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

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.

Bioinspired Capillary Transistors

Inspired by the unidirectional liquid spreading on Nepenthes peristome, Araucaria leaf, butterfly wings, etc., various microfluidic devices have been developed for water collection, irrigation, physical/chemical reaction, and oil-water separation. Despite extensive progress, most natural and artificial structures fail to enhance the Laplace pressure difference or capillary force, thus suffering from a low unidirectional capillary height (<30 mm). In this work, asymmetric re-entrant structures with long overhangs and connected forward/lateral microchannels are fabricated by 3D printing, resulting in a significantly increased unidirectional capillary height of 102.3 mm for water, which approximately corresponds to the theoretical limit. The overhangs can partially overlap the forward microchannels of the front structures without direct contact, thus enhancing the Laplace pressure difference and capillary force simultaneously. Based on asymmetric and symmetric re-entrant structures, capillary transistors are proposed and realized to programmably adjust the capillary direction, height, and width, which are envisioned to function as switches/valves and amplifiers/attenuators for highly efficient liquid patterning, desalination, and biochemical microreaction in 3D space.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

4D Printing of Butterfly Scale-Inspired Structures for Wide-Angle Directional Liquid Transport

Unidirectional liquid transport has been extensively explored for water/fog harvesting, electrochemical sensing, and desalination. However, current research mainly focuses on linear liquid transport (transport angle α = 0°), which exhibits hindered lateral liquid spreading and low unidirectional transport efficiency. Inspired by the wide-angle (0° < α < 180°) liquid transport on butterfly wings, this work successfully achieves linear (α = 0°), wide-angle, and even ultra-wide-angle (α = 180°) liquid transport by four-dimensional (4D) printing of butterfly scale-inspired re-entrant structures. These asymmetric re-entrant structures can achieve unidirectional liquid transport, and their layout can control the Laplace pressure in the forward (structure-tilting) and lateral directions to adjust the transport angle. Specifically, high transport efficiency and programmable forward/lateral transport paths are simultaneously achieved by the ultra-wide-angle transport, where liquid fills the lateral path before being transported forward. Moreover, the ultra-wide-angle transport is also validated in 3D space, which provides an innovative platform for advanced biochemical microreaction, large-area evaporation, and self-propelled oil-water separation.

Design, fabrication, and applications of bioinspired slippery surfaces

Bioinspired slippery surfaces (BSSs) have attracted considerable attention owing to their antifouling, drag reduction, and self-cleaning properties. Accordingly, various technical terms have been proposed for describing BSSs based on specific surface characteristics. However, the terminology can often be confusing, with similar-sounding terms having different meanings. Additionally, some terms fail to fully or accurately describe BSS characteristics, such as the surface wettability of lubricants (hydrophilic or hydrophobic), surface wettability anisotropy (anisotropic or isotropic), and substrate morphology (porous or smooth). Therefore, a timely and thorough review is required to clarify and distinguish the various terms used in BSS literature. This review initially categorizes BSSs into four types: slippery solid surfaces (SSSs), slippery liquid-infused surfaces (SLISs), slippery liquid-like surfaces (SLLSs), and slippery liquid-solid surfaces (SLSSs). Because SLISs have been the primary research focus in this field, we thoroughly review their design and fabrication principles, which can also be applied to the other three types of BSS. Furthermore, we discuss the existing BSS fabrication methods, smart BSS systems, antifouling applications, limitations of BSS, and future research directions. By providing comprehensive and accurate definitions of various BSS types, this review aims to assist researchers in conveying their results more clearly and gaining a better understanding of the literature.

Overflow Control for Sustainable Development by Superwetting Surface with Biomimetic Structure

Liquid flowing around a solid edge, i.e., overflow, is a commonly observed flow behavior. Recent research into surface wetting properties and microstructure-controlled overflow behavior has attracted much attention. Achieving controllable macroscale liquid dynamics by manipulating the micro-nanoscale liquid overflow has stimulated diverse scientific interest and fostered widespread use in practical applications. In this review, we outline the evolution of overflow and present a critical survey of the mechanism of surface wetting properties and microstructure-controlled liquid overflow in multilength scales ranging from centimeter to micro and even nanoscale. We summarize the latest progress in utilizing the mechanisms to manipulate liquid overflow and achieve macroscale liquid dynamics and in emerging applications to manipulate overflow for sustainable development in various fields, along with challenges and perspectives.

