Directional pumping of water and oil microdroplets on slippery surface

Liquid-like Surface Chemistry Meets Structured Textures: A Synergistic Approach to Advanced Repellent Materials

Liquid-repellent surfaces have advanced significantly over two decades. While super-liquid-repellent surfaces with micro/nano-textures dominate the field, liquid-like smooth surfaces (LLSS) grafted with highly flexible molecule chains offer a compelling alternative, enabling near-ideal dynamic droplet repellency with ultralow contact angle hysteresis (CAH). Prior LLSS studies have focused on optimizing molecular structures, grafting densities, and mechanical stability, enabling applications in anti-fouling, liquid harvesting, and drag reduction. However, innovation challenges and performance bottlenecks hinder practical scalability. This review highlights a transformative approach developed in recent years: integrating liquid-like surface chemistry with structured surfaces to overcome existing limitations. We outline the key requirements for achieving liquid-like surfaces, their structure-related features and unique interface properties including low CAH, reduced adhesion, enhanced slippage, and nucleation inhibition. By synergizing liquid-like chemistry and surface textures, we categorize pioneering works into application-driven areas such as microscopic residue suppression, enhanced droplet mobility, optimized membrane separation, sustainable fabrics and condensation heat transfer. This composite strategy not only deepens fundamental understanding of liquid-like wetting mechanisms but also broadens real-world applicability. We conclude with perspectives on future challenges and opportunities, positioning this promising material system as a frontier in functional interfacial materials.

Material Design for Durable Lubricant-Infused Surfaces That Can Reduce Liquid and Solid Fouling

Liquid and solid fouling is a pervasive problem in numerous natural and industrial settings, significantly impacting energy efficiency, greenhouse emissions, operational costs, equipment lifespan, and human health. Inspired by pitcher plants, recently developed lubricant-infused surfaces (LISs) demonstrate resistance to both liquid and solid accretion under diverse environmental conditions, offering a potential solution to combat various foulants such as ice, bacteria, and mineral deposits. However, the commercial viability for most fouling-resistant LISs has thus far been compromised due to the challenges associated with maintaining a stable lubricant layer during operation. This review aims to address this important concern by providing systematic material design guidelines for fabricating durable LISs. We discuss fundamental design principles, methods for evaluating fouling resistance, and strategies to prevent lubricant loss. By presenting a comprehensive design methodology for this important class of materials, this review aims to aid future advancements in the field of antifouling surfaces, potentially impacting a variety of industries ranging from marine engineering to medical device manufacturing.

Advances in small droplets manipulation on bio-inspired slippery surfaces: chances and challenges

The manipulation of droplets with non-destructive, efficient, and high-precision features is of great importance in several fields, including microfluidics and biomedicine. The lubrication layer of bioinspired slippery surfaces demonstrates remarkable stability and self-restoration capabilities when subjected to external perturbations. Consequently, research into the manipulation of droplets on slippery surfaces has continued to make progress. This paper presents a review of the methods of droplet manipulation on bioinspired slippery surfaces. It begins by outlining the basic theory of slippery surfaces and the mechanism of droplet motion on slippery surfaces. Furthermore, droplet manipulation methods on slippery surfaces are classified into active and passive approaches based on the presence of external stimuli (e.g., heat, light, electricity, and magnetism). Finally, an outlook is provided on the current challenges facing droplet manipulation on slippery surfaces, and potential solution ideas are presented.

3D-printed spines for programmable liquid topographies and micromanipulation

Manipulating floating objects, whether solid or liquid, from microscopic to mesoscopic sizes, is crucial in various microfluidics and microfabrication applications. While capillary menisci naturally self-assemble and transport floating particles, their shapes and sizes are limited by the properties of the fluid and the objects involved. We herein harness the superposition of capillary menisci to curve liquid interfaces controllably. By using 3D-printed spines piercing the interface, we can finely adjust height gradients across the liquid surface to create specific liquid topographies. Thus, our method becomes a powerful tool for manipulating floating objects into programmable paths. Combining experimental demonstrations, numerical simulations, and theoretical modeling, we study the liquid elevation created by specific spine dispositions and the three-dimensional manipulation of submillimetric particles. Multiple examples showcase the method's potential applications, including sorting and capturing particles, which could pave the way for cleaning fluid interfaces.

