Bioinspired superwetting oil-water separation strategy: toward the era of openness.

Bioinspired superwetting oil-water separation strategies have received significant attention for their potential in addressing global water scarcity and aquatic pollution challenges. Over the past two decades, the field has rapidly developed, reaching a pivotal phase of innovation in the oil-water separation process. However, many groundbreaking studies have not received extensive scientific recognition. In this review, we systematically examine the application of bioinspired superwetting materials for complex multiscale oil-water separation. We discuss the development of 2D membrane filtration and 3D sponge adsorption materials in confined spaces, summarizing the core separation mechanisms, key research findings, and the evolutionary logic of these materials. Additionally, we highlight emerging open-space separation strategies, emphasizing several novel dynamic separation devices of significant importance. We evaluate and compare the design concepts, separation principles, materials used, comprehensive performance, and existing challenges of these diverse strategies. Finally, we summarize these advantages, critical bottlenecks, and prospects of this field and propose potential solutions for real oil-water separation processes from a general perspective.

Capillarity Constructed Open Siphon for Sustainable Drainage.

Siphon is an effective method to transfer liquid from a higher to a lower level, which has many applications in hygienic design, clinical apparatus, and hydraulic engineering. Traditional operation requires energy to overcome gravity and establish flow in a closed system. Achieving sustainable high flux siphon drainage without energy input remains a challenge due to viscous dissipation. Here, an unexpected open siphon behavior on the South American pitcher plant Heliamphora minor consisting of trichomes covered pitcher and a wedge-shaped sheath is examined. Exploiting the concept of Digital Twin, a new biomimetic research method by transforming the biological sample to a virtual 3D model is proposed and unveiled that maintained connection of wicking on sub-millimeter long trichomes due to asymmetric pressure distribution and ascending in wedge sheath under unbalanced pressure forms continuous surface flow. Exploring this mechanism, a biomimetic siphon device achieving continuous high flux exposed to ambient air is constructed. Besides, particles floating on the meniscus in the outside wedge move under a curvature gradient as water ascends, which implies a biological nutrient capture method and new dust collection manner in the drainage system. Applying the underlying principle enhances the siphon efficiency of floor drains and has the potential for other liquid transfer device design improvements.

Pontederia crassipes inspired bottom overflow for fast and stable drainage.

Fast and stable water drainage is essential for living organisms, drainage plane construction, and protection of infrastructure from damage during rainfall. Unlike traditional anti-overflow drainage methods that rely on hydrophobic or sharped edges, this study demonstrates a bottom overflow-induced drainage model inspired by the water path employed by Pontederia crassipes leaves, leading to fast and stable drainage. A superhydrophilic bottom surface guides water to overflow and pin at the bottom of a thin sheet, resulting in dripping at a higher frequency and reduced water retention. This bottom drainage idea assists large-scale thin sheets to function as efficient and stable drainage surfaces in simulated rain environments. The flexible thin sheet can also be feasibly attached to dusty substrates to effectively remove dusty rainwater with slight dust residue. The bioinspired approach presented herein suggests a promising potential for efficient water drainage on outdoor functional photovoltaic surfaces, such as solar panels and radomes, thus ensuring effective energy conversion and stable signal transmission.

Liquid harvesting and transport on multiscaled curvatures.

Various creatures, such as spider silk and cacti, have harnessed their surface structures to collect fog for survival. These surfaces typically stay dry and have a large contact hysteresis enabling them to move a condensed water droplet, resulting in an intermittent transport state and a relatively reduced speed. In contrast to these creatures, here we demonstrate that Nepenthes alata offers a remarkably integrated system on its peristome surface to harvest water continuously in a humid environment. Multicurvature structures are equipped on the peristome to collect and transport water continuously in three steps: nucleation of droplets on the ratchet teeth, self-pumping of water collection that steadily increases by the concavity, and transport of the acquired water to overflow the whole arch channel of the peristome. The water-wetted peristome surface can further enhance the water transport speed by ∼300 times. The biomimetic design expands the application fields in water and organic fogs gathering to the evaporation tower, laboratory, kitchen, and chemical industry.

Finger directed surface charges for local droplet motion.

