Topological liquid diode

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.

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

Stretch-Controlled Branch Shape Microstructures for Switchable Unidirectional Self-Driven Spreading of Oil Droplets

Unidirectional transport of liquids has attracted the attention of researchers in recent years for its wide application foreground. However, it is still a challenge to control the spreading of liquid, especially for oils with relatively high viscosity. In this paper, a flexible surface textured with branch-shaped microstructures is proposed. These asymmetric microstructures exhibit excellent unidirectional spreading behaviors for various oils. By suitably stretching the flexible surface to different stretch ratios, the spreading length of the oil droplets can be controlled. Moreover, the ongoing forward spreading of oil droplets can be suspended dynamically when the surface is stretched to 40%. Corresponding mechanism analysis demonstrates that surface stretching can narrow and close the microvalves between adjacent branches, which restrain the flow of the precursor film and the primary droplet. The switchable unidirectional spreading behavior enables the surface with such microstructures to be used for oil transportation, oil-water separation, and controllable lubrication.

Flexible liquid-diode microtubes from multimodal microfluidics

Directional transport of liquids is of great importance in energy saving, chemical/biomedical engineering, and microfluidics applications. Despite considerable progress in engineering different open surfaces to achieve liquid manipulation, the realization of diode-like liquid transport in enclosed spaces is still challenging. Here, a flexible diode microtube is presented for directional liquid transport within confined spaces using pulsed microfluidics. The microtubes exhibit sophisticated microstructures on the inner wall, replicated from a precisely controlled flow configuration in the microfluidic channel. Under the effect of asymmetric pinning and unbalanced Laplace pressure, such microtubes enable directional liquid transport in closed channels. More importantly, by integrating in situ flow lithography with the microfluidic system, segmented liquid diodes are fabricated as assembly units for the construction of fluidic-electronic circuits that perform logic operations. These results demonstrate the capacity of the present liquid-diode microtubes for flexible, directional, and programmable liquid transport. We believe that it can open an avenue for designing advanced fluidic circuit-based devices toward versatile practical applications.

Barbed arrow-like structure membrane with ultra-high rectification coefficient enables ultra-fast, highly-sensitive lateral-flow assay of cTnI

Acute myocardial infarction (AMI) has become a public health disease threatening public life safety due to its high mortality. The lateral-flow assay (LFA) of a typical cardiac biomarker, troponin I (cTnI), is essential for the timely warnings of AMI. However, it is a challenge to achieve an ultra-fast and highly-sensitive assay for cTnI (hs-cTnI) using current LFA, due to the limited performance of chromatographic membranes. Here, we propose a barbed arrow-like structure membrane (BAS Mem), which enables the unidirectional, fast flow and low-residual of liquid. The liquid is rectified through the forces generated by the sidewalls of the barbed arrow-like grooves. The rectification coefficient of liquid flow on BAS Mem is 14.5 (highest to date). Using BAS Mem to replace the conventional chromatographic membrane, we prepare batches of lateral-flow strips and achieve LFA of cTnI within 240 s, with a limit of detection of 1.97 ng mL-1. The lateral-flow strips exhibit a specificity of 100%, a sensitivity of 93.3% in detecting 25 samples of suspected AMI patients. The lateral-flow strips show great performance in providing reliable results for clinical diagnosis, with the potential to provide early warnings for AMI.

Assembly-Induced Dynamic Structural Color in a Host-Guest System for Time-Dependent Anticounterfeiting and Double-Lock Encryption

Manipulation of periodic micro/nanostructures in polymer film is of great importance for academics and industrial applications in anticounterfeiting. However, with the increasing demand on information security, materials with time-dependent features are urgently required, especially the material where the same information can appear more than once on the time scale. Here, one concise strategy to realize time-dependent anticounterfeiting and "double-lock" information encryption based on a host-guest system is proposed, with one photoresponsive azopolymer as the host and one liquid-crystalline molecule as the guest. The system exhibits a tunable mass transport in pre-designed periodic micro/nanostructures by tailoring the process of cis-to-trans recovery of azo groups and assembly of mesogenic trans-isomers, resulting in a dynamic structural color in film. Taking advantage of this extraordinary feature, time-dependent dynamic anticounterfeiting has been achieved. More importantly, the time of each state's appearance in the whole process can be modulated by changing the host-guest ratio. Combining the manipulatable process of mass transport with the unique decoding method, the stored information in film can be decrypted correctly. This work provides an unprecedented dynamic approach for advanced anticounterfeiting technology with a higher level of security and high-end applications in information encryption.

Unidirectional transport of both wettable and nonwettable liquids on an asymmetrically concave structured surface

Unidirectional liquid transport (UDLT) has been widely used in various fields as an important process for transferring both mass and energy. However, UDLT driven by a structural gradient has been witnessed for a long time only in wettable liquids. For nonwettable liquids, UDLT can hardly proceed merely by a structural gradient. Herein, we propose an asymmetrically concave structured surface (AMC-surface), featuring tip-to-base periodically arranged pyramid-shaped concave structures with a certain degree of overlap, which enables the UDLT of both wettable and nonwettable liquids. For wettable liquids, the capillary force along each corner leads to the UDLT pointing toward the base side of the concave pyramid, while for nonwettable liquids, the UDLT is attributable to the static liquid pressure overwhelming the repulsive Laplace pressure induced by the asymmetric grooves and overlapping part. As a result, both wettable and nonwettable liquids transport spontaneously and unidirectionally on the AMC-surface with no energy input. Moreover, the concave structure endows good mechanical stability and can be easily prepared using a facile nail-punching approach over a large area. We also demonstrated its application in a continuous chemical reaction in a confined area. We envision that the unique UDLT behavior on the as-developed AMC-surface will shed new light on the programmable manipulation of various liquids.

