Biological and Engineered Topological Droplet Rectifiers

Bioinspired colloidal crystal hydrogel pressure sensors with Janus wettability for uterus cervical canal tension perception

The pursuit of flexible, sensitive, and cost-effective pressure sensors plays a pivotal role in medical diagnostics, particularly in the domain of cervical health monitoring. However, significant challenges remain in the economical production of flexible piezoresistive materials and the integration of microstructures aimed at enhancing sensor sensitivity. This urge highlights the use of innovative, stable hydrogel films that demonstrate robust adherence to soft biological tissues, thereby enabling prolonged bio-signal monitoring. In this study, we introduce an innovative integration of a flexible pressure electrical signal sensor with structural color hydrogel scaffolds. This integration leverages the tunability of the inverse opal structure to fine-tune the scaffold's adherence to the endocervical wall under varying environmental conditions and to amplify the sensitivity of pressure measurements. Our findings indicate that this novel approach holds promise for substantial enhancements in the manufacturing and functional capabilities of cervical pressure sensors, potentially revolutionizing personalized medical treatments and improving patient monitoring.

Treefrog-Inspired Flexible Electrode with High Permeability, Stable Adhesion, and Robust Durability

Long-term continuous monitoring (LTCM) of physiological electrical signals is an effective means for detecting several cardiovascular diseases. However, the integrated challenges of stable adhesion, low impedance, and robust durability under different skin conditions significantly hinder the application of flexible electrodes in LTCM. This paper proposes a structured electrode inspired by the treefrog web, comprising dispersed pillars at the bottom and asymmetric cone holes at the top. Attachment structures with a dispersed pillar improve the contact stability (adhesion increases 2.79/13.16 times in dry/wet conditions compared to an electrode without structure). Improved permeable duct structure provides high permeability (12 times compared to cotton). Due to high adhesion and permeability, the electrode's durability is 40 times larger than commercial Ag/AgCl electrodes. The treefrog web-like electrode has great advantages in permeability, adhesion, and durability, resulting in prospects for application in physiological electrical signal detection and a new design idea for LTCM wearable dry electrodes.

Reconfigurability-Encoded Hierarchical Rectifiers for Versatile 3D Liquid Manipulation

Manipulating small-volume liquids is crucial in natural processes and industrial applications. However, most liquid manipulation technologies involve complex energy inputs or non-adjustable wetting gradient surfaces. Here, a simple and adjustable 3D liquid manipulation paradigm is reported to control liquid behaviors by coupling liquid-air-solid interfacial energy with programmable magnetic fields. This paradigm centers around a hierarchical rectifier with magnetized microratchets, using Laplace pressure asymmetry to enable multimodal directional steering of various surface tension liquids (23-72 mN m-1). The scale-dependent effect in microratchet design shows its superiority in handling small-volume liquids across three orders of magnitude (100-103 µL). Under programmed magnetic fields, the rectifier can reconfigure its morphology to harness interfacial energy to exhibit richer liquid behaviors without dynamic real-time control. Reconfigured rectifiers show improved rectification performance in the inertia-dominant fluid regime, i.e., a remarkable 2000-fold increase in the critical Weber number for pure ethanol. Moreover, the rectifier's switchable reconfigurations offer flexible control over liquid transport directions and spatiotemporally controllable 3D liquid manipulation reminiscent of inchworm motions. This scalable liquid manipulation paradigm promotes versatile engineering and biochemistry applications, e.g., portable liquid purity testing (screening resolution <1 mN m-1), logical open-channel microfluidics, and automated chemical reaction platforms.

Smart materials for light control of droplets

Droplet manipulation plays a critical role in both fundamental research and practical applications, especially when combined with smart materials and external fields to achieve multifunctional droplet manipulation. Light control of droplets has emerged as a significant and widely used strategy, driven primarily by photochemistry, photomechanics, light-induced Marangoni effects, and light-induced electric effects. This approach allowing for droplet manipulation with high spatial and temporal resolution, all while maintaining a remote and non-contact mode of operation. This review aims to provide a comprehensive overview of the mechanisms underlying light control of droplets, the design of smart materials for this purpose, and the diverse range of applications enabled by this technique. These applications include merging, splitting, releasing, forwarding, backward movement, and rotation of droplets, as well as chemical reactions, droplet robots, and microfluidics. By presenting this information, we aim to establish a unified framework that guides the sustainable development of light control of droplets. Additionally, this review addresses the challenges associated with light control of droplets and suggests potential directions for future development.

