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

Erasable and Programmable Unidirectional Liquid Transport on Multiple Asymmetric Slippery Microstructures

Unidirectional fluid delivery on an asymmetric microstructure has garnered significant attention due to its advantages, such as being pumpless, multifunctional, and easy to integrate. However, the precise design of continuous flow in unidirectional transport and on-surface recyclable manipulation remains challenging. Here, we present a 3D-printed array of multiple asymmetric microstructures decorated with a nanoparticle-based sprayable slippery coating that can achieve promising liquid control for both droplets and flow. Inspired by two biological modes, lizard skin and butterfly wings, we designed two types of asymmetric microstructures, i.e., triangular protrusions and tilted microcones. Both asymmetric microstructures exhibit unidirectional wettability, such as directional droplet sliding and liquid spreading. Benefiting from the improved water repellency of the slippery surface, asymmetric microstructures can unlock more functions compared to previous examples, such as erasable liquid channels, programmable flow control, and modular fluidics. By tuning the motion resistance of the asymmetric microstructure, a complex and diverse pathway for the unidirectional channel is achieved, such as ordered flow transport and flow-based logic circuits for LED lighting. This contribution unifies two types of asymmetric microstructures in one system, providing a deeper analysis of resistance tuning in unidirectional fluid transport. We envision that these slippery asymmetric microstructures will diversify liquid-manipulating interfaces for the development of advanced fluidic devices.

Directional Water Transport in Flexible Nanochannels

Directional liquid transport in nanochannels composed of two-dimensional (2D) materials shows great potential in applications such as liquid diodes and biochemical nanosensors. However, experimental research on water transport at the nanoscale is challenging, and most previous theoretical studies have focused on rigid nanochannels while overlooking the coupling effect of channel flexibility on liquid transport. Considering the deformation of the 2D material, molecular dynamics simulations and theoretical analysis are employed to assess the transport of water droplets in a flexible 2D material nanochannel. The applicability of the Laplace equation at the nanoscale is verified first. And the deformation of the nanochannel confined with water droplets is analyzed using membrane theory, considering a significant interfacial effect at the nanoscale. Regardless of the wettability, thickness, and droplet size of the flexible graphene nanochannel, the droplets consistently move spontaneously toward the center of the channel, except in the case of a critical wettability state, where the water contact angle (WCA) is close to 90°. The passive transport mechanism is further explained from the perspective of the driving force on the droplet and the evolution of the system potential energy, both deduced from the asymmetric deformation of the channel. Relevant results reveal that for both hydrophilic and hydrophobic channels, as the droplet moves toward the channel center, the driving force and system potential energy decrease, reaching zero and a minimum at the center, respectively. However, in the cases of a critical wettability state, the driving force and potential energy remain zero and constant, respectively.

Biomimetic high-efficiency fog collector based on fractal spiral structure

Fog collection provides a promising solution to the freshwater shortage. However, the efficiency of conventional fog collection apparatus is significantly reduced under the complex and variable natural conditions. Furthermore, fog collectors are usually plagued by intricate designs and inadequate durability, resulting in degradation of their structural and surface integrity over time. This deterioration consequently impacts the efficiency of mist collection. In the present study, for preparing a one-piece, simple and efficient fog collector that is insensitive to the direction of the fog flow, a variety of bionic structures were combined with a spiral spring structure designed by the fractal concept. The maximum fog collection efficiency of the finished product is about 0.29 g∙min-1∙cm-2, which can be up to 4.53 times that of the equivalent fog collection planes and 1.81 times that of the unmodified original structure. In addition, the preparation process is straightforward with one-step forming, while the collector maintains a high fog collecting level after various durability tests.

Janus Textile: Advancing Wearable Technology for Autonomous Sweat Management and Beyond

To alleviate the discomfort caused by excessive sweating, there is a growing emphasis on developing wearable textiles that can evacuate sweat autonomously. These advanced fabrics, unlike their absorbent and retention-prone predecessors, harness the Janus structure-distinguished by its asymmetric wettability-to facilitate one-way transport of liquid. This unique characteristic has significant potential in addressing issues related to excessive bodily moisture and propelling the realm of smart wearables. This review offers a comprehensive overview of the advancements in Janus-structured textiles within the wearable field, delving into the mechanisms behind their unidirectional liquid transport, which rely on chemical gradient and curvature gradient strategies, alongside the methodologies for achieving asymmetric wettability. It further spotlights the multifaceted applications of Janus-based textiles in wearables, including moisture and thermal management, wound care, and sweat analysis. In addition to examining existing hurdles, the review also explores avenues for future innovation, envisioning a new era of Janus textiles tailored for personalized comfort and health monitoring capabilities.