Enabling Self-Adaptive Water-Energy-Balance of Photothermal Water Diode Evaporator: Dynamically Maximizing Energy Utilization Under the Ever-Changing Sunlight

Maintaining a match between input solar energy and required energy by water supply management is key to achieving efficient interfacial solar-driven evaporation (ISDE). In practice, the solar radiation flux is constantly changing throughout the day, so keeping a dynamic water-energy-balance of ISDE is a big challenge. Herein, a photothermal water diode (WD) evaporator concept is proposed by an integrated hydrophilic/hydrophobic Janus absorber to overcome the issue. Due to the unique unidirectional water transport properties induced by asymmetric wettability, a self-adaptive balance between photothermal energy input and water uptake is established, thus realizing the energy matching and utilization maximization. The experimental and simulation results exhibit that with the increase of sunlight intensity, the water supply speed is significantly accelerated due to the dynamic management and self-regulation on water replenishment. Therefore, an excellent evaporation rate of up to 2.14 kg m-2 h-1 with a high efficiency of 93.7% under 1 sun illumination is achieved. This water diode engineering with Janus wettability provides a novel strategy and extends the path for designing solar evaporation systems with diverse water supply properties, which shows great potential in different environmental conditions.

Infinite Self-Propulsion of Circularly On/Discharged Droplets

Self-propulsion of droplets in a controlled and long path at a high-speed is crucial for organic synthesis, pathological diagnosis and programable lab-on-a-chip. To date, extensive efforts have been made to achieve droplet self-propulsion by asymmetric gradient, yet, existing structural, chemical, or charge density gradients can only last for a while (<50 mm). Here, this work designs a symmetrical waved alternating potential (WAP) on a superhydrophobic surface to charge or discharge the droplets during the transport process. By deeply studying the motion mechanisms for neutral droplets and charged droplets, the circularly on/discharged droplets achieve the infinite self-propulsion (>1000 mm) with an ultrahigh velocity of meters per second. In addition, after permutation and combination of two motion styles of the droplets, it can be competent for more interesting work, such as liquid diode and liquid logic gate. Being assembled into a microfluidic chip, the strategy would be applied in chemical synthesis, cell culture, and diagnostic kits.

A three-dimensional liquid diode for soft, integrated permeable electronics

Wearable electronics with great breathability enable a comfortable wearing experience and facilitate continuous biosignal monitoring over extended periods1-3. However, current research on permeable electronics is predominantly at the stage of electrode and substrate development, which is far behind practical applications with comprehensive integration with diverse electronic components (for example, circuitry, electronics, encapsulation)4-8. Achieving permeability and multifunctionality in a singular, integrated wearable electronic system remains a formidable challenge. Here we present a general strategy for integrated moisture-permeable wearable electronics based on three-dimensional liquid diode (3D LD) configurations. By constructing spatially heterogeneous wettability, the 3D LD unidirectionally self-pumps the sweat from the skin to the outlet at a maximum flow rate of 11.6 ml cm-2 min-1, 4,000 times greater than the physiological sweat rate during exercise, presenting exceptional skin-friendliness, user comfort and stable signal-reading behaviour even under sweating conditions. A detachable design incorporating a replaceable vapour/sweat-discharging substrate enables the reuse of soft circuitry/electronics, increasing its sustainability and cost-effectiveness. We demonstrated this fundamental technology in both advanced skin-integrated electronics and textile-integrated electronics, highlighting its potential for scalable, user-friendly wearable devices.

Recent progress on the development of bioinspired surfaces with high aspect ratio microarray structures: From fabrication to applications

Surfaces with high aspect ratio microarray structures can implement sophisticated assignment in typical fields including microfluidics, sensor, biomedicine, et al. via regulating their deformation or the material properties. Inspired by natural materials and systems, for example sea cockroaches, water spiders, cacti, lotus leaves, rice leaves, and cedar leaves, many researchers have focused on microneedle functional surface studies. When the surface with high aspect ratio microarray structures is stimulated by the external fields, such as optical, electric, thermal, magnetic, the high aspect ratio microarray structures can undergo hydrophilic and hydrophobic switching or shape change, which may be gifted the surfaces with the ability to perform complex task, including directional liquid/air transport, targeted drug delivery, microfluidic chip sensing. In this review, the fabrication principles of various surfaces with high aspect ratio microarray structures are classified and summarized. Mechanisms of liquid manipulation on hydrophilic/hydrophobic surfaces with high aspect ratio microarray structures are clarified based on Wenzel model, Cassie model, Laplace pressure theories and so on. Then the intelligent control strategies have been demonstrated. The applications in microfluidic, drug delivery, patch sensors have been discussed. Finally, current challenges and new insights of future prospects for dynamic manipulation of liquid/air based on biomimetic surface with high aspect ratio microarray structures are also addressed.

Steerable mass transport in a photoresponsive system for advanced anticounterfeiting

Numerous anticounterfeiting platforms using photoresponsive materials have been designed to improve information security, enabling applications in anticounterfeiting technology. However, fabricating sophisticated micro/nanostructures using bidirectional mass transport to achieve advanced anticounterfeiting remains challenging. Here, we propose one strategy to achieve steerable mass transport in a photoresponsive system with the assistance of solvent vapor at room temperature. Upon optimizing the host-guest ratio and the width of photoisomerized areas, wettability gradient is acquired just photo-patterning once, then bidirectional mass transport is realized due to the competition of mass transport induced by surface energy gradient of the material itself and flow of the solvent on the film surface with wettability gradient. Taking advantage of the interaction between solvent and film surface with wettability gradient, this bidirectional polymer flow has been successfully applied in multi-mode anticounterfeiting. This work paves a promising avenue toward high-level information storage in soft materials, demonstrating the potential applications in anticounterfeiting.