Bionic Surfaces for Fog Collection: A Comprehensive Review of Natural Organisms and Bioinspired Strategies

Water scarcity has become a critical global threat, particularly in arid and underdeveloped regions. However, certain insects and plants have evolved the capability to obtain water from fog under these arid conditions. Bionic fog collection, characterized by passive harvesting, minimal energy requirements, and low maintenance costs, has proven to be an efficient method for water harvesting, offering a sustainable water source. This review introduces two superwettable surfaces, namely, superhydrophilic and superhydrophobic surfaces, detailing their preparation methods and applications in fog collection. The fog collection mechanisms of three typical natural organisms, Namib Desert beetles, spider silk, and cactus, along with their bionic surfaces for fog collection devices, are discussed. Additionally, other biological surfaces exhibiting fog transport properties are presented. The main challenges regarding the fabrication and application of bionic fog collection are summarized. Furthermore, we firmly believe that environmentally friendly, low-cost, and stable fog collection materials or devices hold promising prospects for future applications.

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.

Temperature-responsive peristome-structured smart surface for the unidirectional controllable motion of large droplets

The manipulation of fast, unidirectional motion for large droplets shows important applications in the fields of fog collection and biochemical reactions. However, driving large droplets (>5 μL) to move directionally and quickly remains challenging due to the nonnegligible volume force. Herein, we fabricated a scalable, bionic peristome substrate with a microcavity width of 180 μm using a 3D printing method, which could unidirectionally drive a large water droplet (~8 μL) at a speed reaching 12.5 mm/s by temperature-responsive wettability. The substrate surface was grafted with PNIPAAm, which could reversibly change its wettability in response to temperature, thereby enabling a temperature-responsive smart surface that could regulate droplet movement in real-time by changing the temperature. A series of temperature-responsive smart patterns were designed to induce water transport along specific paths to further realize controllable droplet motion with the antibacterial treatment of predesignated areas. The ability to achieve temperature-responsive unidirectional motion and dynamic control of droplet movement could allow programmable fluidic biosensors and precision medical devices. A temperature-responsive smart surface was produced to control the unidirectional motion of large droplets between spreading and pinning movement by changing the surface wettability.

Unveiling the Interactions between Water Molecule Clusters and Conical Structures via Molecular Dynamics Simulations

Water scarcity presents a pressing global challenge, necessitating innovative solutions, such as the collection of water from the air using conical structures. However, current research primarily focuses on mist collection rather than on nanoscale clusters of water molecules. Under standard atmospheric conditions, water vapor predominantly exists as imperceptible clusters. Therefore, it is crucial to investigate the interactions between these water molecule clusters and conical structures, particularly regarding whether the conical shape induces Laplace pressure difference on the adhering cluster formations. To gain deeper insights and determine optimal droplet collection structures, we conducted molecular dynamics simulations to investigate interactions between water molecule clusters and conical structures. Our investigations focused on studying the interactions between conical structures and water molecule clusters with varying densities, as well as the impact of surface energies on the collection of water by these conical structures. Notably, our simulations unveiled the significant roles played by van der Waals forces and Laplace pressure in the process of collecting water molecule clusters. Furthermore, our simulations revealed that Janus conical structures, featuring two distinct surface energy regions, played a crucial role in promoting the aggregation of water molecules, resulting in the formation of larger droplets. This aggregation was driven by surface tension gradients, which arise from the contrasting wetting properties in different regions of the Janus structure. As a consequence, under the influence of gravitational forces, these larger droplets could eventually detach from the structure. Through the combined effects of surface tension gradients and gravitational forces, Janus conical structures offer a promising avenue for enhancing the collection efficiency of water from the air. Our research sheds light on the fundamental mechanisms governing water molecule cluster-based water collection and provides valuable insights for the design of more efficient and effective water collection systems.

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.

Bioinspired Green Fabricating Design of Multidimensional Surfaces for Atmospheric Water Harvesting

Across the globe, the quest for clean water is escalating for both households as well as agricultural exigencies. With the industrial revolution and swift population growth, the contamination of natural water bodies has impacted the lives of more than two billion people around the world. A spectrum of water-saving solutions has been examined. Nonetheless, most of them are either energy-inefficient or limited to only a particular region. Thus, the pursuit of clean and potable drinking water is an assignment that invites collective discourse from scientists, policymakers, and innovators. In this connection, the presence of moisture in the atmosphere is considered one of the major sources of potential freshwater. Thus, fishing in atmospheric water is a mammoth opportunity. Atmospheric water harvesting (AWH) by some plants and animals in nature (particularly in deserts or arid regions) at low humidity serves as an inspiration for crafting state-of-the-art water harvesting structures and surfaces to buffer the menace of acute water scarcity. Though a lot of research articles and reviews have been reported on bioinspired structures with applications in water and energy harvesting, the area is still open for significant improvisation. This work will address the multidimensional-based AWH ability of natural surfaces or fabricated structures without the involvement of toxic chemicals. Moreover, the review will discuss the availability of clean technologies for emulating fascinating natural surfaces on an industrial scale. In the end, the current challenges and the future scope of bioinspired water harvesters will be discussed for pushing greener technologies to confront climate change.