Compositional-Asymmetry-Induced Transition of Directional Liquid Transport on Tilted and Janusian Nanohair Arrays

Anisotropic wetting on certain surfaces endowed with structural asymmetry or compositional gradients commonly impedes the directional adjustment of liquid transport. We report here that directional liquid transport (DLT) against the tilt direction of nanohair and in the reverse direction was achieved on tilted-nanohair arrays (TNAs) and tilted-Janusian-nanohair arrays (TJNAs), respectively. Janusian compositional asymmetry on the surface of TJNAs was created by plasma polymer deposition on structurally asymmetric TNAs previously fabricated by Faraday-cage-assisted plasma nanotexturing. The structurally asymmetric TNAs led to DLT against the tilting direction due to the asymmetric wetting under the capillary imbibition between tilted nanohairs and the preferential coalescence of liquid against the tilt direction. The Janusian compositional asymmetry of TJNAs changing the capillarity imbibition condition between tilted nanohairs resulted in the transition of the liquid spreading direction along the tilt direction. The spreading direction along and against the tilt direction is predicted through a comprehensive analysis of the structural and compositional asymmetries of the TNAs and TJNAs.

Superwetting Gels: Wetting Principles, Applications, and Challenges

Along with the in-depth understanding of wetting behaviors in nature, superwetting gels have received a lot of attention in the past decade. The viscoelasticity of gel materials makes wetting characteristics different from those of rigid materials and brings diverse functionality. In this Review, we summarize the current progress in principles of gel wettability from two aspects: wetting on gels and wetting of gels. Distinct from rigid substrates, the viscoelasticity and solid-liquid coexistence of gel materials introduce additional factors, including surface tension and deformation, resulting in various wetting phenomena. Besides, the similarity between gels and tissues broadens its applications in biomedical devices and smart interfacial regulation. We further conclude the current application that utilizes superwetting gels. Finally, we provide our perspective for future research directions.

Bioinspired Electrostatic Capture-and-Release System for Precise Microdroplet Manipulation

The application of physical fields is crucial for droplet generation and manipulation, underpinning technologies like printing, microfluidic biochips, drug delivery, and flexible sensors. Despite advancements, precise micro/nanoscale droplet generation and accurate microfluidic reactions remain challenging. Inspired by the liquid ejection mechanisms in microscopic organisms, an electrostatic manipulator for the precise capture, emission, and transport of microdroplets is proposed. This approach enables the controlled and periodic emission of nanoscale daughter droplets from microscale parent droplets, achieved through dielectric pinning on surfaces and electrostatic field-driven forces. Results show precise nanoscale droplet release on inert polymer surfaces, enabling directional, contamination-free liquid manipulation. Moreover, leveraging surface treatment techniques and robust electrostatic force-driven transportation, a versatile strategy for droplet generation and manipulation, spanning from microfluidic devices to chemical reaction operations. The novel droplet manipulation phenomena and control strategies can advance the fields of electrostatic-based microfluidics, materials fabrication, and beyond.

Reversible Lubricating Layer for Improving the Endoscope Field with High Transparency and Antipollution Property

Endoscopes, a minimally invasive medical tool, are susceptible to impaired visibility due to the adhesion of biological fluids. However, traditional self-cleaning coatings face limitations in terms of transparency and sustainability, making it difficult to apply them to lenses. Inspired by the phospholipid layer of the eye, a reversible lubricating layer (RL-layer) with low-adhesion and high-transparency properties is reported. Even coagulated blood can be easily removed due to the low surface tension and easy swing of the brushes. In addition, the silicon-oxygen bonds can be cleaved by fluoride ions, thereby achieving reversible modification and showing potential in sustainable usage. To prove the concept, biomimetic endoscopic lenses are employed in vivo experiments. The results indicate that it consistently maintains a clear vision in narrow spaces containing mucus and exhibits excellent biocompatibility. The reversible lubricating layer provides valuable reference significance for self-cleaning coatings in other fields.