Water droplets are expected to be employed as animated soft matter to mimic the behaviours of both nonliving objects and small living organisms. Local water droplet motion has attracted considerable interest and has expanded into various application areas because of its close relationship with processes associated with life. However, few approaches have been capable of independently manipulating local droplet motion without loss on a substrate due to the difficulty in shaping and focusing the motion route. Here, we demonstrate a non-contact electrostatic-powered local water motion strategy. The gradient of electrostatic charges in space guides the local drop motion without liquid loss in a controlled motion path. The local droplet motion on surfaces with varied wettabilities is discussed and compared. A unipolar electrostatic field is theoretically simulated. This work can introduce a finger-directed surface charge pattern and local droplet motion as a new variable in many droplet robot schemes and inspire next-generation liquid devices.

Peristome-Mimetic Curved Surface for Spontaneous and Directional Separation of Micro Water-in-Oil Drops.

Separation of micro-scaled water-in-oil droplets is important in environmental protection, bioassays, and saving functional inks. So far, bulk oil-water separation has been achieved by membrane separation and sponge absorption, but micro-drop separation still remains a challenge. Herein we report that instead of the "plug-and-go" separation model, tiny water-in-oil droplets can be separated into pure water and oil droplets through "go-in-opposite ways" on curved peristome-mimetic surfaces, in milliseconds, without energy input. More importantly, this overflow controlled method can be applied to handle oil-in-oil droplets with surface tension differences as low as 14.7 mN m-1 and viscous liquids with viscosities as high as hundreds centipoises, which markedly increases the range of applicable liquids for micro-scaled separation. Furthermore, the curved peristome-mimetic surface guides the separated drops in different directions with high efficiency.

Efficient agricultural drip irrigation inspired by fig leaf morphology.

Irrigation is limited by water scarcity. Here, we show how a drip irrigation system inspired by the leaf of the fig tree Ficus religiosa (also known as the bodhi tree) can improve irrigation efficiency. The reverse curvature of the leaf regulates the convergence process of multiple water streams, while its long-tail apex allows for fast water drainage with the droplet separation centroid beyond the leaf apex. We explain why drip frequency increases after the break-up of contact line pinning at the apex tip by using scaling laws for drip volume and analyzing drainage dynamics. We build a drip irrigation emitter inspired by the bodhi leaf apex and compare the germination efficiency of wheat, cotton, and maize under different irrigation modes. These results show that the proposed bodhi-leaf-apex-mimetic (BLAM) drip irrigation can improve water saving while ensuring germination and seedling growth.

Dual-bionic superwetting gears with liquid directional steering for oil-water separation.

Developing an effective and sustainable method for separating and purifying oily wastewater is a significant challenge. Conventional separation membrane and sponge systems are limited in their long-term usage due to weak antifouling abilities and poor processing capacity for systems with multiple oils. In this study, we present a dual-bionic superwetting gears overflow system with liquid steering abilities, which enables the separation of oil-in-water emulsions into pure phases. This is achieved through the synergistic effect of surface superwettability and complementary topological structures. By applying the surface energy matching principle, water and oil in the mixture rapidly and continuously spread on preferential gear surfaces, forming distinct liquid films that repel each other. The topological structures of the gears facilitate the overflow and rapid transfer of the liquid films, resulting in a high separation flux with the assistance of rotational motion. Importantly, this separation model mitigates the decrease in separation flux caused by fouling and maintains a consistently high separation efficiency for multiple oils with varying densities and surface tensions.

Sustained Agricultural Spraying: From Leaf Wettability to Dynamic Droplet Impact Behavior.

Crop production and quality safety system have the potential to nurture human health and improve environmental sustainability. Providing a growing global population with sufficient and healthy food is an immediate challenge. However, this system largely depends on the spraying of agrochemicals. Crop leaves are covered with different microstructures, exhibiting distinct hydrophilic, hydrophobic, or even superhydrophobic wetting characteristics, thus leading to various deposition difficulties of sprayed droplets. Here, the relationship between wettability and surface microstructure in different crop leaves from biological and interfacial structural perspectives is systematically demonstrated. A relational model is proposed in which complex microstructures lead to stronger leaf hydrophobicity. And adding surfactant with a faster dynamically migrating velocity and reducing droplet size can improve agrochemical precise deposition. These contribute toward highly accurate and efficient targeted applications with fewer agrochemicals use and promote sustainable models of eco-friendly agriculture systems.

Introduction to the biomimetic design of interfacial materials for water overflow control.