Bioinspired On-Demand Directional Droplet Manipulation Surfaces

In this study, we exploited the properties of nature-inspired hierarchical structures to propose surfaces capable of on-demand directional droplet manipulation. A microline polydimethylsiloxane structure that simulated a bamboo leaf was fabricated, and silica particles were embedded onto its surface to create hierarchical structures. The as-fabricated multiscale line structures exhibited anisotropic wetting properties along the advancing direction. As the embedded particle size increased, the perpendicular roll-off angle (ROA) decreased and the anisotropic roll-off characteristic disappeared, adopting lotus-leaf characteristics. Consequently, the fabricated surface exhibited characteristics of both bamboo and lotus leaves. The roll off could be controlled through different ROAs by changing the particles size of silica on the same surface.

Surface Charge Density Gradient Printing To Drive Droplet Transport: A Numerical Study

Traditional strategies, such as morphological or chemical gradients, struggle to realize the high-velocity and long-distance transport for droplets on a solid surface because of the pinning hydrodynamic equilibrium. Thus, there is a continuing challenge for practical technology to drive droplet transport over the last decades. The surface charge density (SCD) gradient printing method overcame the theoretical limit of traditional strategies and tackled this challenge [Nat. Mater. 2019, 18: 936], which utilized the asymmetric electric force to realize the high-velocity and long-distance droplet transport along a preprinted SCD gradient pathway. In the present work, by coupling the electrostatics and the hydrodynamics, we developed an unexplored numerical model for the water droplet transporting along the charged superhydrophobic surface. Subsequently, the effects of SCD gradients on the droplet transport were systematically discussed, and an optimized method for SCD gradient printing was proposed according to the numerical results. The present approach can provide early guidance for the SCD gradient printing to drive droplet transport on a solid surface.

Biomimetic Patch with Wicking-Breathable and Multi-mechanism Adhesion for Bioelectrical Signal Monitoring

Wearable bioelectrical monitoring devices can provide long-term human health information such as electrocardiogram and other physiological signals. It is a crucial part of the remote medical system. These can provide prediction for the diagnosis and treatment of cardiovascular disease and access to timely treatment. However, the patch comfort of the wearable monitoring devices in long-term contact with the skin have been a technical bottleneck of the hardware. In this study, the biomimetic patch with wicking-breathable and multi-mechanism adhesion performance to achieve adaptability and comfortability to human skin has been reported. The patch was designed based on a conical through-hole and hexagonal microgroove to directionally transport sweat from skin to air which gives the patch the breathable performance. The breathable and drainage capability of the biomimetic patch was experimentally verified by analyzing the conical through-hole and hexagonal microgroove with the structural mechanism of wicking. Multi-mechanism adhesion of the Ag/Ni microneedle array and PDMS-t adhesion material ensures the stability of patch signal acquisition. This study provides a new way for enhancing the breathability and adaptability of the patch to realize accurate bioelectrical signal monitoring under sweat conditions on human skin.

A review on control of droplet motion based on wettability modulation: principles, design strategies, recent progress, and applications

The transport of liquid droplets plays an essential role in various applications. Modulating the wettability of the material surface is crucial in transporting droplets without external energy, adhesion loss, or intense controllability requirements. Although several studies have investigated droplet manipulation, its design principles have not been categorized considering the mechanical perspective. This review categorizes liquid droplet transport strategies based on wettability modulation into those involving (i) application of driving force to a droplet on non-sticking surfaces, (ii) formation of gradient surface chemistry/structure, and (iii) formation of anisotropic surface chemistry/structure. Accordingly, reported biological and artificial examples, cutting-edge applications, and future perspectives are summarized.

Robust and Transparent Lossless Directional Omniphobic Ultra-Thin Sticker-Type Film with Re-entrant Micro-Stripe Arrays

Directional droplet-sliding control without wetting the surface is immensely required in advanced surface engineering, including biological and chemical analyses or green technology. However, the development of robust and transparent thin sticker-type directional omniphobic films for portable usage in smart microfluidic platforms is rare. In this study, we report a novel perfluoropolyether (PFPE) directional omniphobic film (PDOF). The PDOF is a robust and transparent ultra-thin sticker-type film that can control the anisotropic sliding of various liquid droplets on the surface. The PFPE is a chemically stable and turgid material compared to polydimethylsiloxane (PDMS), which is often used to fabricate liquid-repellent thin films. A well-designed fabrication criterion through adhesion engineering in the soft-molding process was developed using the PFPE to obtain a PDOF with a thickness of 56 μm, with re-entrant micro-stripe structures on the surface. The fabricated PDOF showed intriguing liquid sliding properties based on the direction and spacing of the microstructures. This aspect is defined as an anisotropic factor.