Photothermal-Driven Droplet Manipulation: A Perspective

Optofluidics, which utilizes the interactions between light and fluids to realize various functions, has garnered increasing attention owing to the advantages of operational simplicity, exceptional flexibility, rapid response, etc. As one of the typical light-fluid interactions, the localized photothermal effect serving as a stimulus has been widely used for fluid manipulation. Particularly, significant progress on photothermal-driven droplet manipulation has been made. In this perspective, recent advancements in localized photothermal effect driven droplet manipulation are summarized. First, the photothermal manipulation of droplets on open surfaces is outlined. An attractive droplet manipulation of light droplet levitation above the gas-liquid interface via localized photothermal effect is then discussed. Besides, the photothermal-driven manipulation of droplets in an immiscible liquid phase is also discussed. Although promising, further development of photothermal-driven droplet manipulation is still needed. The challenges and perspectives of this light droplet manipulation strategy for broad implementation are summarized, which will help future studies and applications.

Fluid Unidirectional Transport Induced by Structure and Ambient Elements across Porous Materials: From Principles to Applications

Spontaneous or nonspontaneous unidirectional fluid transport across multidimension can occur under specific structural designs and ambient elements for porous materials. While existing reviews have extensively summarized unidirectional fluid transport on surfaces, there is an absence of literature summarizing fluid's unidirectional transport across porous materials. This review introduces wetting phenomena observed on natural biological surfaces or porous structures. Subsequently, it offers an overview of diverse principles and potential applications in this field, emphasizing various physical and chemical structural designs (surface energy, capillary size, topographic curvature) and ambient elements (underwater, under oil, pressure, and solar energy). Applications encompass moisture-wicking fabric, sensors, skincare, fog collection, oil-water separation, electrochemistry, liquid-based gating, and solar evaporators. Additionally, significant principles and formulas from various studies are compelled to offer readers valuable references. Simultaneously, potential advantages and challenges are critically assessed in these applications and the perspectives are presented.

Janus Flexible Device with Microcone Channels for Sampling and Analysis of Biological Microfluidics

Controlling the spontaneous directional transport of droplets plays an important role in the application of microchemical reactions and microdroplet detection. Although the relevant technologies have been widely studied, the existing spontaneous droplet transport strategies still face problems of complex structure, single function, and poor flexibility. Inspired by the spontaneous droplet transport strategy in nature, an asymmetric wettability surface with microcone channels (AWS-MC) is prepared on a flexible fabric by combining surface modification and femtosecond laser manufacturing technology. On this surface, the capillary force and Laplace pressure induced by the wettability gradient and the geometric structure gradient drive the droplet transport from the hydrophobic surface to the hydrophilic surface. Notably, droplets in adjacent hydrophilic regions do not exchange substances even if the gap in the hydrophilic region is only 1 mm, which provides an ideal platform for numerous detections by a single drop. The droplet transport strategy does not require external energy and can adapt to the manipulation of various droplet types. Application of this surface in the blood of organisms is demonstrated. This work provides an effective method for microdroplet-directed self-transport and microdroplet detection.

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.

Droplet Manipulation on Bioinspired Slippery Surfaces: From Design Principle to Biomedical Applications

Droplet manipulation with high efficiency, high flexibility, and programmability, is essential for various applications in biomedical sciences and engineering. Bioinspired liquid-infused slippery surfaces (LIS), with exceptional interfacial properties, have led to expanding research for droplet manipulation. In this review, an overview of actuation principles is presented to illustrate how materials or systems can be designed for droplet manipulation on LIS. Recent progress on new manipulation methods on LIS is also summarized and their prospective applications in anti-biofouling and pathogen control, biosensing, and the development of digital microfluidics are presented. Finally, an outlook is made on the key challenges and opportunities for droplet manipulation on LIS.

Efficient fabrication of bioinspired soft, ridged-slippery surfaces with large-range anisotropic wettability for droplet manipulation

The droplet lossless directional motion control on slippery surfaces holds immense promise for applications in microfluidic chips, hazardous substance detection, chemical dispensing, etc. However, a significant challenge in this domain lies in efficiently developing soft, slippery surfaces with large-range anisotropic wettability and compatibility for curved scenarios. This study addressed this challenge through a quick 3D printing-assisted method to produce soft, ridged-slippery surfaces (SRSSs) as the droplet manipulation platform. The SRSSs demonstrated substantial anisotropic rolling resistances, measuring 116.9 μN in the perpendicular direction and 7.7 μN in the parallel direction, exhibiting a ratio of 15.2. Combining several extents of anisotropic wettability on a soft substrate could realize diverse reagent manipulation functions. Furthermore, these SRSSs showcased high compatibility with various droplet constituents, impressive liquid impact resistance, self-repair capability, and mechanical durability and thermal durability, ensuring exceptional applicability. As proofs of concept, the SRSSs were successfully applied in droplet control and classification for heavy metal ion detection, mechanical arm-based droplet grab and release, and cross-species transport, showcasing their remarkable versatility, compatibility, and practicality in advanced droplet microfluidic chips and water harvesting applications.