Synergistic Inclusion Effects of Hard Magnetic Nanorods on the Magnetomechanical Actuation of Soft Magnetic Microsphere-Based Polymer Composites

The magnetomechanical actuation of micropillars is developed for the contactless manipulation of miniaturized actuators and microtextured surfaces. Anisotropic geometry of micropillars can significantly enhance the magnetic actuation compared with their isotropic counterparts by directional stress distributions. However, this strategy is not viable for triangular micropillars owing to insufficient anisotropy. In this study, a significant improvement in the magnetic actuation of triangular micropillars using composite magnetic particles is reported. A minute and optimal amount of hard magnetic gamma-ferrite nanorods are hybridized with soft magnetic iron microspheres to generate synergistic effects of magnetic coupling and percolation phenomenon on the magnetic actuation of polymer composites. The addition of 1 wt% face-centered cubic-phased gamma-ferrite nanorods suppresses the magnetic coupling interference of body-centered cubic-phased iron microspheres. Furthermore, the nanorods reduce the percolation threshold by participating in the percolation of the microspheres. A systematic compositional study on the magnetization and magnetorheological properties reveals that the coupling effect dominates the percolation effect at a low magnetic field, whereas the percolation effect governs the magnetic actuation at a high magnetic field. This hybrid approach can help in designing material constituents for effective magnetic actuators and robotic systems that can sensitively respond to an external magnetic field.

Electrosprayed Environment-Friendly Dry Triode-Like Facial Masks for Skincare

The cosmetics industry has a worrying impact on the environment, including the plastics used in products and packaging and environmentally unfriendly additives. In this study, we present an environment-friendly triode-like facial mask (TFM) that utilizes only green and degradable raw materials, nontoxic and harmless solvents, and electric energy to achieve distinct switchable directional water transport properties, avoids a wet storage environment, and reduces excessive packaging. The TFM demonstrates droplet stability when not in contact with the skin while facilitating rapid liquid transfer (15 μL) within durations of 2.8 s (dry skin) and 1.9 s (moist skin) upon contact. We elucidate the underlying mechanism behind this triode-like behavior, emphasizing the synergistic interaction of the wettability gradient, Gibbs pinning, and additional circumferential capillary force. Moreover, the TFM exhibits a reduction in the proportion of aging cells, decreasing from 44.33 to 13.75%, while simultaneously providing antibacterial and skin-beautifying effects. The TFM brings a novel experience while also holding the potential to reduce environmental pollution in the production, packaging, use, and recycling of cosmetics products.

Long-Distance Continuous Self-Transport of a Droplet by Merging Droplets on a Graphene-Covered Multibranch Gradient Groove Surface

Although the self-transport of liquid droplets by a gradient-textured substrate can break away from the energy input, the long distance and even continuous spontaneous motion of droplets will be limited by the length in the surface-gradient direction. This article introduces a novel design with a monolayer graphene-covered multibranch gradient groove surface (GMGGS). The design aims to achieve long-distance, continuous self-transport of a mercury (Hg) droplet by merging with other mercury droplets, and the process is carried out using molecular dynamics (MD) simulation. This method achieves the merging of mercury droplets through the structure of multibranch gradient grooves, and we have observed that the merged mercury droplet can be reaccelerated in the gradient groove. The results demonstrate that droplet merging allows for control over the surface morphology variations of mercury droplets within the gradient groove. This creates a forward pressure difference, which leads to reacceleration of the mercury droplets. In light of this mechanism, the trunk droplet can achieve long-distance continuous self-transport on the GMGGS by continuously merging with branch droplets. These findings will broaden our comprehension of droplet merging and self-transport behavior, offering corresponding theoretical support for the long-distance continuous self-transport of droplets.

Heterogenous Slippery Surfaces: Enabling Spontaneous and Rapid Transport of Viscous Liquids with Viscosities Exceeding 10 000 mPa s

Superhydrophobic and slippery lubricant-infused surfaces have garnered significant attention for their potential to passively transport low-viscosity liquids like water (1 mPa s). Despite exciting progress, these designs have proven ineffective for transporting high-viscosity liquids such as polydimethylsiloxane (5500 mPa s) due to their inherent limitations imposed by the homogenous surface design, resulting in high viscous drags and compromised capillary forces. Here, a heterogenous water-infused divergent surface (WIDS) is proposed that achieves spontaneous, rapid, and long-distance transport of viscous liquids. WIDS reduces viscous drag by spatially isolating the viscous liquids and surface roughness through its heterogenous, slippery topological design, and generates capillary forces through its heterogenous wetting distributions. The essential role of surface heterogeneity in viscous liquid transport is theoretically and experimentally verified. Remarkably, such a heterogenous paradigm enables transporting liquids with viscosities exceeding 12 500 mPa s, which is two orders of magnitude higher than state-of-the-art techniques. Furthermore, this heterogenous design is generic for various viscous liquids and can be made flexible, making it promising for various systems that require viscous liquid management, such as micropatterning.

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.

Sweat Gland-Inspired Skin-like Fabric with Directional Water Transport and Durability for Efficient Personal Moisture Management

Directional water transport textiles are an energy-free approach to improving the comfort of the human body. However, existing strategies mainly focus on enhancing the capacity of directional water transport, complicating the preparation process and limiting the long-term durability of textiles. Herein, a skin-like fabric inspired by sweat glands was prepared in one step by patterning printed hydrophobic paste on the fabric. This skin-like fabric has achieved the desired one-way water transport index (R, 721%), air permeability of 104 mm s-1, and water vapor transmission rate (298 g m-2 h-1). More significantly, due to the strong chemical bonds between the fabric and the coating, the skin-like fabric exhibited a high weight retention of 99.4% after 400 abrasion cycles and stable performance (R, 658%) after 25 h of washing. This work proposes a reliable way to prepare high-performance fabrics with durability, which show great potential for applications in functional textiles for personal moisture management.