Droplet Bouncing: Fundamentals, Regulations, and Applications

Droplet impact is a ubiquitous phenomenon in nature, daily life, and industrial processes. It is thus crucial to tune the impact outcomes for various applications. As a special outcome of droplet impact, the bouncing of droplets keeps the form of the droplets after the impact and minimizes the energy loss during the impact, being beneficial in many applications. A unified understanding of droplet bouncing is in high demand for effective development of new techniques to serve applications. This review shows the fundamentals, regulations, and applications of millimeter-sized droplet bouncing on solid surfaces and same/miscible liquids (liquid pool and another droplet). Regulation methods and current applications are summarized, and potential directions are proposed.

Wall-Confined Spreading Dynamics on the Surface of Surfactant Solution

A liquid spreading over another is a universal physical process in the nature, which was investigated by the scaling law to reveal the underlying mechanical mechanism over the decades. However, scaling laws are restricted to piecewise physical stages, respectively. It is a challenge to present a full physical picture for a dynamic spreading process covering a wide-spectrum speed. We propose a general wall-confined spreading dynamics (WCSD) model originating from molecular kinetic theory (MKT). It creatively illustrates the order and domination between driving energy and energy dissipation (or transfer) using a phase diagram according to theory and experiments. This work reveals the deep mechanical mechanism of WCSD which provides an indirect guidance on the solution processing methods of two-dimensional molecular crystals (2DMCs) growth.

Durably Self-Sustained Droplet on a Fully Miscible Liquid Film

Droplets impacting onto a solid or liquid surface inducing wetting, floatation, splash, coalescence, etc. is ubiquitous in nature and industrial processes. Here, we report that liquid droplets exhibit spherical caps upon contact with a fully miscible liquid film of lower surface tension, despite the spontaneous mixing of the two liquids. Such a spherical cap on a continuous liquid surface sustains a long lifespan up to minutes before ultimately merging into the film. Benefiting from large viscous forces in a thin film as a result of spatial confinement, the surface flow is substantially suppressed. Therefore, the surface tension gradient responsible for this phenomenon is maintained because the normal diffusion of film liquid into the droplet can timely dilute film liquid supplied by uphill Marangoni flow at the droplet surface. The present finding removes the conventional cognition that droplet coalescence is prompt on fully miscible continuous liquid surfaces, thus benefiting design of new types of microfluidic devices.

Simple but Efficient Method To Transport Droplets on Arbitrarily Controllable Paths

The flexible manipulation of droplets manifests a wide spectrum of applications, such as micro-flow control, drug-targeted therapy, and microelectromechanical system heat dissipation. How to realize the efficient control of droplets has become a problem of concern. In this paper, a simple method that can realize the transport of droplets along any controllable path is proposed. It not only has a simple preparation process and clear transport mechanism but is also easy to realize in manipulation technology. A magnetic-sensitive surface is prepared by filling a polymer matrix with magnetic particles and immersing in a lubricant. Under the action of an external magnetic field, rough microstructures are generated locally on the surface, forming the wettability gradient with the area far away from the field. Moving the magnetic field, the wettability gradient region moves accordingly and drives droplets to transport. To better control the transport path of droplets or realize a more complex path design, a ring-shaped magnetic field is further adopted, during which the droplet is automatically located in the ring-shaped region and moves with the movement of the ring-shaped magnetic field. The present technique is simple and easy to implement, which should be helpful in the field of precise regulation of the droplet position.

Materials Engineering for Atmospheric Water Harvesting: Progress and Perspectives

Atmospheric water harvesting (AWH) is emerging as a promising strategy to produce fresh water from abundant airborne moisture to overcome the global clean water shortage. The ubiquitous moisture resources allow AWH to be free from geographical restrictions and potentially realize decentralized applications, making it a vital parallel or supplementary freshwater production approach to liquid water resource-based technologies. Recent advances in regulating chemical properties and micro/nanostructures of moisture-harvesting materials have demonstrated new possibilities to promote enhanced device performance and new understandings. This perspective aims to provide a timely overview on the state-of-the-art materials design and how they serve as the active components in AWH. First, the key processes of AWH, including vapor condensation, droplet nucleation, growth, and departure are outlined, and the desired material properties based on the fundamental mechanisms are discussed. Then, how tailoring materials-water interactions at the molecular level play a vital role in realizing high water uptake and low energy consumption is shown. Last, the challenges and outlook on further improving AWH from material designs and system engineering aspects are highlighted.

Smart surfaces with reversibly switchable wettability: Concepts, synthesis and applications

As a growing hot research topic, manufacturing smart switchable surfaces has attracted much attention in the past a few years. The state-of-the-art study on reversibly switchable wettability of smart surfaces has been presented in this systematic review. External stimuli are brought about to render the alteration in chemical conformation and surface morphology to drive the wettability switch. Here, starting from the fundamental theories related to the surfaces wetting principles, highlights on different triggers for switchable wettability, such as pH, light, ions, temperature, electric field, gas, mechanical force, and multi-stimuli are discussed. Different applications that have various wettability requirement are targeted, including oil-water separation, droplets manipulation, patterning, liquid transport, and so on. This review aims to provide a deep insight into responsive interfacial science and offer guidance for smart surface engineering. It ends with a summary of current challenges, future opportunities, and potential solutions on smart switch of wettability on superwetting surfaces.