On-chip droplet analysis and cell spheroid screening by capillary wrapping enabled shape-adaptive ferrofluid transporters

Non-invasive droplet manipulation with no physical damage to the sample is important for the practical value of manipulation tools in multidisciplinary applications from biochemical analysis and diagnostics to cell engineering. It is a challenge to achieve this for most existing photothermal, electric stimuli, and magnetic field-based technologies. Herein, we present a droplet handling toolbox, the ferrofluid transporter, for non-invasive droplet manipulation in an oil environment. It involves the transport of droplets with high robustness and efficiency owing to low interfacial friction. This capability caters to various scenarios including droplets with varying components and solid cargo. Moreover, we fabricated a droplet array by transporter positioning and achieved droplet gating and sorting for complex manipulation in the droplet array. Benefiting from the ease of scale-up and high biocompatibility, the transporter-based droplet array can serve as a digital microfluidic platform for on-chip droplet-based bioanalysis, cell spheroid culture, and downstream drug screening tests.

Emergent Collective Motion of Self-Propelled Condensate Droplets

Recently, there is much interest in droplet condensation on soft or liquid or liquidlike substrates. Droplets can deform soft and liquid interfaces resulting in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, interdroplet attraction). Here, we describe a unique collective motion of condensate water droplets that emerges spontaneously when a solid substrate is covered with a thin oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. We show that this self-propulsion (with speeds in the 0.1-1  mm s^{-1} range) is fueled by the interfacial energy release upon merging with newly condensed but much smaller droplets. The resultant collective motion spans multiple length scales from submillimeter to several centimeters, with potentially important heat-transfer and water-harvesting applications.

Multifunctional droplet handling on surface-charge-graphic-decorated porous papers

Developing a fluidic platform that combines high-throughput with reconfigurability is essential for a wide range of cutting-edge applications, but achieving both capabilities simultaneously remains a significant challenge. Herein, we propose a novel and unique method for droplet manipulation via drawing surface charge graphics on electrode-free papers in a contactless way. We find that opposite charge graphics can be written and retained on the surface layer of porous insulating paper by a controlled charge depositing method. The retained charge graphics result in high-resolution patterning of electrostatic potential wells (EPWs) on the hydrophobic porous surface, allowing for digital and high-throughput droplet handling. Since the charge graphics can be written/projected dynamically and simultaneously in large areas, allowing for on-demand and real-time reconfiguration of EPWs, we are able to develop a charge-graphic fluidic platform with both high reconfigurability and high throughput. The advantages and application potential of the platform have been demonstrated in chemical detection and dynamically controllable fluidic networks.

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.

Pumping and sliding of droplets steered by a hydrogel pattern for atmospheric water harvesting

Atmospheric water harvesting is an emerging strategy for decentralized and potable water supplies. However, water nucleation and microdroplet coalescence on condensing surfaces often result in surface flooding owing to the lack of a sufficient directional driving force for shedding. Herein, inspired by the fascinating properties of lizards and catfish, we present a condensing surface with engineered hydrogel patterns that enable rapid and sustainable water harvesting through the directional pumping and drag-reduced sliding of water droplets. The movement of microscale condensed droplets is synergistically driven by the surface energy gradient and difference in Laplace pressure induced by the arch hydrogel patterns. Meanwhile, the superhydrophilic hydrogel surface can strongly bond inner-layer water molecules to form a lubricant film that reduces drag and facilitates the sliding of droplets off the condensing surface. Thus, this strategy is promising for various water purification techniques based on liquid-vapor phase-change processes.

Overview of the design of bionic fine hierarchical structures for fog collection

Nature always uses its special wisdom to construct elegant and suitable schemes. Consequently, organisms in the flora and fauna are endowed with fine hierarchical structures (HS) to adapt to the harsh environment due to many years of evolution. Water is one of the most important resources; however, easy access to it is one the biggest challenges faced by human beings. In this case, fog collection (FC) is considered an efficient method to collect water, where bionic HS can be the bridge to efficiently facilitate the process of the FC. In this review, firstly, we discuss the basic principles of FC. Secondly, the role of HS in FC is analyzed in terms of the microstructure of typical examples of plants and animals. Simultaneously, the water-harvesting function of HS in a relatively new organism, fungal filament, is also presented. Thirdly, the HS design in each representative work is analyzed from a biomimetic perspective (single to multiple biomimetic approaches). The role of HS in FC, and then the FC performance of each work are analyzed in order of spatial dimension from a bionic perspective. Finally, the challenges at this stage and the outlook for the future are presented.