Wicking assisted condenser platform with patterned wettability for space application

Vapor condensation is extensively used in applications that demand the exchange of a substantial amount of heat energy or the vapor-liquid phase conversion. In conventional condensers, the condensate removal from a subcooled surface is caused by gravity force. This restricts the use of such condensers in space applications or horizontal orientations. The current study demonstrates proof-of-concept of a novel plate-type condenser platform for passively removing condensate from a horizontally oriented surface to the surrounding wicking reservoir without gravity. The condensing surface is engineered with patterned wettabilities, which enables the continuous migration of condensate from the inner region of the condenser surface to the side edges via surface energy gradient. The surrounding wicking reservoir facilitates the continuous absorption of condensate from the side edges. The condensation dynamics on different substrates with patterned wettabilities are investigated, and their condensation heat transfer performance is compared. The continuous migration of condensate drops from a superhydrophobic to a superhydrophilic area can rejuvenate the nucleation sites in the superhydrophobic area, resulting in increased heat transport. The proposed condenser design with engineered wettability can be used for temperature and humidity management applications in space.

Self-Adaptive Droplet Bouncing on a Dual Gradient Surface

Rapid detachment of impacting droplets from underlying substrate is highly preferred for mass, momentum, and energy exchange in many practical applications. Driven by this, the past several years have witnessed a surge in engineering macrotexture to reduce solid-liquid contact time. Despite these advances, these strategies in reducing contact time necessitate the elegant control of either the spatial location for droplet contact or the range of impacting velocity. Here, this work circumvents these limitations by designing a dual gradient surface consisting of a vertical spacing gradient made of tapered pillar arrays and a lateral curvature gradient characterized as macroscopic convex. This design enables the impacting droplets to self-adapt to asymmetric or pancake bouncing mode accordingly, which renders significant contact time reduction (up to ≈70%) for a broad range of impacting velocities (≈0.4-1.4 m s-1 ) irrespective of the spatial impacting location. This new design provides a new insight for designing liquid-repellent surfaces, and offers opportunities for applications including dropwise condensation, energy conversion, and anti-icing.

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.

Programmable Microfluidics Enabled by 3D Printed Bionic Janus Porous Matrics for Microfluidic Logic Chips

Numerous structures have been functionally optimized for directional liquid transport in nature. Inspired by lush trees' xylem that enable liquid directional transportation from rhizomes to the tip of trees, a new kind of programmable microfluidic porous matrices using projection micro-stereolithography (PµSL) based 3D printing technique is fabricated. Structural matrices with internal superhydrophilicity and external hydrophobicity are assembled for ultra-fast liquid rising enabled by capillary force. Moreover, the unidirectional microfluidic performance of the bionic porous matrices can be theoretically optimized by adjusting its geometric parameters. Most significantly, the successive programmable flow of liquid in a preferred direction inside the bionic porous matrices with tailored wettability is achieved, validating by a precisely printed liquid displayer and a microfluidic logic chip. The programmable and functional microfluidic matrices promise applications of patterned liquid flow, displayer, logic chip, cell screening, gas-liquid separation, and so on.

Non-Hermitian Chiral Heat Transport

Exceptional point (EP) has been captivated as a concept of interpreting eigenvalue degeneracy and eigenstate exchange in non-Hermitian physics. The chirality in the vicinity of EP is intrinsically preserved and usually immune to external bias or perturbation, resulting in the robustness of asymmetric backscattering and directional emission in classical wave fields. Despite recent progress in non-Hermitian thermal diffusion, all state-of-the-art approaches fail to exhibit chiral states or directional robustness in heat transport. Here we report the first discovery of chiral heat transport, which is manifested only in the vicinity of EP but suppressed at the EP of a thermal system. The chiral heat transport demonstrates significant robustness against drastically varying advections and thermal perturbations imposed. Our results reveal the chirality in heat transport process and provide a novel strategy for manipulating mass, charge, and diffusive light.

Bioinspired Underwater Superoleophilic Two-Dimensional Surface with Asymmetric Oleophobic Barriers for Unidirectional and Long-Distance Oil Transport

Unidirectional and long-distance liquid transport is critically important to a range of practical applications, e.g., water harvesting, microfluidics, and chemical reactions. Great efforts have been made on liquid manipulation; most of which, however, are limited in the air environment. It is still a great challenge to achieve unidirectional and long-distance oil transport in an aqueous environment. Herein, we have successfully fabricated an underwater superoleophilic two-dimensional surface (USTS) with asymmetric oleophobic barriers to arbitrarily manipulate oil in aqueous medium. The behavior of oil on USTS was carefully investigated, of which the unidirectional spreading capability was originated from the anisotropic spreading resistance resulted from the asymmetric oleophobic barriers. Accordingly, an underwater oil/water separation device has been developed, which can achieve continuous and efficient oil/water separation and further prevent the secondary pollution caused by oil volatilization.