A Janus Fabric with Hexagonal Microcavity Channels for Efficient Urine Transport and Accurate Physiological Monitoring

The current research on wearable electrochemical sensors for urine monitoring is relatively rare, which is primarily limited by the lack of an active management mechanism to effectively manipulate the transportation of excessive liquids. In this work, a Janus fabric with hexagonal microcavity channels (HMJ-FT) was assembled based on a disposable facial towel, which was further introduced to develop the wearable electrochemical sensor (HMJ-Sensor) for the directional manipulation of urine transportation and simultaneous detection of dopamine (DA) and uric acid (UA). The designed hexagonal microcavity structure can synergistically promote horizontal migration and vertical transport of liquid, thus ensuring efficient and rapid manipulation of urine transportation and preventing its accumulation and reflux, which are essential for accurate, real-time monitoring. Therefore, the constructed HMJ-Sensor demonstrated a lower limit of detection (LOD) compared to most reported wearable sensors, which is 10.0770 nM for DA and 1.4100 nM for UA, respectively. Additionally, it also has the widest detection range known to date (DA: 0.0360-4000 μM; UA: 0.0050-6000 μM), which can better adapt to the large volume of urine transport and significant fluctuations on urine concentration in practical applications. After being subjected to a 120-day storage period along with multiple bending, rubbing, and washing treatments, the HMJ-Sensor maintained its excellent detection performance, indicating its high stability and reliability. This work not only provided a novel strategy for the manipulation of urine transport but also enhanced the detection capabilities of urine monitoring, which holds significant potential for boosting wearable applications and medical monitoring in physiological and clinical settings.

Advancing Efficiency in Solar-Driven Interfacial Evaporation: Strategies and Applications

Solar-driven interfacial evaporation (SDIE) has emerged as a promising technology for addressing global water scarcity by utilizing solar-thermal conversion and evaporation at the air/material/water interface. The exceptional performance of these systems has attracted significant interest; it is imperative to establish rigorous and scientific standards for evaluating effectiveness, optimizing system design, and ensuring efficient practical applications. In this Review, we propose consensus criteria for accurately assessing system performance and guiding future advancements. We then explore the fundamental mechanisms driving system synergy, emphasizing how material compositions, microscopic hierarchical material structures, and macroscopic three-dimensional spatial architecture designs enhance solar absorption and photothermal conversion; balance heat confinement with water pathway optimization; manage salt resistance; and regulate enthalpy during vaporization. These matched coordination strategies are crucial for maximizing the target SDIE efficiency. Additionally, we investigate the practical applications of SDIE technologies, focusing on cutting-edge progress and versatile water purification, combined with atmospheric water harvesting, salt collection, electric generation, and photothermal deicing. Finally, we highlight the challenges and exciting opportunities for advancing research, emphasizing future efforts to integrate fundamental principles, system-level collaboration, and application-driven approaches to boost sustainable and highly efficient water and energy technologies. By linking system performance evaluation with optimization strategies for influencing factors, we offer a comprehensive overview of the field and a future outlook that promotes highly efficient clean water production and synergistic applications.

Bioinspired Superwettable Surfaces and Materials for Liquid Motion Control

Directional fluid dynamics has garnered increasing attention because of its extensive applications in diverse fields including water harvesting, anti-icing, and microfluidic manipulation. Natural organisms have evolved a myriad of surfaces with specialized functions that manipulate liquids by virtue of their surface structure and chemical composition. These surfaces provide an extremely rich source of inspiration for controlled fluid transfer. The study of the fundamentals of what happens between droplets and functional surfaces and the close interactions is essential for the development of technologies and solutions in different fields. Exploring the inherent workings of droplet manipulation on natural biosurfaces can inspire the design and development of superwettable materials. This review deepens the understanding of directed fluid dynamics by summarizing interface fluid dynamics theory and mechanisms. It presents the fundamental principles of directed fluid dynamics on typical natural biological surfaces. Additionally, it elucidates the fluid dynamics behavior and applications of a diverse set of smart functional surfaces inspired by natural organisms. Simultaneously, it shares its view on superwetting interface liquid dynamics challenges and opportunities, pushing for next-generation biomimetic superwettable materials.