Robust reverse-electrowetting based energy harvesting on slippery surface

Reversed-electrowetting based droplet electricity generator (REWOD-DEG) shows merits in high power densities, tunable output formats, and wide adaptability to diverse mechanical energies. However, the surface charge trapping and dielectric failure, which are also common challenges for electrowetting system, hinders the development of reliable REWOD-DEGs for long-term running. We innovatively introduce a slippery lubricant-infused porous surface (SLIPS) into REWOD-DEG. Benefits from the significant inhibitory effect for surface charge trapping and ambient contamination, self-healing characteristic given by SLIPS, and robust reversed-electrowetting based energy harvesting were achieved. The SLIPS enhanced REWOD-DEG experienced 100 days of intermittent energy harvesting without deterioration. In addition, the device shows robust performances when exposed to a variety of extreme working conditions, like low temperature, pH, humidity, fouling, and even scratching. This work may address the core application challenges of REWOD based devices, and inspire the development of other robust droplet-based electricity generators.

Droplet transportation on photosensitive lubricant-impregnated slippery surfaces in response to the light induced Marangoni effect and asymmetrical wetting ridges

Flexible control of droplet transportation is crucial in various applications but is constrained by liquid-solid friction. The development of biomimetic lubricant-impregnated slippery surfaces provides a new solution for flexible manipulation of droplet transportation. Herein, a light strategy is reported for flexibly controlling droplet transportation on photosensitive lubricant-impregnated slippery surfaces. Owing to the localized heating effect of a focused laser beam via photothermal conversion, the resultant thermal Marangoni flow and horizontal component of the surface tension associated with the asymmetric wetting ridges are together responsible for actuating droplet transportation. It is found that the asymmetry of the wetting ridge is dominated by the thickness of the infused oil layer, which directly affects the droplet transportation. The feasibility of this light strategy is also demonstrated by uphill movement, droplet coalescence, and chemical reaction. This study provides a new design for potential applications in open droplet microfluidics, analytical chemistry, diagnosis, etc.

Research progress of bionic fog collection surfaces based on special structures from natural organisms

With the increasing shortage of water resources, people are seeking more innovative ways to collect fog to meet the growing need for production and the demand for livelihood. It has been proven that fog collection is efficient for collecting water in dry but foggy areas. As a hot research topic in recent years, bionic surfaces with fog collection functions have attracted widespread attention in practical applications and basic research. By studying natural organisms and bionic surfaces, more avenues are provided for the development of fog collection devices. Firstly, starting from biological prototypes, this article explored the structural characteristics and fog collection mechanisms of natural organisms such as spider silk, desert beetles, cactus, Nepenthes and other animals and plants (Sarracenia, shorebird and wheat awn), revealing the fog collection mechanism of the natural organisms based on microstructures. Secondly, based on the theory of interfacial tension, we would delve into the fog collection function's theoretical basis and wetting model, expounding the fog collection mechanism from a theoretical perspective. Thirdly, a detailed introduction was given to prepare bionic surfaces and recently explore fog collection devices. For bionic surfaces of a single biological prototype, the fog collection efficiency is about 2000-4000 mg cm-2 h-1. For bionic surfaces of multiple biological prototypes, the fog collection efficiency reaches 7000 mg cm-2 h-1. Finally, a critical analysis was conducted on the current challenges and future developments, aiming to promote the next generation of fog collection devices from a scientific perspective from research to practical applications.

Magnetic Janus origami robot for cross-scale droplet omni-manipulation

The versatile manipulation of cross-scale droplets is essential in many fields. Magnetic excitation is widely used for droplet manipulation due to its distinguishing merits. However, facile magnetic actuation strategies are still lacked to realize versatile multiscale droplet manipulation. Here, a type of magnetically actuated Janus origami robot is readily fabricated for versatile cross-scale droplet manipulation including three-dimensional transport, merging, splitting, dispensing and release of daughter droplets, stirring and remote heating. The robot allows untethered droplet manipulation from ~3.2 nL to ~51.14 μL. It enables splitting of droplet, precise dispensing (minimum of ~3.2 nL) and release (minimum of ~30.2 nL) of daughter droplets. The combination of magnetically controlled rotation and photothermal properties further endows the robot with the ability to stir and heat droplets remotely. Finally, the application of the robot in polymerase chain reaction (PCR) is explored. The extraction and purification of nucleic acids can be successfully achieved.