Curvature Adjustable Liquid Transport on Anisotropic Microstructured Elastic Film

Directional liquid transport is expected via adjusting chemical components, surface morphology, and external stimuli and is critical for practical applications. Although many studies have been conducted, there are still challenges to achieving real-time transformation of liquid transport direction on the material surface. Herein, we demonstrate a strategy to achieve curvature responsive anisotropic wetting on the elastic film with V-shaped prism microarray (VPM) microstructure, which can be used to control the direction of liquid transport. The results reveal that the curvature change of an elastic film can adjust the arrangement of V-shaped prisms on the elastic film. Correspondingly, the liquid wetting trend will change and even the moving direction reverses with varying arrangements of the V-shaped prisms on the elastic film. Meanwhile, surface hydrophobicity of the VPM elastic film also affects the liquid wetting trend and even shows the opposite transport direction of the liquid, which is up to the water wetting state on the VPM elastic film. Based on these results, the VPM elastic film can serve as a valve to control the liquid transport direction and is promising in the application of liquid directional harvest, chemical reaction, microfluidic, etc.

Microfluidic solutions for biofluids handling in on-skin wearable systems

On-skin wearable systems for biofluid sampling and biomarker sensing can revolutionize the current practices in healthcare monitoring and personalized medicine. However, there is still a long path toward complete market adoption and acceptance of this fascinating technology. Accordingly, microfluidic science and technology can provide excellent solutions for bridging the gap between basic research and clinical research. The research gap has led to the emerging field of epidermal microfluidics. Moreover, recent advances in the fabrication of highly flexible and stretchable microfluidic systems have revived the concept of micro elastofluidics, which can provide viable solutions for on-skin wearable biofluid handling. In this context, this review highlights the current state-of-the-art platforms in this field and discusses the potential technologies that can be used for on-skin wearable devices. Toward this aim, we first compare various microfluidic platforms that could be used for on-skin wearable devices. These platforms include semiconductor-based, polymer-based, liquid metal-based, paper-based, and textile-based microfluidics. Next, we discuss how these platforms can enhance the stretchability of on-skin wearable biosensors at the device level. Next, potential microfluidic solutions for collecting, transporting, and controlling the biofluids are discussed. The application of finger-powered micropumps as a viable solution for precise and on-demand biofluid pumping is highlighted. Finally, we present the future directions of this field by emphasizing the applications of droplet-based microfluidics, stretchable continuous-flow micro elastofluidics, stretchable superhydrophobic surfaces, liquid beads as a form of digital micro elastofluidics, and topological liquid diodes that received less attention but have enormous potential to be integrated into on-skin wearable devices.

Engineered Switchable-Wettability Surfaces for Multi-Path Directional Transportation of Droplets and Subaqueous Bubbles

Conventional engineered surfaces for fluid manipulation are hindered by the set wettability, and thus they can only achieve spontaneous transport of single-phase fluid, namely liquid or gas. Moreover, fluid transport systems that are robust to path defects have yet to be fully explored. Here, unprecedentedly, a universal wettability switching strategy is developed for achieving programmable directional transport of both droplets and subaqueous bubbles on a dumbbell-patterned functional surface (DPFS), featuring in strong robustness, high efficiency, and effective cost. By tuning the superwettability of DPFS through octadecyltrichlorosilane treatment and ultraviolet-C selective irradiation, the transport fluid can alternate between liquid and gas. The material's switchable superwettability regulates the fluid directed dynamics within the confined pattern, in which the sustaining fluid propelling relies on the surface energy difference between the starting and terminal sites. This enables the construction of multiple channels, which works synergistically with ultralow-volume-loss transport to impart the fluidic system with strong robustness against path defects. Underlying the completion of complex microfluidics tasks, spatially-selective cooling devices and subaqueous gas microreactors are successfully demonstrated. This energy-consumption-free fluid transport system opens a new avenue for on-chip programmable fluid manipulation, promoting innovative applications requiring rational control of two-phase fluid transport.

Wetting-State-Induced Turning of Water Droplet Moving Direction on the Surface

The spontaneous directional movement of water droplets on a wedge-shaped groove has gained extensive attention due to the advantage of not requiring energy input and its potential wide applications. However, manipulating the direction of movement of water droplets on a wedge-shaped groove has been not fully achieved, and the fundamental understanding of its underlying mechanism remains unclear. Here, molecular dynamics simulations and theoretical analyses are combined to reveal the mechanism of movement in opposite directions of a water droplet at the same position on the wedge-shaped groove interface. It is shown that the moving direction of the water droplet is related to its wetting state on the surface, i.e., the Wenzel and the Cassie states. A water droplet initially in the Wenzel and Cassie states will move toward the diverging and the converging ends, respectively. This phenomenon is attributed to the opposite roles played by the groove substrate and the upper layers in the two wetting states. Moreover, it is found that the water droplet is likely to move faster on a surface with a higher groove, larger opening angle and stronger hydrophobicity. These findings are expected to be of benefit for fully understanding droplet movement and shedding light on the regulation of the direction of movement of the droplets on the groove surface.

Wetting Effect on Patterned Substrates

A droplet deposited on a solid substrate leads to the wetting phenomenon. A natural observation is the lotus effect, known for its superhydrophobicity. This special feature is engendered by the structured microstructure of the lotus leaf, namely, surface heterogeneity, as explained by the quintessential Cassie-Wenzel theory (CWT). In this work, recent designs of functional substrates are overviewed based on the CWT via manipulating the contact area between the liquid and the solid substrate as well as the intrinsic Young's contact angle. Moreover, the limitation of the CWT is discussed. When the droplet size is comparable to the surface heterogeneity, anisotropic wetting morphology often appears, which is beyond the scope of the Cassie-Wenzel work. In this case, several recent studies addressing the anisotropic wetting effect on chemically and mechanically patterned substrates are elucidated. Surface designs for anisotropic wetting morphologies are summarized with respect to the shape and the arrangement of the surface heterogeneity, the droplet volume, the deposition position of the droplet, as well as the mean curvature of the surface heterogeneity. A thermodynamic interpretation for the wetting effect and the corresponding open questions are presented at the end.