The overflow behavior of liquid at a curved surface or a surface border is a common phenomenon in many circumstances of daily life and industry. Accurate control of overflow is beneficial for improving the convenience of our daily life and efficiency of production, thus has great importance not only for enhancing existing applications but for creating new products. The effect of surface wettability on overflow has not been paid enough attention in the past, however, as the development of construction techniques, especially 3D printing, of superwettable surfaces and the understanding of the dynamic interfacial wetting properties, the potential of surface wettability and structure on the overflow control has gained great recognition in the recent decade. On this basis, the feature article will outline the understanding evolution of the overflow phenomenon, and summarise the current research on the control of overflow behavior from aspects including the bioinspired idea, the fabrication of superwettable surfaces, the development of control techniques, the exploration of control mechanisms, etc., and provide an outlook for the accurate control of overflow by surfactant and additives, along with challenges and perspectives.

3D Printing a Biomimetic Bridge-Arch Solar Evaporator for Eliminating Salt Accumulation with Desalination and Agricultural Applications.

Solar-driven water evaporation has been considered a sustainable method to obtain clean water through desalination. However, its further application is limited by the complicated preparation strategy, poor salt rejection, and durability. Herein, inspired by superfast water transportation of the Nepenthes alata peristome surface and continuous bridge-arch design in architecture, a biomimetic 3D bridge-arch solar evaporator is proposed to induce Marangoni flow for long-term salt rejection. The formed double-layer 3D liquid film on the evaporator is composed of a confined water film for water supplementation and a free-flowing water film with ultrafast directional Marangoni convection for salt rejection, which functions cooperatively to endow the 3D evaporator with all-in-one function including superior solar-driven water evaporation (1.64 kg m-2 h-1 , 91% efficiency for pure water), efficient solar desalination, and long-term salt-rejecting property (continuous 200 h in 10 wt% saline water) without any post-cleaning treatment. The design principle of the 3D structures is provided for extending the application of Marangoni-driven salt rejection and the investigation of structure-design-induced liquid film control in the solar desalination field. Furthermore, excellent mechanical and chemical stability is proved, where a self-sustainable and solar-powered desalination-cultivation platform is developed, indicating promising application for agricultural cultivation.

Highly efficient three-dimensional solar evaporator for high salinity desalination by localized crystallization.

Solar-driven water evaporation represents an environmentally benign method of water purification/desalination. However, the efficiency is limited by increased salt concentration and accumulation. Here, we propose an energy reutilizing strategy based on a bio-mimetic 3D structure. The spontaneously formed water film, with thickness inhomogeneity and temperature gradient, fully utilizes the input energy through Marangoni effect and results in localized salt crystallization. Solar-driven water evaporation rate of 2.63 kg m-2 h-1, with energy efficiency of >96% under one sun illumination and under high salinity (25 wt% NaCl), and water collecting rate of 1.72 kg m-2 h-1 are achieved in purifying natural seawater in a closed system. The crystalized salt freely stands on the 3D evaporator and can be easily removed. Additionally, energy efficiency and water evaporation are not influenced by salt accumulation thanks to an expanded water film inside the salt, indicating the potential for sustainable and practical applications.

Uniform Spread of High-Speed Drops on Superhydrophobic Surface by Live-Oligomeric Surfactant Jamming.

Inkjet printing of water-based inks on superhydrophobic surfaces is important in high-resolution bioarray detection, chemical analysis, and high-performance electronic circuits and devices. Obtaining uniform spreading of a drop on a superhydrophobic surface is still a challenge. Uniform round drop spreading and high-resolution inkjet printing patterns are demonstrated on superhydrophobic surfaces without splash or rebound after high-speed impacting by introducing live-oligomeric surfactant adhesion. During impact, the live-oligomeric surfactant molecules aggregate into dynamic, wormlike micelle networks, which jam at the solid-liquid interface by entangling with the surface micro/nanostructures to pin the contact line and jam at the spreading periphery to keep the uniform spreading lamellar shape. This efficient uniform spreading of high-speed impact drops opens a promising avenue to control drop impact dynamics and achieve high-resolution printing.

Time-Dependent Liquid Transport on a Biomimetic Topological Surface.