Droplet attraction and coalescence mechanism on textured oil-impregnated surfaces

Droplets residing on textured oil-impregnated surfaces form a wetting ridge due to the imbalance of interfacial forces at the contact line, leading to a wealth of phenomena not seen on traditional lotus-leaf-inspired non-wetting surfaces. Here, we show that the wetting ridge leads to long-range attraction between millimeter-sized droplets, which coalesce in three distinct stages: droplet attraction, lubricant draining, and droplet merging. Our experiments and model show that the magnitude of the velocity and acceleration at which droplets approach each other horizontally is the same as the vertical oil rise velocity and acceleration in the wetting ridge. Moreover, the droplet coalescence mechanism can be modeled using the classical mass-spring system. The insights gained from this work will inform future fundamental studies on remote droplet interaction on textured oil-impregnated surfaces for optimizing water harvesting and condensation heat transfer.

High-speed directional transport of condensate droplets on superhydrophobic saw-tooth surfaces

HYPOTHESIS

Most droplets on high-efficiency condensing surfaces have radii of less than 100 μm, but conventional droplet transport methods (such as wettability-gradient surfaces and structural-curvature-gradient surfaces) that rely on the unbalanced force of three-phase lines can only transport millimeter-sized droplets efficiently. Regulating high-speed directional transport of condensate droplets is still challenging. Therefore, we present a method for condensate droplet transportation, based on the reaction force of the superhydrophobic saw-tooth surfaces to the liquid bridge, the condensate droplets could be transported at high speed and over long distances.

EXPERIMENTS

The superhydrophobic saw-tooth surfaces are fabricated by femtosecond laser ablation and chemical etching. Condensation experiments and luminescent particle characterization experiments on different surfaces are conducted. Aided by the theoretical analysis, we illustrate the remarkable performance of condensate droplet transportation on saw-tooth surfaces.

FINDINGS

Compared with conventional methods, our method improves the transport velocity and relative transport distance by 1-2 orders of magnitude and achieves directional transport of the smallest condensate droplet of about 2 μm. Furthermore, the superhydrophobic saw-tooth surfaces enable multi-hop directional jumping of condensate droplets, leading to cross-scale increases in transport distances from microns to decimeters.

Active Droplet Transport Induced by Moving Meniscus on a Slippery Magnetic Responsive Micropillar Array

Intelligent droplet manipulation plays a crucial role in both scientific research and industrial technology. Inspired by nature, meniscus driving is an ingenious way to spontaneously transport droplets. However, the shortages of short-range transport and droplet coalescence limit its application. Here, an active droplet manipulation strategy based on the slippery magnetic responsive micropillar array (SMRMA) is reported. With the aid of a magnetic field, the micropillar array bends and induces the infusing oil to form a moving meniscus, which can attract nearby droplets and transport them for a long range. Significantly, clustered droplets on SMRMA can be isolated by micropillars, avoiding droplet coalescence. Moreover, through adjusting the arrangement of the micropillars of SMRMA, multi-functional droplet manipulation such as unidirectional droplet transport, multi-droplet transport, droplet mixing, and droplet screening can be achieved. This work provides a promising approach for intelligent droplet manipulation and unfolds broad application prospects in microfluidics, microchemical reaction, biomedical engineering, and other fields.