Slippery Shape Memory Tube for Smart Droplet Transportation

Recently, research about controllable droplet transportation in tubes has aroused increased interest. However, existing strategies mainly depend on the elastic tube's shape variation that needs constant external stimuli. Meanwhile, these reported tubes are only suitable for wetting liquids. To achieve the transportation of diverse liquids, different coatings are needed to modify the tube's inner surface to realize complete wetting of different liquids. Herein, we advance a design principle by combining a shape memory polymer (SMP) tube and Nepenthes pitcher plant-inspired slippery surface, which can solve the above-mentioned problems. The SMP offers a tunable tube shape owing to its shape memory effect (SME); the slippery surface reduces the adhesion and expands the applicable range of liquids. Transportation of both water and oils in a wide range of surface tension values can be smartly controlled. The results show that not only the transportation speed and direction can be adjusted but also diverse modes including round-trip transportation, segmented transportation, and antigravity transportation can be achieved. Moreover, applications of the tube in batch inspection of different droplets and step-by-step control of multiple microreactions are also displayed. This work reports a strategy for droplet transportation control based on the tube's SME, which initiates some fresh ideas for designing new superwetting materials toward smart liquid transportation.

High-Performance Directional Water Transport Using a Two-Dimensional Periodic Janus Gradient Structure

Numerous materials in micro- or nanoscale hierarchical structures with surface gradients serve as the enablers in directional liquid transportation. However, concurrent high-speed and long-range liquid transport is yet to be fully realized so far. Here, an overall-improved approach is achieved in both water transport distance and velocity aspects using a 2D periodic Janus gradient structure, which is inspired by the Janus-wettable desert beetle back, tapered asymmetric cacti spine, and periodic Nepenthes alata microcavity. This 2D channel can efficiently regulate the kinetics of liquid transport within its confined structure, in which the terminal potential well and periodic Janus topological structure enable sustaining water propelling through a long distance. In addition, the rapidly formed aqueous film facilitates a high initial momentum and fast transport of liquid droplets along the channel, achieving an averaged velocity of over 400 mm s^-1 and a maximum normalized transport distance of 23.4 for a 3 µL droplet, as well as an ultralow liquid volume loss of 6.02% upon high-flux water transport. This scalable, controllable, and easy-fabricable 2D water transport system provides an insightful pathway in realizing high-performance water manipulation and possibly facilitates substantial innovative applications in multidisciplinary fields.

Patterning Wettability for Open-Surface Fluidic Manipulation: Fundamentals and Applications

Effective manipulation of liquids on open surfaces without external energy input is indispensable for the advancement of point-of-care diagnostic devices. Open-surface microfluidics has the potential to benefit health care, especially in the developing world. This review highlights the prospects for harnessing capillary forces on surface-microfluidic platforms, chiefly by inducing smooth gradients or sharp steps of wettability on substrates, to elicit passive liquid transport and higher-order fluidic manipulations without off-the-chip energy sources. A broad spectrum of the recent progress in the emerging field of passive surface microfluidics is highlighted, and its promise for developing facile, low-cost, easy-to-operate microfluidic devices is discussed in light of recent applications, not only in the domain of biomedical microfluidics but also in the general areas of energy and water conservation.

Recent advances of bio-inspired anti-icing surfaces

The need for improved anti-icing surfaces is the demand of the time and closely related to many important aspects of our lives as surface icing threatens not only industrial production but also human safety. Freezing on a cold surface is usually a heterogeneous nucleation process induced by the substrate. Creating an anti-icing surface is mainly achieved by changing surface morphology and chemistry to regulate the interaction between the surface and the water/ice to inhibit freezing on the surface. In this paper, recent research progress in the creation of biomimetic anti-icing surfaces is reviewed. Firstly, basic strategies of bionic anti-icing are introduced, and then bionic anti-icing surface strategies are reviewed according to four aspects: the process of ice formation, including condensate self-removing, inhibiting ice nucleation, reducing ice adhesion, and melting accumulated ice on the surface. The remaining challenges and the direction of future development of biomimetic anti-icing surfaces are also discussed.

Three-Dimensional Open Water Microchannel Transpiration Mimetics

The key problem that hinders the water transportation performance and application of microchannels is the annoying gaslock. Realizing liquid transport without the gaslock requires a specially designed pump and a channel system, as well as the reduction of gas concentration in liquids. In nature, to eat viscous nectar with high efficiency, hummingbirds use their open geometric tongue for nectar-sucking. Inspired by hummingbirds' tongue, we report a bionic open microchannel that discharges unwanted gas inside the microchannel from the opening without influencing its fluidic performance. The opening can also be used for extrusion of oil droplets in microchannels, indicating great potential applications in oil-water separation and chemical slow release, especially for bubble discharge in microchannels. Most significantly, a mimicked "leaf" with our bionic open microchannnels exhibits marvelous "transpiration" performance when irradiated by a laser. Our work provides a new strategy for the fabrication of open microchannels and sheds light on potential applications of multiphase phenomena in microchannels including oil-water separation, phase change heat and mass transfer, solar vapor generation, and precisely controllable drug delivery.

Adhesion behaviors of water droplets on bioinspired superhydrophobic surfaces

The adhesion behaviors of droplets on surfaces are attracting increasing attention due to their various applications. Many bioinspired superhydrophobic surfaces with different adhesion states have been constructed in order to mimic the functions of natural surfaces such as a lotus leaf, a rose petal, butterfly wings, etc. In this review, we first present a brief introduction to the fundamental theories of the adhesion behaviors of droplets on various surfaces, including low adhesion, high adhesion and anisotropic adhesion states. Then, different techniques to characterize droplet adhesion on these surfaces, including the rotating disk technique, the atomic force microscope cantilever technique, and capillary sensor-based techniques, are described. Wetting behaviors, and the switching between different adhesion states on bioinspired surfaces, are also summarized and discussed. Subsequently, the diverse applications of bioinspired surfaces, including water collection, liquid transport, drag reduction, and oil/water separation, are discussed. Finally, the challenges of using liquid adhesion behaviors on various surfaces, and future applications of these surfaces, are discussed.