Liquid drops impacting on a solid surface is a familiar phenomenon. On rainy days, it is quite important for leaves to drain off impacting raindrops. Water can bounce off or flow down a water-repellent leaf easily, but with difficulty on a hydrophilic leaf. Here, we show an interesting phenomenon in which impacting drops on the hydrophilic pitcher rim of Nepenthes alata can spread outward to prohibit water filling the pitcher tank. We mimic the peristome surface through a designed 3D printing and replicating way and report a time-dependently switchable liquid transport based on biomimetic topological structures, where surface curvature can work synergistically with the surface microtextures to manipulate the switchable spreading performance. Motived by this strange behavior, we construct a large-scaled peristome-mimetic surface in a 3D profile, demonstrating the ability to reduce the need to mop or to squeegee drops that form during the drop impacting process on pipes or other curved surfaces in food processing, moisture transfer, heat management, etc.

Janus Gradient Meshes for Continuous Separation and Collection of Flowing Oils under Water.

Gradient meshes with Janus wettabilities are fabricated to stably separate and collect spilled oils from a range of flowing oily wastewater. Here, we demonstrate an overflow with separation methodology, which combines selective oil overflow and membrane separation, to separate low content oils from dynamic flowing oil-water mixtures by a curved gradient mesh that covered on a solid edge. The microscaled air-oil-water-solid four-phase wetting state during the oil-water separation process is visualized and demonstrated. The fundamental understanding of this overflow with separation system and the superior gradient mesh materials would enable us to construct a wide variety of separation devices out of traditional designs and advance related applications, such as wastewater treatment and fuel purification.

Ballistic Jumping Drops on Superhydrophobic Surfaces via Electrostatic Manipulation.

The ballistic ejection of liquid drops by electrostatic manipulating has both fundamental and practical implications, from raindrops in thunderclouds to self-cleaning, anti-icing, condensation, and heat transfer enhancements. In this paper, the ballistic jumping behavior of liquid drops from a superhydrophobic surface is investigated. Powered by the repulsion of the same kind of charges, water drops can jump from the surface. The electrostatic acting time for the jumping of a microliter supercooled drop only takes several milliseconds, even shorter than the time for icing. In addition, one can control the ballistic jumping direction precisely by the relative position above the electrostatic field. The approach offers a facile method that can be used to manipulate the ballistic drop jumping via an electrostatic field, opening the possibility of energy efficient drop detaching techniques in various applications.

Liquids Unidirectional Transport on Dual-Scale Arrays.

Liquids unidirectional transport has cutting-edge applications ranging from fog collection, oil-water separation, to microfluidic devices. Despite extensive progresses, existing man-made surfaces with asymmetric wettability or micro/nanoscales structures are still limited by complex fabrication techniques or obscure essential transport mechanisms to achieve unidirectional transport with both high speeds and large volumes. Here, we demonstrate the three-dimensional printed micro/macro dual-scale arrays for rapid, spontaneous, and continuous unidirectional transport. We reveal the essential directional transport mechanism via a Laplace pressure driven theory. The relationship between liquid unidirectional transport and surface morphology parameter is systematically explored. Threshold values to achieve unidirectional transport are determined. Significantly, dual-scale arrays even facilitate liquid's uphill running, microfluidics patterning, and liquid shunting in target directions without external energy input. Free combination of dual-scale island arrays modules, just like LEGO bricks, achieves fast liquid transport on demand. This dual-scale island array can be used to build smart laboratory-on-a-chip devices, printable microfluidic integration systems, and advanced biochemistry microreactors.

Drop Cargo Transfer via Unidirectional Lubricant Spreading on Peristome-Mimetic Surface.

To promote drop mobility, lubricating the gap between liquid drop and solid surface is a facile method which has been widely exploited by nature. Examples include lotus and rice leaves using entrapped air to "lubricate" water and Nepenthes pitcher plant using a slippery water layer to trap insects. Inspired by these, here, we report a strategy for transporting drop cargoes via the unidirectional spreading of immiscible lubricants on the peristome-mimetic surface. Oleophilic/hydrophobic peristome-mimetic surfaces were fabricated through replicating three-dimensional printed samples. The peristome-mimetic surface, via unidirectional immiscible hexadecane spreading, can transport a wide diversity of drop cargoes over a long distance with no loss with controllable drop volumes and velocities, hence mixing multiphase liquids and even reacting liquids. We anticipate this unidirectional drop cargo transport technique will find use in microfluidics, microreactors, water harvesting systems, etc.