwater superoleophobicity, various functional interfaces have recently been derived. Such heterogeneous oil wettability underwater is attributed to the entrapment of liquid water in an extremely hydrophilic interface decorated with micro- and nanostructures. Designing underwater superoleophobic surfaces with differences in the force of oil adhesion is important for extending its potential utilizations in various and relevant applications. While underwater non-adhesive superoleophobicity enables applications like oil-liquid separation, self-cleaning, anti-fouling, anti-platelet adhesion, etc., the underwater superoleophobic interfaces embedded with the controlled force of oil adhesion remain crucial for another set of applications-including no-loss oil droplet manipulation, transfer, chemical toxin sensing, etc. This review discusses various strategies for deriving such underwater superoleophobic surfaces, emphasizing the need for co-optimizing appropriate surface nanoarchitectonics and hydrophilic chemistry and illustrating strategies for addressing durability and scalability challenges. Further, this review reveals the dominant role of chemical modulations over topography optimization for precise and orthogonal control on both oil wettability and force of oil adhesion. Additionally, strategic post-functionalization approaches are highlighted that enable instrument-free and naked-eye detection of physiological biomarkers and environmental toxins. It also depicts approaches to deriving mechanically durable underwater superoleophobic coatings-improving their suitability for more realistic application in outdoor conditions.

Underwater Adhesion of Oil Droplets to Surfaces Grafted with Hydrophilic Polymer Brushes: Many-Body Dissipative-Particle Simulations

A droplet probe atomic force microscopy (AFM) simulation using many-body dissipative particle dynamics is employed to investigate the effects of polymer hydrophilicity and grafting density on the underwater adhesion behavior of oil droplets at hydrophilic polymer brush layers. The results show that increasing the grafting density suppresses oil penetration, leading to reduced adhesion. Moreover, a higher polymer hydrophilicity weakens the sensitivity of adhesion behavior to grafting density, causing adhesion forces to converge under strong polymer-water interactions. Further analysis reveals that interfacial properties, including contact area, penetration depth, and contribution of polymers at the interface, significantly influence oil droplet adhesion. Additionally, polymer hydrophilicity plays a key role in regulating the relative contribution of polymers at the contact interface, ultimately determining the adhesion and its sensitivity to interfacial properties.

Role of chemistry in bio-inspired liquid wettability

Chemistry and topography are the two distinct available tools for customizing different bio-inspired liquid wettability including superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superoleophilicity, and liquid infused slippery property. In nature, various living species possessing super and special liquid wettability inherently comprises of distinctly patterned surface topography decorated with low/high surface energy. Inspired from the topographically diverse natural species, the variation in surface topography has been the dominant approach for constructing bio-inspired antiwetting interfaces. However, recently, the modulation of chemistry has emerged as a facile route for the controlled tailoring of a wide range of bio-inspired liquid wettability. This review article aims to summarize the various reports published over the years that has elaborated the distinctive importance of both chemistry and topography in imparting and modulating various bio-inspired wettability. Moreover, this article outlines some obvious advantages of chemical modulation approach over topographical variation. For example, the strategic use of the chemical approach has allowed the facile, simultaneous, and independent tailoring of both liquid wettability and other relevant physical properties. We have also discussed the design of different antiwetting patterned and stimuli-responsive interfaces following the strategic and precise alteration of chemistry for various prospective applications.

Silicon carbide microfiltration membranes for oil-water separation: Pore structure-dependent wettability matters

Both the pore size and surface properties of silicon carbide (SiC) membranes are demonstrated to significantly affect their separation efficiency when used for oily water treatment. However, the potential influences of open porosity together with the pore size of SiC membranes on their surface properties and oil-water separation performance have rarely been investigated. In this work, porous SiC ceramic membranes with tunable open porosity and pore size were purposely prepared and selected to systematically study the effect of pore structure-dependent wettability on the oil-water separation performance. The measured pure water flux of selected membranes as a function of open porosity (34-48%) and pore size (0.43-0.67 μm) was well-fitted by using a modified H-P equation. Interestingly, the hydrophilicity of SiC membranes was improved with the increase in open porosity and pore size, as evidenced by the gradually decreased dynamic water contact angle and underwater adhesion of oil droplets. Further, the open porosity of SiC membranes was found to contribute more to the improved surface wettability. As a result, the stable flux of SiC membranes in oil-in-water (O/W) emulsions was increased by 24% with the increased open porosity while the oil rejection rate remained above 90%. This work quantitatively reveals the contributions of the pore structure to the surface wettability of ceramic membranes, and thus provides an effective pathway to improve their performance in oil-water separation.