Light-Driven Liquid Conveyors: Manipulating Liquid Mobility and Transporting Solids on Demand

The intelligent transport of materials at interfaces is essential for a wide range of processes, including chemical microreactions, bioanalysis, and microfabrication. Both passive and active methods have been used to transport droplets, among which light-based techniques have attracted much attention because they are noncontact, safe, reversible, and controllable. However, conventional light-driven systems also involve challenges related to low transport ability and instability. Because of these shortcomings, technologies that can transport and manipulate droplets and microsolids on the same surface have yet to be realized. The present work demonstrates a light-driven system referred to as a liquid conveyor that enables the transport of both water droplets and microsolids. After the incorporation of an azobenzene-based molecular motor capable of undergoing photoisomerization into the surface liquid layer of this system, an isomerization gradient was induced by exposure to ultraviolet light emitting diodes that induced flow in this layer. Various parameters were optimized, including the concentration of the molecular motor compound, the light intensity, the viscosity of the liquid layer, and the droplet volume. This process eventually achieved the horizontal transport of droplets in any direction at varied rates. As a consequence of the limited heat buildup, the lack of droplet deformation, and extremely small contact angle hysteresis in this system, microsolids on droplets were also transported. This liquid conveyor is a promising platform for high-throughput omni-liquid/solid manipulation in the fields of biotechnology, chemistry, and mechanical engineering.

Robust bioinspired surfaces and their exploitation for petroleum hydrocarbon remediation

The current improvement in science and engineering, actively dealing with surfaces and interfaces, turns into a functioning control with a thriving advancement propensity. Superlyophobic/superlyophilic phenomena in surface sciences have pulled in broad considerations of researchers and specialists. Inspired by the natural and living organism, researchers have designed different biomimetic materials with exceptional surface wettability, such as the smart wetting of asymmetric spider silk surfaces. These smart materials with superlyophobic/superlyophilic wettability are generally utilized for water assortment, self-cleaning, fluid transportation and separation, and many researchers' domains. Among them, emulsion separation, including division of oil-water blend, mixtures of immiscible liquids and oil-water emulsions, is highlighted by an increasing number of researchers. Numerous materials with one- and two-dimensional morphology, smart surfaces, and super wettability have been effectively designed and utilized in various scientific research applications. We expect that these bioinspired materials with super wettability can have promising applications in practical for emulsion destabilization and liquid transportation.

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.

Wetting-Induced Water Promoted Flow on Tunable Liquid-Liquid Interface-Based Nanopore Membrane System

Membrane separation provides effective methods for solving the global water crisis. Contemporary membrane systems depend on interfacial interactions between liquid and solid membrane matrixes. However, it may lead to a limiting permeate flux due to the large flow resistance at hydrophobic liquid-solid interfaces. Herein, the liquid-liquid interface with improved interface energy is reversibly introduced in membrane systems to boost wetting and reduce transport resistance. A series of interfaces were systematically explored to reveal mechanisms of wetting and boosted flow performances, which are further supported by simulations. Findings of this study highlight that interfacial liquids with lower surface energies, lower viscosities, and higher solubilities can effectively improve water flow without sacrificing rejection performance, achieving by transforming a solid-liquid interface into liquid-liquid interface interaction. It provides a concept to design advanced membrane systems for water purification (e.g., desalination and oil-water separation) and energy conversion processes.

Replica of Bionic Nepenthes Peristome-like and Anti-Fouling Structures for Self-Driving Water and Raman-Enhancing Detection

The flexible, anti-fouling, and bionic surface-enhanced Raman scattering (SERS) biochip, which has a Nepenthes peristome-like structure, was fabricated by photolithography, replicated technology, and thermal evaporation. The pattern of the bionic Nepenthes peristome-like structure was fabricated by two layers of photolithography with SU-8 photoresist. The bionic structure was then replicated by polydimethylsiloxane (PDMS) and grafting the zwitterion polymers (2-methacryloyloxyethyl phosphorylcholine, MPC) by atmospheric plasma polymerization (PDMS-PMPC). The phospholipid monomer of MPC immobilization plays an important role; it can not only improve hydrophilicity, anti-fouling and anti-bacterial properties, and biocompatibility, but it also allows for self-driving and unidirectional water delivery. Ag nanofilms (5 nm) were deposited on a PDMS (PDMS-Ag) substrate by thermal evaporation for SERS detection. Characterizations of the bionic SERS chips were measured by a scanning electron microscope (SEM), optical microscope (OM), X-ray photoelectron spectrometer (XPS), Fourier-transform infrared spectroscopy (FTIR), and contact angle (CA) testing. The results show that the superior anti-fouling capability of proteins and bacteria (E. coli) was found on the PDMS-PMPC substrate. Furthermore, the one-way liquid transfer capability of the bionic SERS chip was successfully demonstrated, which provides for the ability to separate samples during the flow channel, and which was detected by Raman spectroscopy. The SERS intensity (adenine, 10⁻⁴ M) of PDMS-Ag with a bionic structure is ~4 times higher than PDMS-Ag without a bionic structure, due to the multi-reflection of the 3D bionic structure. The high-sensitivity bionic SERS substrate, with its self-driving water capability, has potential for biomolecule separation and detection.