Survey of Micro/Nanofabricated Chemical, Topographical, and Compound Passive Wetting Gradient Surfaces

Surface wetting gradients are desirable due to their ability to passively transport liquid droplets without the aid of gravity. Such surfaces can be prepared through topographical or chemical methods or a compound approach involving both methods. By altering the surface free energy across a surface, a droplet that contacts such a surface will experience an actuation force toward the hydrophilic region. Such transport properties make these surfaces attractive for a range of applications from thermal management to microfluidics to the investigation of biomolecular interactions. This paper reviews passive wetting gradients that have been demonstrated over the last three decades, focusing on the types of surfaces that have been developed to date along with the materials that have been used. The corresponding wetting ranges and physical lengths over which droplet mobility has been achieved on these various types of gradient surfaces are compared to guide future developments.

Microscopic Observation of Preferential Capillary Pumping in Hollow Nanowire Bundles

Numerous studies have focused on designing micro/nanostructured surfaces to improve wicking capability for rapid liquid transport in many industrial applications. Although hierarchical surfaces have been demonstrated to enhance wicking capability, the underlying mechanism of liquid transport remains elusive. Here, we report the preferential capillary pumping on hollow hierarchical surfaces with internal nanostructures, which are different from the conventional solid hierarchical surfaces with external nanostructures. Specifically, capillary pumping preferentially occurs in the nanowire bundles instead of the interconnected V-groove on hollow hierarchical surfaces, observed by confocal laser scanning fluorescence microscopy. Theoretical analysis shows that capillary pumping capability is mainly dependent on the nanowire diameter and results in 15.5 times higher capillary climbing velocity in the nanowire bundles than that in the microscale V-groove. Driven by the Laplace pressure difference between nanowire bundles and V-grooves, the preferential capillary pumping is increased with the reduction of the nanowire diameter. Capillary pumping of the nanowire bundles provides a preferential path for rapid liquid flow, leading to 2 times higher wicking capability of the hollow hierarchical surface comparing with the conventional hierarchical surface. The unique mechanism of preferential capillary pumping revealed in this work paves the way for wicking enhancement and provides an insight into the design of wicking surfaces for high-performance capillary evaporation in a broad range of applications.

Electrostatic tweezer for droplet manipulation

Various physical tweezers for manipulating liquid droplets based on optical, electrical, magnetic, acoustic, or other external fields have emerged and revolutionized research and application in medical, biological, and environmental fields. Despite notable progress, the existing modalities for droplet control and manipulation are still limited by the extra responsive additives and relatively poor controllability in terms of droplet motion behaviors, such as distance, velocity, and direction. Herein, we report a versatile droplet electrostatic tweezer (DEST) for remotely and programmatically trapping or guiding the liquid droplets under diverse conditions, such as in open and closed spaces and on flat and tilted surfaces as well as in oil medium. DEST, leveraging on the coulomb attraction force resulting from its electrostatic induction to a droplet, could manipulate droplets of various compositions, volumes, and arrays on various substrates, offering a potential platform for a series of applications, such as high-throughput surface-enhanced Raman spectroscopy detection with single measuring time less than 20 s.

Contactless Discharge-Driven Droplet Motion on a Nonslippery Polymer Surface

Droplet manipulation is the cornerstone of many modern technologies. It is still challenging to drive the droplet motion on nonslippery surfaces flexibly. We present a droplet manipulation method on nonslippery polymer surfaces based on the corona discharge. With the corona discharge of two-needle electrodes with opposite polarities, the droplet's charge polarity can be switched, which results in the directionally droplet transport on a charged polymer surface with the oscillation. Here, such droplet behaviors are presented in detail. Dependence of the motion on the critical distance and driving distance between the droplet and the needle electrode is revealed. The driving mechanism is verified by experiments and simulations. This work enriches the droplet manipulation techniques on nonslippery surfaces for various applications, such as combinatory chemistry, biochemical, and medical detection.

Regular Self-Actuation of Liquid Metal Nanodroplets in Radial Texture Gradient Surfaces

Liquid metal movement in microfluidic devices generally requires an external stimulus to achieve its motion, which results in many difficulties to precisely manipulate its motion at a nanoscale. Therefore, there is an attempt to control the motion of a liquid metal droplet without the input of an external force. In this paper, we report an approach to achieve the self-actuation of a gallium nanodroplet in radial texture gradients on substrates. The results have proved the validity of this method. It is suggested that there are four stages in the self-motion of the droplet and that the precursor film forming on the second stage plays a pivotal role in the motion. Furthermore, how the impact velocity affects the self-actuation of the nanodroplet on the gradient surface is also studied. We find that the moderate impacting velocity hinders the self-actuation of the gallium nanodroplet. This study is very helpful to regulate the self-actuation on patterned substrates and facilitate their applications in the fields of microfluidics devices, soft robots, and liquid sensors.