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.

Bacterial Superoleophobic Fibrous Matrices: A Naturally Occurring Liquid-Infused System for Oil-Water Separation

Nanocellulose fibers bioengineered by bacteria are a high-performance three-dimensional cross-linked network which can confine a dispersed liquid medium such as water. The strong chemical and physical interactions of dispersed water molecules with the entangled cellulosic network allow these materials to be ideal substrates for effective liquid separation. This type of phenomenon can be characterized as green with no equivalent precedent; its performance and sustainability relative to other cellulose-based or synthetic membranes are shown herein to be superior. In this work, we demonstrated that the renewable bacterial nanocellulosic membrane can be used as a stable liquid-infused system for the development of soft surfaces with superwettability and special adhesion properties and thus address intractable issues normally encountered by solid surfaces.

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.

Anisotropy-induced directional self-transportation of low surface tension liquids: a review

Inspired by natural surfaces such as butterfly wings, cactus leaves, or the Nepenthes alata plant, synthetic materials may be engineered to directionally transport liquids on their surface without external energy input. This advantageous feature has been adopted for various mechanical and chemical processes, e.g. fog harvesting, lubrication, lossless chemical reactions, etc. Many studies have focused on the manipulation and transport of water or aqueous droplets, but significantly fewer have extended their work to low surface tension (LST) liquids, although these fluids are involved in numerous industrial and everyday processes. LST liquids completely wet most surfaces which makes spontaneous transportation an active challenge. This review focuses on recently developed strategies for passively and directionally transporting LST liquids.

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.

Underliquid Superlyophobic Copper-Coated Meshes for the Separation of Immiscible Organic Liquid Mixtures

Superwettable materials have been studied extensively and successfully applied in various forms liquid separation. However, because of low surface tension differences, organic liquids (OLs) exhibit approximate wettability on most of the material surfaces, and the separation of OL mixtures remains a challenge. The current separation method for OL mixtures is mainly dependent on covalent modification to precisely control the surface energy of the membranes, which is extremely complicated. Herein, we demonstrate a novel concept of underliquid superlyophobicity for the separation of immiscible OLs mixtures, which only depend on a relatively stable liquid-repellent interface. Furthermore, the minimum system's free-energy principle was used to explain this wetting behavior. Different from the previous reports, the method of separation of OL mixtures does not involve various low-surface-energy materials, thus it is facile and eco-friendly. Our research provides a general strategy for the efficient separation of immiscible OLs mixtures and is expected to promote the development of superwettable materials for multiphase liquid separation.

NIR-Triggered Photothermal Responsive Coatings with Remote and Localized Tunable Underwater Oil Adhesion

Tunable underwater oil adhesion is a critical issue in interfacial science and industrial applications. Although much progress has been made to date, development of novel smart coating materials that can selectively change the wetting property at different areas is considerably scarce. Here, a simple strategy is proposed to fabricate photothermal responsive coatings, which can change the oil adhesion behavior from low-adhesive rolling state to high-adhesive pinning state for a variety of oily liquids in a remote, local, and reversible manner. Owing to this unique controllability, the adhesion and no-adhesion of oil droplets on the coated surfaces can be easily manipulated by remote and local near-infrared radiation.

Nature-Inspired Liquid Infused Systems for Superwettable Surface Energies

The development of an innovative interfacial wetting strategy known as liquid infused systems offers great promise for the advanced design of superwetting and superantiwetting substrates to overcome the drawbacks of textured surfaces classified under the heading of Cassie/Wenzel states. The potential value of nature-inspired surfaces has significant potential to address scientific and technological challenges within the field of interfacial chemistry. The objective of the current review is to provide insights into a fruitful and young field of research, highlight its historical developments, examine its nature-inspired design principles, gauge recent progress in emerging applications, and offer a fresh perspective for future research.

A robust and stretchable superhydrophobic PDMS/PVDF@KNFs membrane for oil/water separation and flame retardancy

The wide application of superhydrophobic membranes has been limited due to their complicated preparation technology and weak durability. Inspired by the mechanical flexibility of nanofibrous biomaterials, nanofibrils have been successfully generated from Kevlar, which is one of the strongest synthetic fibers, by appropriate hydrothermal treatment. In this study, a robust superhydrophobic PDMS/PVDF@KNFs membrane is prepared via a simple one-step process and subsequent curing without combination with inorganic fillers. The as-prepared PDMS/PVDF@KNFs membrane not only shows efficient oil/water separation ability and oil absorption capacity but also has excellent superhydrophobicity stability after deformation. The resultant membrane shows stretchability, flexibility and flame retardance because of the reinforcing effect and the excellent flame retardancy of Kevlar. We believe that this simple fabrication of PDMS/PVDF@KNFs has promising applications in filtering membranes and wearable devices.