Bioinspired functional SLIPSs and wettability gradient surfaces and their synergistic cooperation and opportunities for enhanced condensate and fluid transport

Bioinspired smart functional surfaces have received increasing attention in recent years owed to their tunable wettability and enhanced droplet transport suggesting them as excellent candidates for industrial and nanotechnology-related applications. More specifically, bioinspired slippery lubricant infused porous surfaces (SLIPSs) have been proposed for their low adhesion enabling continuous dropwise condensation (DWC) even of low-surface tension fluids. In addition, functional surfaces with chemical and/or structural wettability gradients have also been exploited empowering spontaneous droplet transport in a controlled manner. Current research has focused on the better understanding of the mechanisms and intimate interactions taking place between liquid droplets and functional surfaces or on the forces imposed by differences in surface wettability and/or by Laplace pressure owed to chemical or structural gradients. Nonetheless, less attention has been paid to the synergistic cooperation of efficiently driving droplet transport via chemical and/or structural patterns/gradients on a low surface energy/adhesion background imposed by SLIPSs, with the consequent promising potential for microfluidics and condensation heat transfer applications amongst others. This review provides a detailed and timely overview and summary on recent advances and developments on bioinspired SLIPSs and on wettability gradient surfaces with focus on their synergistic cooperation for condensation and fluid transport related applications. Firstly, the fundamental theory and mechanisms governing complex droplet transport on homogeneous, on wettability gradient surfaces and on inclined SLIPSs are introduced. Secondly, recent advances on the fabrication and characterization of SLIPSs and functional surfaces are presented. Then, the condensation performance on such functional surfaces comprising chemical or structural wettability gradients is reviewed and their applications on condensation heat transfer are summarized. Last a summary outlook highlighting the opportunities and challenges on the synergistic cooperation of SLIPSs and wettability gradient surfaces for heat transfer as well as future perspective in modern applications are presented.

Programmable droplet manipulation and wetting with soft magnetic carpets

A set of magnetically responsive, soft hairs, which form a soft magnetic carpet, can be infused with a liquid to achieve switchable wetting. Applying a pattern of magnetic field results in a reconfigurable wetting pattern on the soft magnetic carpet. Combining this switchable wetting with a travelling magnetic field wave can allow us to spatially manipulate droplets. The efficiency of the droplet manipulation depends on the size and the contact angle of the droplet, which allows a pathway to sort and separate different droplets. Temporal and spatial control over multiple droplets allows us to conduct droplet reactions, which has a potential to be used for automated analytical testing and screening.

The ability to regulate interfacial and wetting properties is highly demanded in anti-icing, anti-biofouling, and medical and energy applications. Recent work on liquid-infused systems achieved switching wetting properties, which allow us to turn between slip and pin states. However, patterning the wetting of surfaces in a dynamic fashion still remains a challenge. In this work, we use programmable wetting to activate and propel droplets over large distances. We achieve this with liquid-infused soft magnetic carpets (SMCs) that consist of pillars that are responsive to external magnetic stimuli. Liquid-infused SMCs, which are sticky for a water droplet, become slippery upon application of a magnetic field. Application of a patterned magnetic field results in a patterned wetting on the SMC. A traveling magnetic field wave translates the patterned wetting on the substrate, which allows droplet manipulation. The droplet speed increases with an increased contact angle and with the droplet size, which offers a potential method to sort and separate droplets with respect to their contact angle or size. Furthermore, programmable control of the droplet allows us to conduct reactions by combining droplets loaded with reagents. Such an ability of conducting small-scale reactions on SMCs has the potential to be used for automated analytical testing, diagnostics, and screening, with a potential to reduce the chemical waste.

3D Photovoltaic Router of Water Microdroplets Aiming at Free-Space Microfluidic Transportation

So far, microfluidic navigation based on space-charge modulation is limited in a two-dimensional (2D) substrate plane. In this paper, a three-dimensional (3D) photovoltaic water-microdroplet router based on a superhydrophobic LiNbO₃:Fe crystal is reported. This router employs the repulsive electrostatic force induced by the positive photovoltaic charges generated under focused laser illumination and permits traveling microdroplets to be routed in both in-plane and out-of-plane ways. By analyzing the dynamic process of microdroplet routing, it is found that the microdroplets can gain positive charges through traveling on a superhydrophobic surface and that the positive photovoltaic charges exert an electrophoretic (EP) force on the microdroplets being charged and make them either routed inside the 2D substrate plane or jump out of the 2D plane through electrostatic ballistic ejection. The laser-illumination and microdroplet-size dependence of the deflecting parameters of the in-plane microdroplet routing as well as the jumping trajectory of the out-of-plane routing are investigated. An electrostatic kinetic model is established for both routing ways, and the simulation based on this model predicts well the experimental dependence. A few examples of cascaded free-space microfluidic transportation using the 3D photovoltaic router are demonstrated, showing the potential of this technique in future biological applications.