Asymmetric Mass Transport through Dense Heterogeneous Polymer Membranes: Fundamental Principles, Lessons from Nature, and Artificial Systems

Many organisms rely on directional water transport schemes for the purpose of water retention and collection. Directional transport of water and other fluids is also technologically relevant, for example to harvest water, in separation processes, packaging solutions, functional clothing, and many other applications. One strategy to promote mass transport along a preferential direction is to create compositionally asymmetric, multi-layered, or compositionally graded architectures. In recent years, the investigation of natural and artificial membranes based on this design has attracted growing interest and allowed researchers to develop a good understanding of how the properties of such membranes can be tailored to meet the demands of particular applications. Here a summary of theoretical works on mass transport through dense asymmetric membranes, comprehensive reviews of biological and artificial membranes featuring this design, and a discussion of applications, remaining questions, and opportunities are provided.

Initial-position-driven opposite directional transport of a water droplet on a wedge-shaped groove

The transport direction of water droplets on a functionalized surface is of great significance due to its wide applications in microfluidics technology. The prevailing view is that a water droplet on a wedge-shaped groove always moves towards the wider end. In this paper, however, molecular dynamics simulations show that a water droplet can move towards the narrower end if placed at specific positions. It is found that the direction of water droplet transport on a grooved surface is related to its initial position. The water droplet moves towards the wider end only when it is placed near the wider end initially. If the water droplet is placed near the narrower end, it will move in the opposite direction. The novel phenomenon is attributed to the opposite interactions of the groove substrate and the groove upper layers with water droplets. Two effective models are proposed to exploit the physical origin of different transport directions of water droplets on a wedge-shaped groove surface. The study provides an insight into the design of nanostructured surfaces to effectively control the droplet motion.

Unidirectional water transport on a two-dimensional hydrophilic channel with anisotropic superhydrophobic barriers

Many creatures have a unique anisotropic structure and special wettability on their skins, presenting intriguing water transporting properties. Inspired by the biosphere, a two-dimensional titanium dioxide-based hydrophilic channel possessing anisotropic superhydrophobic barriers was synthesized. This channel demonstrates unidirectional water transporting properties. When water is injected into the channel, fluid tends to spread in a specific direction. An asymmetric spreading resistance is generated by the different interaction modes between the liquid and superhydrophobic barriers. The superhydrophobic barriers are designed as two main styles: line and curve. As for line barriers, the included angle between barrier and horizontal is the key parameter for the unidirectional water transporting ability whereas, for curve barriers, the radius is an important variable. The best design scheme for unidirectional water transporting properties could be found by varying the parameters of these two types of barriers in the channel. Overall, this study is expected to have a significant implication in the water transporting field.

Rapid and Persistent Suction Condensation on Hydrophilic Surfaces for High-Efficiency Water Collection

Water collection by dew condensation emerges as a sustainable solution to water scarcity. However, the transient condensation process that involves droplet nucleation, growth, and transport imposes conflicting requirements on surface properties. It is challenging to satisfy all benefits for different condensation stages simultaneously. By mimicking the structures and functions of moss Rhacocarpus, here, we report the attainment of dropwise condensation for efficient water collection even on a hydrophilic surface gated by a liquid suction mechanism. The Rhacocarpus-inspired porous surface (RIPS), which possesses a three-level wettability gradient, facilitates a rapid, directional, and persistent droplet suction. Such suction condensation enables a low nucleation barrier, frequent surface refreshing, and well-defined maximum droplet shedding radius simultaneously. Thus, a maximum ∼160% enhancement in water collection performance compared to the hydrophobic surface is achieved. Our work provides new insights and a design route for developing engineered materials for a wide range of water-harvesting and phase-change heat-transfer applications.

Dropwise condensation: From fundamentals of wetting, nucleation, and droplet mobility to performance improvement by advanced functional surfaces

As a ubiquitous vapor-liquid phase-change process, dropwise condensation has attracted tremendous research attention owing to its remarkable efficiency of energy transfer and transformative industrial potential. In recent years, advanced functional surfaces, profiting from great progress in modifying micro/nanoscale features and surface chemistry on surfaces, have led to exciting advances in both heat transfer enhancement and fundamental understanding of dropwise condensation. In this review, we discuss the development of some key components for achieving performance improvement of dropwise condensation, including surface wettability, nucleation, droplet mobility, and growth, and discuss how they can be elaborately controlled as desired using surface design. We also present an overview of dropwise condensation heat transfer enhancement on advanced functional surfaces along with the underlying mechanisms, such as jumping condensation on nanostructured superhydrophobic surfaces, and new condensation characteristics (e.g., Laplace pressure-driven droplet motion, hierarchical condensation, and sucking flow condensation) on hierarchically structured surfaces. Finally, the durability, cost, and scalability of specific functional surfaces are focused on for future industrial applications. The existing challenges, alternative strategies, as well as future perspectives, are essential in the fundamental and applied aspects for the practical implementation of dropwise condensation.