Underwater Oil Droplet Splitting on a Patterned Template

Underwater oil droplets stretched and pinned by dual-dot oleophilic patterns on a superoleophobic substrate have been split into two nearly equal-volume daughter droplets using an underwater superoleophobic blade at substantially lower cutting speeds than reported in previous studies. A "liquid exchange model" based on Laplace pressure-driven liquid transport has been proposed to explain the mechanism of the underwater droplet split process. The dependence of droplet geometrical shape (curvature) and liquid properties (surface tension, viscosity) on the critical cutting speed that allows equal-volume split was investigated. Results demonstrate that critical cutting speed increases with increased curvature and surface tension of the split droplet, and decreases with increased droplet viscosity, which agrees with the proposed model. The ability to reproducibly split a single bulk oil droplet into daughter droplets with nearly equal volume facilitates the development of new functions for underwater microreactors.

'Reactive' nano-complex coated medical cotton: a facile avenue for tailored release of small molecules

Controlled and sustained release of drug-like small molecules in an aqueous medium still remains a challenging problem due to rapid infiltration of liquid water in most reported drug release systems. However, internal-superhydrophobicity with an antifouling property extending beyond the surface of a material recently has been recognized as a potential avenue for sustained and extended release of drug-like small molecules. Sluggish removal of metastable trapped air in a superhyrophobic material provides a basis to achieve extended release of encapsulated small molecules. In this article, naturally abundant medical-cotton-extensively used in wound management including control of bleeding, absorbance of secretions and protecting wounds from contamination-is strategically exploited in tailoring (from rapid to extended) the release of small molecules by appropriate modulation of liquid water wettability. Modulation included bio-mimicked adhesive and non-adhesive superhydrophobicity of the medical cotton without erosion of any polymeric material. In this process, amine 'reactive' nano-complexes (RNC) were prepared by just mixing branched poly(ethylenimine) (BPEI) with dipentaerythritol pentaacrylate (5Acl) in ethanol with appropriate compositions. Then they were covalently immobilized on fibrous medical-cotton through a facile and robust 1,4-conjugated addition reaction. Residual acrylate moieties in the immobilized RNC provide an opportunity to tailor water wettability through strategic and appropriate post-chemical modification of RNC-coated medical cotton with a primary amine containing various small molecules. This medical-cotton with tunable wettability was exploited further to control the release rate of small molecules from rapid (<24 h) to sustained (>100 days) times. A volatile solvent induced transient and reversible switching of anti-fouling properties which allowed further varying the amount of post-loading small molecules into the medical cotton up to 2.36 wt% without compromising the embedded anti-wetting property. Thus, our current approach has immense potential to develop appropriate materials for a sustained and controlled release of small molecules from a clinically relevant substrate (i.e., medical-cotton) and may be useful in various bio-medical applications including improving wound management, preventing bacterial infections, better pain management, etc.

Synthesis of 'reactive' and covalent polymeric multilayer coatings with durable superoleophobic and superoleophilic properties under water

Bioinspired underwater super-oil-wettability (superoleophilic/superoleophobic) properties are emerging as a potential avenue for developing smart materials for addressing issues related to healthcare, environment, energy, etc. However, the inherent poor durability of the materials that are mostly developed using polymeric hydrogel, metal oxide coatings and electrostatic multilayers often challenges the application of these wettability properties in practical scenarios. Here, 'amine-reactive' polymeric multilayers of nano-complex were developed to fabricate 'internal' underwater superoleophobic/superoleophilic coatings with impeccable physical/chemical durability. This allows the super-wetting properties to exist beyond the surface of the material and remain intact even after severe physical damage including erosion of the material and continuous exposure to an artificial-marine environment for more than 80 days. Moreover, this current design allowed for independent revalidation of some key hypotheses with direct experimental demonstrations, and provided a basis to develop highly durable super-oil-wettability properties under water. It is believed that this contemporary study will make a worthwhile contribution on developing multifunctional materials for widespread practical applications by exploiting these super-oil-wetting properties.