Surface-Tension-Confined Channel with Biomimetic Microstructures for Unidirectional Liquid Spreading

Unidirectional liquid spreading without energy input is of significant interest for the broad applications in diverse fields such as water harvesting, drop transfer, oil-water separation and microfluidic devices. However, the controllability of liquid motion and the simplification of manufacturing process remain challenges. Inspired by the peristome of Nepenthes alata, a surface-tension-confined (STC) channel with biomimetic microcavities was fabricated facilely through UV exposure photolithography and partial plasma treatment. Perfect asymmetric liquid spreading was achieved by combination of microcavities and hydrophobic boundary, and the stability of pinning effect was demonstrated. The influences of structural features of microcavities on both liquid spreading and liquid pinning were investigated and the underlying mechanism was revealed. We also demonstrated the spontaneous unidirectional transport of liquid in 3D space and on tilting slope. In addition, through changing pits arrangement and wettability pattern, complex liquid motion paths and microreactors were realized. This work will open a new way for liquid manipulation and lab-on-chip applications.

Three-Dimensional Multifunctional Magnetically Responsive Liquid Manipulator Fabricated by Femtosecond Laser Writing and Soft Transfer

Nature-inspired magnetically responsive intelligent topography surfaces have attracted considerable attention owing to their controllable droplet manipulation abilities. However, it is still challenging for magnetically responsive surfaces to realize three-dimensional (3D) droplet/multidroplet transport in both horizontal and vertical directions. Additionally, the droplet horizontal propulsion speed needs to be improved. In this work, a 3D droplet/multidroplet transport strategy based on magnetically responsive microplates array (MMA) actuated by a spatially varying and periodic magnetic field is proposed. The modified superhydrophobic surface can transport droplets rapidly both in horizontal and vertical directions, and it can even realize against-gravity upslope propulsion. The rapid horizontal droplet propulsion (∼58.6 mm/s) is ascribed to the abrupt inversion of the modified surface induced by the specific magnetic field. Furthermore, the nonmagnetically responsive microplates (NMMs)/MMA composite surface is constructed to realize 3D multidroplet manipulation. The implementations of MMA in manipulation of continuous fluids and liquid metal are further demonstrated, providing a valuable platform for microfluidic applications.

Bioinspired Smart Liquid Directional Transport Control

Developments in bioinspired superwetting materials have triggered technological revolutions in many disciplines. One representative area is liquid directional transport dominated by interface properties, which has experienced rapid progress recently. To improve the controllability, scientists try to use the external field, such as light, electricity, thermal, and so on, to assist or achieve controllable smart, responsive liquid directional transport. However, there are still some intractable problems and challenges behind prosperity. Here, we summarize the relevant basic theory of surface wettability and the processes of the development of bioinspired superwetting materials. We discuss the different essential mechanisms of liquid directional transport. Furthermore, smart external field-controlled fluid directional transport is the primary focus of this feature article. We briefly put forward our views on some outstanding problems, existing challenges, and trends in this field.

Droplet Sorting and Manipulation on Patterned Two-Phase Slippery Lubricant-Infused Surface

Slippery lubricant-infused surfaces are composite materials consisting of a solid matrix permanently infused by a lubricant. Such surfaces have proved to be highly repellent to various liquids immiscible with the lubricant. Depending on the underlying surface chemistry, different lubricants can be used, including perfluorinated or alkylated oils. Here, we construct patterned slippery surfaces that consist of virtual channels permanently impregnated with an organic oil and surrounded by areas permanently impregnated with a perfluorinated oil. We demonstrate that water droplets preferentially occupy the organic-oil-lubricated virtual channels. Based on a simple model, we evaluate the forces acting on droplets crossing over to the regions impregnated with perfluorinated oil and show that the cloaking of the droplets plays an important role. We study the actuation of droplets in virtual oil-in-oil channels based on gravity and magnetic fields. Finally, we construct a variety of organic-oil-lubricated channel architectures permitting droplet sorting according to size. We believe that this novel approach for the formation of virtual all-liquid surface-tension-confined channels based on lubricant-infused surfaces, channel networks, or patterns will advance the field of droplet-based microfluidics. The approach presented can be potentially useful for applications in biotechnology, diagnostics, or analytical chemistry.