A Review of Smart Superwetting Surfaces Based on Shape-Memory Micro/Nanostructures

Bioinspired smart superwetting surfaces with special wettability have aroused great attention from fundamental research to technological applications including self-cleaning, oil-water separation, anti-icing/corrosion/fogging, drag reduction, cell engineering, liquid manipulation, and so on. However, most of the reported smart superwetting surfaces switch their wettability by reversibly changing surface chemistry rather than surface microstructure. Compared with surface chemistry, the regulation of surface microstructure is more difficult and can bring novel functions to the surfaces. As a kind of stimulus-responsive material, shape-memory polymer (SMP) has become an excellent candidate for preparing smart superwetting surfaces owing to its unique shape transformation property. This review systematically summarizes the recent progress of smart superwetting SMP surfaces including fabrication methods, smart superwetting phenomena, and related application fields. The smart superwettabilities, such as superhydrophobicity/superomniphobicity with tunable adhesion, reversible switching between superhydrophobicity and superhydrophilicity, switchable isotropic/anisotropic wetting, slippery surface with tunable wettability, and underwater superaerophobicity/superoleophobicity with tunable adhesion, can be obtained on SMP micro/nanostructures by regulating the surface morphology. Finally, the challenges and future prospects of smart superwetting SMP surfaces are discussed.

Long-Term Retention Microbubbles with Three-Layer Structure for Floating Intravesical Instillation Delivery

Intravesical instillation is an effective treatment for bladder cancer. However, clinical anticancer agents always suffer rapid excretion by periodic urination, leading to low therapeutic efficacy. Prolonging the retention time of drugs in the bladder is the key challenge for intravesical instillation treatment. Herein, a facile and powerful surface cross-linking-freeze drying strategy is proposed to generate ultra-stable albumin bovine air microbubbles (BSA-MBs) that can float and adhere to the bladder wall to overcome the excretion of urination and exhibit a remarkable property of long-term retention in the bladder. More noteworthy, BSA-MBs are endowed with a specific three-layer structure, namely, the outer membrane, middle drug loading layer and inner air core, which makes them have a low density to easily float and possess a high drug loading capacity. Based on their unique superiorities, the therapeutic potential of doxorubicin (DOX)-loaded BSA-MBs (DOX-MBs) is exemplified by intravesical instillation for bladder cancer. After injection into the bladder, DOX-MBs can remain in the bladder for a long time and sustain the release of DOX in urine, exhibiting potent anticancer efficacy. Consequently, the prolonged retention of BSA-MBs in the bladder renders them as an effective floating drug delivery system for intravesical instillation therapy.

Noncontact Charge Shielding Knife for Liquid Microfluidics

Multibehavioral droplet manipulation in a precise and programmed manner is crucial for stoichiometry, biological virus detection, and intelligent lab-on-a-chip. Apart from fundamental navigation, merging, splitting, and dispensing of the droplets are required for being combined in a microfluidic chip as well. Yet, existing active manipulations including strategies from light to magnetism are arduous to use to split liquids on superwetting surfaces without mass loss and contamination, because of the high cohesion and Coanda effect. Here, we demonstrate a charge shielding mechanism (CSM) for platforms to integrate with a series of functions. In response to attachment of shielding layers from the bottom, the instantaneous and repeatable change of local potential on our platform achieves the desired loss-free manipulation of droplets, with a wide-ranging surface tension from 25.7 mN m^-1 to 87.6 mN m^-1, functioning as a noncontact air knife to cleave, guide, rotate, and collect reactive monomers on demand. With further refinement of the surface circuit, the droplets, just as the electron, can be programmed to be transported directionally at extremely high speeds of 100 mm s^-1. This new generation of microfluidics is expected to be applied in the field of bioanalysis, chemical synthesis, and diagnostic kit.

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.

Directional Self-Transportation of Droplets on Superwetting Wedge-Shaped Surface in Air and Underliquid Environments

The directional self-transportation of droplets has aroused great attention in microfluidic systems. However, most reported surfaces are mainly designed for driving water droplets to move in air, displaying low adaptability in complex environments. This work presents a wedge-shaped surface with multiple superwettability, i.e., superhydrophilicity/superoleophilicity and underwater superoleophobicity/underoil superhydrophobicity, fabricated by electrodeposition of a metal-organic framework on a copper sheet. This surface exhibited excellent performance for driving droplet self-transportation, regardless of the droplet type (water or oil) and environmental media (air or underliquids). In air, the wedge-shaped surface with wedge angle of 9.2° could move droplets of water and dodecane up to 24.5 mm and 17.9 mm, respectively. The movement of water droplet under dodecane, however, dropped from 24.5 mm to 22.1 mm, while the dodecane droplet underwater increased from 17.9 mm to 20.3 mm in moving displacement, indicating the underliquid environment is in favor of manipulation of oil droplets. Furthermore, the droplet convergence, transportation, and separation were achieved on the well-designed multiple wedge tracks in air with a total movement distance up to 60.0 mm. The test of micro-oil droplets collecting under water demonstrated that a sponge with two wedges has 2.1 times the oil droplet collection capacity over that of the sponge only, providing a new strategy for efficient treatment of the micro-oil droplets contaminated water.