Enhancing Waterproof Food Packaging with Janus Structure: Lotus Leaf Biomimicry and Polyphenol Particle Technology for Vegetable Preservation

With the increasing demand for improved food preservation, conventional waterproof food packaging has proven inadequate because of its limited functionality. Although incorporating features such as antibacterial and antioxidant properties into packaging enhances protection, it can compromise the hydrophobicity of the involved material, thereby increasing the risk of contamination from external sources. To address this challenge, a robust and reliable barrier capable of simultaneously integrating multiple protective functions is required. This research synthesizes polyphenol particles via metal ion coordination and multiple hydrogen bondings to enhance antioxidant and antimicrobial properties. In addition, inspired by the asymmetric wettability of Janus-structured lotus leaves, this study develops biomimetic multifunctional Janus electrospun fibers via electrostatic spinning. These fibers exhibit exceptional properties, including superhydrophobic, antifouling, ultraviolet-blocking, pH sensitivity, antioxidation, antimicrobial, and freshness retention properties. Experiments and mesoscopic capillary flow simulations elucidate the waterproofing ability and underlying mechanisms of the Janus electrospun fibers, demonstrating their function as hydrophobic shields for preventing water penetration. Overall, this study provides a reference for high-performance waterproof food packaging to enhance vegetable preservation.

Boosting the oxygen reduction activity on metal surfaces by fine-tuning interfacial water with midinfrared stimulation

Heterogeneous catalysis at the metal surface generally involves the transport of molecules through the interfacial water layer to access the surface, which is a rate-determining step at the nanoscale. In this study, taking the oxygen reduction reaction on a metal electrode in aqueous solution as an example, using accurate molecular dynamic simulations, we propose a novel long-range regulation strategy in which midinfrared stimulation (MIRS) with a frequency of approximately 1,000 cm-1 is applied to nonthermally induce the structural transition of interfacial water from an ordered to disordered state, facilitating the access of oxygen molecules to metal surfaces at room temperature and increasing the oxygen reduction activity 50-fold. Impressively, the theoretical prediction is confirmed by the experimental observation of a significant discharge voltage increase in zinc-air batteries under MIRS. This MIRS approach can be seamlessly integrated into existing strategies, offering a new approach for accelerating heterogeneous reactions and gas sensing within the interfacial water 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.

Machine learning predicts atomistic structures of multielement solid surfaces for heterogeneous catalysts in variable environments

Solid surfaces usually reach thermodynamic equilibrium through particle exchange with their environment under reactive conditions. A prerequisite for understanding their functionalities is detailed knowledge of the surface composition and atomistic geometry under working conditions. Owing to the large number of possible Miller indices and terminations involved in multielement solids, extensive sampling of the compositional and conformational space needed for reliable surface energy estimation is beyond the scope of ab initio calculations. Here, we demonstrate, using the case of iron carbides in environments with varied carbon chemical potentials, that the stable surface composition and geometry of multielement solids under reactive conditions, which involve large compositional and conformational spaces, can be predicted at ab initio accuracy using an approach that combines the bond valence model, Gaussian process regression, and ab initio thermodynamics. Determining the atomistic structure of surfaces under working conditions paves the way toward identifying the true active sites of multielement catalysts in heterogeneous catalysis.

Understanding and Utilizing Droplet Impact on Superhydrophobic Surfaces: Phenomena, Mechanisms, Regulations, Applications, and Beyond

Droplet impact is a ubiquitous liquid behavior that closely tied to human life and production, making indispensable impacts on the big world. Nature-inspired superhydrophobic surfaces provide a powerful platform for regulating droplet impact dynamics. The collision between classic phenomena of droplet impact and the advanced manufacture of superhydrophobic surfaces is lighting up the future. Accurately understanding, predicting, and tailoring droplet dynamic behaviors on superhydrophobic surfaces are progressive steps to integrate the droplet impact into versatile applications and further improve the efficiency. In this review, the progress on phenomena, mechanisms, regulations, and applications of droplet impact on superhydrophobic surfaces, bridging the gap between droplet impact, superhydrophobic surfaces, and engineering applications are comprehensively summarized. It is highlighted that droplet contact and rebound are two focal points, and their fundamentals and dynamic regulations on elaborately designed superhydrophobic surfaces are discussed in detail. For the first time, diverse applications are classified into four categories according to the requirements for droplet contact and rebound. The remaining challenges are also pointed out and future directions to trigger subsequent research on droplet impact from both scientific and applied perspectives are outlined. The review is expected to provide a general framework for understanding and utilizing droplet impact.