Biotemplated Fabrication of a Multifunctional Superwettable Shape Memory Film for Wearable Sensing Electronics and Smart Liquid Droplet Manipulation

Wearable superwettable surfaces with dynamic tunable wettability and self-healability are promising for advanced wearable electronics, whereas have been rarely reported. Herein, a flexible superhydrophobic shape memory film (SSMF) with switchable surface wettability and high strain sensitivity has been conveniently fabricated. The surface topography of the SSMF can be finely adjusted by a reversible stretching (bending)/recovery way, which makes it feasible to control the surface-switchable adhesive superhydrophobicity by simple body movements, demonstrating great advantages in selective droplet manipulation and smart control of droplet movement. Moreover, benefitting from the hierarchical micro/nanostructures and outstanding sensing performance, the flexible SSMFs with good adaptivity and durability can serve as smart wearable sensors attached to human skin to achieve full-range and real-time detection of human motions and intelligent control of Internet of Things. More interestingly, the unique dynamic dewetting property enables the sensors to work in a humid environment or rainy days. Overall, this work successfully integrates dynamic tunable superwettability into design of intelligent wearable electronics with multifunctions. The obtained SSMF-based wearable surface with dynamic dewetting properties reveals great potential in versatile application fields such as liquid-repellent electronics, wearable droplet manipulators, and all-weather intelligent actuators.

Directional Liquid Transport from the Cold Region to the Hot Region on a Topological Surface

Manifested from the "tears of wine" to the "coffee-ring effect", the directional transport of a liquid governed by the Marangoni effect is highly pervasive in our daily life and has brought a great number of applications. Similar to this surface tension gradient-dominated process, the fluid preferentially flows from the hot region to the cold region. In contrast to this perception, in this study, we report that water liquid deposited on a specially designed topological surface can flow from the low-temperature region to the high-temperature region in a spontaneous, long-range, and unidirectional manner. We show that such a behavior is mainly owing to a strong topological effect that outweighs the thermal gradient imposed along the surface. Moreover, the specific temperature range applied on the topological surface for the occurrence of such a unidirectional liquid transport phenomenon is also identified. Our findings would find important insights for developing next-generation cooling devices where a rapid flow from the condensation region to the evaporation/boiling region is preferred.

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.

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.

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.

Bioinspired Surfaces With Switchable Wettability

The surface wettability of plants exhibits many unique advantages, which enhances the environmental adaptability of plants. In view of the rapid development of responsive materials, smart surfaces have been explored extensively to regulate surface wettability through external stimuli. Herein, we summarized recent advancements in bioinspired surfaces with switchable wettability. Typical bioinspired surfaces with switchable wettability and their emerging applications have been reviewed. In the end, we have discussed the remaining challenges and provided perspective on future development.

Directional liquid dynamics of interfaces with superwettability

Natural creatures use their surface structures to control directional liquid dynamics for survival. Learning from nature, artificial superwetting materials have triggered technological revolutions in many disciplines. To improve controllability, researchers have attempted to use external fields, such as thermal, light, magnetic, and electric fields, to assist or achieve controllable liquid dynamics. Emerging directional liquid transport applications have prosperously advanced in recent years but still present some challenges. This review discusses and summarizes the field of directional liquid dynamics on natural creatures and artificial surfaces with superwettabilities and ventures to propose several potential strategies to construct directional liquid transport systems for fog collection, 3D printing, energy devices, separation, soft machine, and sensor devices, which are useful for driving liquid transport or motility.

Field-Induced Wettability Gradients for No-Loss Transport of Oil Droplets on Slippery Surfaces

Transporting oil droplets is crucial for a wide range of industrial and biomedical applications but remains highly challenging due to the large contact angle hysteresis on most solid surfaces. A liquid-infused slippery surface has a low hysteresis contact angle and is a highly promising platform if sufficient wettability gradient can be created. Current strategies used to create wettability gradient typically rely on the engineering of the chemical composition or geometrical structure. However, these strategies are inefficient on a slippery surface because the infused liquid tends to conceal the gradient in the chemical composition and small-scale geometrical structure. Magnifying the structure, on the other hand, will significantly distort the surface topography, which is unwanted in practice. In this study, we address this challenge by introducing a field-induced wettability gradient on a flat slippery surface. By printing radial electrodes array, we can pattern the electric field, which induces gradient contact angles. Theoretical analysis and experimental results reveal that the droplet transport behavior can be captured by a nondimensional electric Bond number. Our surface enables no-loss transport of various types of droplets, which we expect to find important applications such as heat transfer, anticontamination, microfluidics, and biochemical analysis.