Unidirectional self-actuation transport of a liquid metal nanodroplet in a two-plate confinement microchannel

Controllable directional transport of a liquid metal nanodroplet in a microchannel has been a challenge in the field of nanosensors, nanofluidics, and nanofabrication. In this paper, we report a novel design that the self-actuation of a gallium nanodroplet in a two-plate confinement microchannel could be achieved via a continuous wetting gradient. More importantly, suitable channel parameters could be used to manipulate the dynamic behavior of the gallium nanodroplet. The self-actuation transport in the two-plate confinement microchannel is the result of the competition between the driving force from the difference of the Laplace pressure and energy dissipation from the viscous resistance. Furthermore, we have identified the conditions to assess whether the droplet will pass through the contractive cross-section or not. This work can provide guidance for manipulating liquid metal nanodroplets in microchannels.

A confined-etching strategy for intrinsic anisotropic surface wetting patterning

Anisotropic functional patterned surfaces have shown significant applications in microfluidics, biomedicine and optoelectronics. However, surface patterning relies heavily on high-end apparatuses and expensive moulds/masks and photoresists. Decomposition behaviors of polymers have been widely studied in material science, but as-created chemical and physical structural changes have been rarely considered as an opportunity for wettability manipulation. Here, a facile mask-free confined-etching strategy is reported for intrinsic wettable surface patterning. With printing technology, the surface wetting state is regulated, enabling the chemical etching of setting locations and efficient fabrication of complex patterns. Notably, the created anisotropic patterns can be used for realizing water-responsive information storage and encryption as well as fabricating flexible electrodes. Featuring advantages of simple operation and economic friendliness, this patterning approach brings a bright prospect in developing functional materials with versatile applications.

Anisotropic functional patterned surfaces have shown significant applications in microfluidics, biomedicine, and optoelectronics. Here, authors demonstrate a fast and mask-free etching method for accurate surface patterning by confined decomposition, enabling the efficient fabrication of complex patterns.

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.

Polygonal non-wetting droplets on microtextured surfaces

Understanding the interactions between liquids and solids is important for many areas of science and technology. Microtextured surfaces have been extensively studied in microfluidics, DNA technologies, and micro-manufacturing. For these applications, the ability to precisely control the shape, size and location of the liquid via textured surfaces is of particular importance for the design of fluidic-based systems. However, this has been passively realized in the wetting state thanks to the pinning of the contact line, leaving the non-wetting counterpart challenging due to the low liquid affinity. In this work, confinement is imposed on droplets located on well-designed shapes and arrangements of microtextured surfaces. An active way to shape non-wetting water and liquid metal droplets into various polygons ranging from triangles, squares, rectangles, to hexagons is developed. The results suggest that energy barriers in different directions account for the movement of the contact lines and the formation of polygonal shapes. By characterizing the curvature of the liquid-vapour meniscus, the morphology of the droplet is correlated to its volume, thickness, and contact angle. The developed liquid-based patterning strategy under active regulation with low adhesion looks promising for low-cost micromanufacturing technology, DNA microarrays, and digital lab-on-a-chip.

Exploring the interactions between liquids and solids is critical for improving control over fluidic systems. Here, authors develop an active way to tailor various polygonal shapes of non-wetting droplet on microtextured surfaces, resulting from the anisotropic energy barriers of the contact line.

Locust-Inspired Direction-Dependent Transport Based on a Magnetic-Responsive Asymmetric-Microplate-Arrayed Surface

Inspired by the highly efficient jumping mechanism of locusts, a magnetic-responsive asymmetric-microplate-arrayed surface is designed. Elastic energy can be stored in the microplate and rapidly released by loading and removing a magnetic field. Similar to the bouncing behavior of the locust, objects deposited on the surface of the microplate-arrayed surface will bounce suddenly. It is found that the continuous transport behavior can be induced in the moving magnetic field and the direction-dependent transport is well achieved by preparing the secondary microstructure. The results show that both the weight and transport velocity of the transported object in the forward transport direction are much greater than those in the reverse transport direction. Furthermore, the anisotropic transport property can be strengthened with the increase of the height of the secondary structure. Such surfaces can transport objects with either soft or hard stiffness, as well as objects with different geometric configurations, and the transport path can be arbitrarily programmed. Based on the transport mechanism, a flexible microconvey belt is further designed, which can transport objects in any controlled direction. Such a simple technique can provide new design ideas for directional microtransport requirements.

Precise Controlling of Friction and Adhesion on Reprogrammable Shape Memory Micropillars

Microstructured surfaces with stimuli-responsive performances have aroused great attention in recent years, but it still remains a significant challenge to endow surfaces with precisely controlled morphological changes in microstructures, so as to get the precise control of regional properties (e.g., friction, adhesion). Herein, a kind of carbonyl iron particle-doped shape memory polyurethane micropillar with precisely controllable morphological changes is realized, upon remote near-infrared light (NIR) irradiation. Owing to the reversible transition of micropillars between bent and upright states, the micro-structured surface exhibits precisely controllable low-to-high friction transitions, together with the changes of friction coefficient ranging from ∼0.8 to ∼1.2. Hence, the changes of the surface friction even within an extremely small area can be precisely targeted, under local NIR laser irradiation. Moreover, the water droplet adhesion force of the surface can be reversibly switched between ∼160 and ∼760 μN, demonstrating the application potential in precisely controllable wettability. These features indicate that the smart stimuli-responsive micropillar arrays would be amenable to a variety of applications that require remote, selective, and on-demand responses, such as a refreshable Braille display system, micro-particle motion control, lab-on-a-chip, and microfluidics.