Preparation of Superhydrophilic and Underwater Superoleophobic Nanofiber-Based Meshes from Waste Glass for Multifunctional Oil/Water Separation

The deterioration of water resources due to oil pollution, arising from oil spills, industrial oily wastewater discharge, etc., urgently requires the development of novel functional materials for highly efficient water remediation. Recently, superhydrophilic and underwater superoleophobic materials have drawn significant attention due to their low oil adhesion and selective oil/water separation. However, it is still a challenge to prepare low-cost, environmentally friendly, and multifunctional materials with superhydrophilicity and underwater superoleophobicity, which can be stably used for oil/water separation under harsh working conditions. Here, the preparation of nanofiber-based meshes derived from waste glass through a green and sustainable route is demonstrated. The resulting meshes exhibit excellent performance in the selective separation of a wide range of oil/water mixtures. Importantly, these meshes can also maintain the superwetting property and high oil/water separation efficiency under various harsh conditions. Furthermore, the as-prepared mesh can remove water-soluble contaminants simultaneously during the oil/water separation process, leading to multifunctional water purification. The low-cost and environmentally friendly fabrication, harsh-environment resistance, and multifunctional characteristics make these nanofiber-based meshes promising toward oil/water separation under practical conditions.

Underwater Transparent Miniature "Mechanical Hand" Based on Femtosecond Laser-Induced Controllable Oil-Adhesive Patterned Glass for Oil Droplet Manipulation

Development of underwater superoleophobic surfaces has captured the imagination of researchers because of their applications; especially, oil manipulation based on such surfaces has attracted much attention. Here, we show a simple and effective way to fabricate an underwater transparent miniature "mechanical hand" based on controllable oil-adhesive patterned glass using a femtosecond laser. The underwater oil-adhesive force of the patterned glasses that compose the "mechanical hand" device can be controlled from ultralow to ultrahigh by adjusting the ratio of the untreated flat glass area to the laser-ablated rough area. These surfaces also showed favorable transparency in water. Various oils such as chloroform, hexadecane, n-dodecane, decane, liquid paraffin, and petroleum ether were tested, and their repellency against the as-prepared surfaces in water medium was confirmed. Moreover, the "mechanical hand" was used to implement oil transportation, fusion, and rapid capture, which can be applied in the construction of microfluidic devices, in situ detectors, and bioreactors.

Functionalization of Biodegradable PLA Nonwoven Fabric as Superoleophilic and Superhydrophobic Material for Efficient Oil Absorption and Oil/Water Separation

Although the construction of superwettability materials for oil/water separation has been developed rapidly, the postprocess of the used separation materials themselves is still a thorny problem due to their nondegradation in the natural environment. In this work, we reported the functionalization of polylactic acid (PLA) nonwoven fabric as superoleophilic and superhydrophobic material for efficient treatment of oily wastewater with eco-friendly post-treatment due to the well-known biodegradable nature of PLA matrix.

Superoleophobic surfaces

This review systematically summarizes the recent developments of superoleophobic surfaces, focusing on their design, fabrication, characteristics, functions, and important applications.

One-step fabricated wettable gradient surface for controlled directional underwater oil-droplet transport

Controlled self-propelling of the underwater oil droplet is achieved by a one-step anodic oxidation method.

Underwater superoleophobicity, anti-oil and ultra-broadband enhanced absorption of metallic surfaces produced by a femtosecond laser inspired by fish and chameleons

Reported here is the bio-inspired and robust function of underwater superoleophobic, anti-oil metallic surfaces with ultra-broadband enhanced optical absorption obtained through femtosecond laser micromachining. Three distinct surface structures are fabricated using a wide variety of processing parameters. Underwater superoleophobic and anti-oil surfaces containing coral-like microstructures with nanoparticles and mount-like microstructures are achieved. These properties of the as-prepared surfaces exhibit good chemical stability when exposed to various types of oils and when immersed in water with a wide range of pH values. Moreover, coral-like microstructures with nanoparticle surfaces show strongly enhanced optical absorption over a broadband wavelength range from 0.2-25 μm. The potential mechanism for the excellent performance of the coral-like microstructures with a nanoparticle surface is also discussed. This multifunctional surface has potential applications in military submarines, amphibious military aircraft and tanks, and underwater anti-oil optical counter-reconnaissance devices.

Construction of superhydrophilic and under-water superoleophobic carbon-based membranes for water purification

A novel hybrid Ag/PAA-CNTs membrane with oil/water separation performance and antibacterial functions is presented, which may open a new window to achieve multifunctional materials for water treatment.