Counterintuitive Ballistic and Directional Liquid Transport on a Flexible Droplet Rectifier

Achieving the directional and long-range droplet transport on solid surfaces is widely preferred for many practical applications but has proven to be challenging. Particularly, directionality and transport distance of droplets on hydrophobic surfaces are mutually exclusive. Here, we report that drain fly, a ubiquitous insect maintaining nonwetting property even in very high humidity, develops a unique ballistic droplet transport mechanism to meet these demanding challenges. The drain fly serves as a flexible rectifier to allow for a directional and long-range propagation as well as self-removal of a droplet, thus suppressing unwanted liquid flooding. Further investigation reveals that this phenomenon is owing to the synergistic conjunction of multiscale roughness, structural periodicity, and flexibility, which rectifies the random and localized droplet nucleation (nanoscale and microscale) into a directed and global migration (millimeter-scale). The mechanism we have identified opens up a new approach toward the design of artificial rectifiers for broad applications.

Critical Pressure Asymmetry in the Enclosed Fluid Diode

Joint physically and chemically pattered surfaces can provide efficient and passive manipulation of fluid flow. The ability of many of these surfaces to allow only unidirectional flow means they are often termed fluid diodes. Synthetic analogues of these are enabling technologies from sustainable water collection via fog harvesting to improved wound dressings. One key fluid diode geometry features a pore sandwiched between two absorbent substrates-an important design for applications that require liquid capture while preventing back-flow. However, the enclosed pore is particularly challenging to design as an effective fluid diode due to the need for both a low Laplace pressure for liquid entering the pore and a high Laplace pressure to liquid leaving. Here, we calculate the Laplace pressure for fluid traveling in both directions on a range of conical pore designs with a chemical gradient. We show that this chemical gradient is in general required to achieve the largest critical pressure differences between incoming and outgoing liquids. Finally, we discuss the optimization strategy to maximize this critical pressure asymmetry.

Bamboo-joint-like platforms for fast, long-distance, directional, and spontaneous transport of fluids

Spontaneous transport of fluids without external force offers an enabling tool for a wide spectrum of fields. However, the development of a universal spontaneous transport platform for liquids remains a challenge. In this work, a novel bamboo-joint-like platform with tapered micro-tubes as transport units is presented, which not only enables the spontaneous transport and extrusion of liquids but also enables customized and optional assembly of transport devices. Spontaneous transport characterized with long-distance, anti-gravity transport, directional transport, and liquid extrusion characteristics was found to show excellent transport capacity. The results indicated that both transport distance and speed varied periodically with time, which was mainly due to the difference in curvature caused by asymmetric structure and capillary force. The desired spontaneous transportation was successfully obtained even when the supply rate speed was up to 632.5 μl/min and length of platform reached a scale of hundreds of millimeters. Transport units were easily fabricated via a commercially available 3D printing technique, so that the customized and directional spontaneous directional transport can be realized for liquid distribution, serpentine loop transportation, and speed control. With the comprehensive use of transport units and connectors, it is very easy to implement self-service construction of a universal complex multi-functional transportation platform.

Rectification of Mobile Leidenfrost Droplets by Planar Ratchets

The self-transportation of mobile Leidenfrost droplets with well-defined direction and velocity on millimetric ratchets is one of the most representative and spectacular phenomena in droplet dynamics. Despite extensive progress in the ability to control the spatiotemporal propagation of droplets, it remains elusive how the individual ratchet units, as well as the interactions within their arrays, are translated into the collective droplet dynamics. Here, simple planar ratchets characterized by uniform height normal to the surface are designed. It is revealed that on planar ratchets, the transport dynamics of Leidenfrost droplets is dependent not only on individual units, but also on the elegant coordination within their arrays dictated by their topography. The design of planar ratchets enriches the fundamental understanding of how the surface topography is translated into dynamic and collective droplet transport behaviors, and also imparts higher applicability in microelectromechanical system based fluidic devices.

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.

Magnetic Actuation Multifunctional Platform Combining Microdroplets Delivery and Stirring

Multifunctional droplets manipulation devices are in urgent need for various laboratory operations such as chemical reaction and biological analysis. However, most current techniques that achieved a controllable droplet transport system mainly rely on passive diffusion for mixing, limiting their practical applications. Here, we develop a magnetic controlled dimple on slippery surface (MCDSS) that enables arbitrary direction or even uphill droplet transport through the synergy between gravitational force and asymmetrical droplet deformation. Further experiments demonstrate that our system could also be used for stirring microdroplets and accelerating the mixing speed by more than one hundred times. In addition, the microstir strategy could help to avoid locally uneven production of precipitation or gas in heterogeneous reactions. This combination of droplet delivery and agitation may have a promising future for application in various fields, for example, laboratory-on-a-chip platforms and microengines.