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.

Enhanced Flexibility of the Segmented Honey Bee Tongue with Hydrophobic Tongue Hairs

Fibrous surfaces in nature have already exhibited excellent functions that are normally ascribed to the synergistic effects of special structures and material properties. The honey bee tongue, foraging liquid food in nature, has a unique segmented surface covered with dense hairs. Since honey bees are capable of using their tongue to adapt to possibly the broadest range of feeding environments to exploit every possible source of liquids, the surface properties of the tongue, especially the covering hairs, would likely represent an evolutionary optimization. In this paper, we show that their tongue hairs are stiff and hydrophobic, the latter of which is highly unexpected as the structure is designed for liquid capturing. We found that such hydrophobicity can prevent those stiff hairs from being adhered to the soft tongue surface, which could significantly enhance the deformability of the tongue when honey bees feed at various surfaces and promote their adaptability to different environments. These findings bridge the relationship between surface wettability and structural characteristics, which may shed new light on designing flexible microstructured fiber systems to transport viscous liquids.

A Biocompatible Vibration-Actuated Omni-Droplets Rectifier with Large Volume Range Fabricated by Femtosecond Laser

High-performance droplet transport is crucial for diverse applications including biomedical detection, chemical micro-reaction, and droplet microfluidics. Despite extensive progress, traditional passive and active strategies are restricted to limited liquid types, small droplet volume ranges, and poor biocompatibilities. Moreover, more challenges occur for biological fluids due to large viscosity and low surface tension. Here, a vibration-actuated omni-droplets rectifier (VAODR) consisting of slippery ratchet arrays fabricated by femtosecond laser and vibration platforms is reported. Through the relative competition between the asymmetric adhesive resistance originating from the lubricant meniscus on the VAODR and the periodic inertial driving force originating from isotropic vibration, the fast (up to ≈60 mm s^-1 ), programmable, and robust transport of droplets is achieved for a large volume range (0.05-2000 µL, V_max /V_min  ≈ 40 000) and in various transport modes including transport of liquid slugs in tubes, programmable and sequential transport, and bidirectional transport. This VAODR is general to a high diversity of biological and medical fluids, and thus can be used for biomedical detection including ABO blood-group tests and anticancer drugs screening. These strategies provide a complementary and promising platform for maneuvering omni-droplets that are fundamental to biomedical applications and other high-throughput omni-droplet operation fields.

Multi-Bioinspired Janus Copper Mesh for Improved Gravity-Irrelevant Directional Water Droplet and Flow Transport

A conceptually novel multi-bioinspired strategy based on structures and functions derived from the Namib desert beetle and lotus leaf is proposed in this paper. The proposed scheme synergistically combines the features of alternating wettability patterns and asymmetric wettability for improved directional water transport. Consequently, a Janus copper mesh, which substantially outperforms other single-bioinspired synthetic materials, is produced. The Janus copper mesh achieves directional self-transportation of tiny water droplets and continuous water flow in a gravity-irrelevant or an anti-gravity manner without energy consumption. This depends on the asymmetric wettability and alternating hydrophobic-hydrophilic wettability patterns on the hydrophobic surface of the mesh. In particular, Janus copper shows remarkable selective directional water transport in a water-oil system, rendering it a promising candidate for practical applications.

Underoil Directional Self-Transportation of Water Droplets on a TiO2-Coated Conical Spine

Directional self-transportation of tiny droplets is significant in many fields. However, almost all existing studies focus on the phenomenon in air, and to realize similar performance in complex environments, such as oil, is still extremely rare. Here, we report a TiO₂-coated conical spine (TCS) and demonstrate underoil directional self-transportation of water droplets on its surface. It is found that high surface hydrophilicity resulting from UV irradiation is necessary to achieve the self-transportation of water in oil. The critical water contact angle in oil is about 57°, and the maximal transport velocity can reach 1.4 mm/s. Mechanism analysis reveals that the excellent self-transportation property is ascribed to the combined effect between the Laplace force (F_L) caused by the conical gradient structure and the hysteresis reduction resulting from the high hydrophilicity. Moreover, based on the special underoil self-transportation performance, a droplet-based microreaction and demulsification of water-in-oil emulsions were demonstrated using the TCS. This work reports the self-transportation of water in oil, which could provide some fresh ideas for designing new superwetting self-transportation materials.

Turning traditionally nonwetting surfaces wetting for even ultra-high surface energy liquids

We present a surface-engineering approach that turns all liquids highly wetting, including ultra-high surface tension fluids such as mercury. Previously, highly wetting behavior was only possible for intrinsically wetting liquid/material combinations through surface roughening to enable the so-called Wenzel and hemiwicking states, in which liquid fills the surface structures and causes a droplet to exhibit a low contact angle when contacting the surface. Here, we show that roughness made of reentrant structures allows for a metastable hemiwicking state even for nonwetting liquids. Our surface energy model reveals that with liquid filled in the structure, the reentrant feature creates a local energy barrier, which prevents liquid depletion from surface structures regardless of the intrinsic wettability. We experimentally demonstrated this concept with microfabricated reentrant channels. Notably, we show an apparent contact angle as low as 35° for mercury on structured silicon surfaces with fluorinated coatings, on which the intrinsic contact angle of mercury is 143°, turning a highly nonwetting liquid/material combination highly wetting through surface engineering. Our work enables highly wetting behavior for previously inaccessible material/liquid combinations and thus expands the design space for various thermofluidic applications.