3D actuation of foam-core liquid metal droplets

Precise manipulation of liquid metal (LM) droplets possesses the potential to enable a wide range of applications in reconfigurable electronics, robotics, and microelectromechanical systems. Although a variety of methods have been explored to actuate LM droplets on a 2D plane, versatile 3D manipulation remains a challenge due to the difficulty in overcoming their heavy weight. Here, foam-core liquid metal (FCLM) droplets that can maintain the surface properties of LM while significantly reducing the density are developed, enabling 3D manipulation in an electrolyte. The FCLM droplet is fabricated by coating LM on the surface of a copper-grafted foam sphere. The actuation of the FCLM droplet is realized by electrically inducing Marangoni flow on the LM surface. Two motion modes of the FCLM droplet are observed and studied and the actuation performance is characterized. Multiple FCLM droplets can be readily controlled to form 3D structures, demonstrating their potential to be further developed to form collaborative robots for enabling wider applications.

Marangoni-induced reversal of meniscus-climbing microdroplets

Small water droplets or particles located at an oil meniscus typically climb the meniscus due to unbalanced capillary forces. Here, we introduce a size-dependent reversal of this meniscus-climbing behavior, where upon cooling of the underlying substrate, droplets of different sizes concurrently ascend and descend the meniscus. We show that microscopic Marangoni convection cells within the oil meniscus are responsible for this phenomenon. While dynamics of relatively larger water microdroplets are still dominated by unbalanced capillary forces and hence ascend the meniscus, smaller droplets are carried by the surface flow and consequently descend the meniscus. We further demonstrate that the magnitude and direction of the convection cells depend on the meniscus geometry and the substrate temperature and introduce a modified Marangoni number that well predicts their strength. Our findings provide a new approach to manipulating droplets on a liquid meniscus that could have applications in material self-assembly, biological sensing and testing, or phase change heat transfer.

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.

Magnetocontrollable droplet mobility on liquid crystal-infused porous surfaces

Magnetocontrollable droplet mobility on surfaces of both solids and simple fluids have been widely used in a wide range of applications. However, little is understood about the effect of the magnetic field on the wettability and mobility of droplets on structured fluids. Here, we report the manipulation of the dynamic behaviors of water droplets on a film of thermotropic liquid crystals (LCs). We find that the static wetting behavior and static friction of water droplets on a 4'-octyl-4-biphenylcarbonitrile (8CB) film strongly depend on the LC mesophases, and that a magnetic field caused no measurable change to these properties. However, we find that the droplet dynamics can be affected by a magnetic field as it slides on a nematic 8CB film, but not on isotropic 8CB, and is dependent on both the direction and strength of the magnetic field. By measuring the dynamic friction of a droplet sliding on a nematic 8CB film, we find that a magnetic field alters the internal orientational ordering of the 8CB which in turn affects its viscosity. We support this interpretation with a scaling argument using the LC magnetic coherence length that includes (i) the elastic energy from the long-range orientational ordering of 8CB and (ii) the free energy from the interaction between 8CB and a magnetic field. Overall, these results advance our understanding of droplet mobility on LC films and enable new designs for responsive surfaces that can manipulate the mobility of water droplets.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (further details of the stability of LCIPS against water-induced dewetting, the interfacial tension and contact angle measurement using a goniometer, the estimation of the thickness of LC wrapping layer at air-water interface on droplets, SEM measurements, the average sliding velocity of a water droplet on 5CB, E7, silicone oil, and mineral oil films with and without a magnetic field, representative force diagram (F _d versus time) of a 3-µL water droplet moving at a speed of 0.1 mm/s on a nematic 8CB film, F _dynamic acting on 3 µL water droplets moving at speeds of 0.1-1 mm/s on an isotropic 8CB film, the calculated magnetic coherence length as a function of the magnitude of the magnetic field applied to the nematic LCIPS, and the apparent advancing and receding contact angles of a moving water droplet on nematic LCIPS as a function of time, and polarized light micrographs (top view) of a nematic 8CB film between two DMOAP-functionalized glass slides before and after applying a horizontal magnetic field) is available in the online version of this article at 10.1007/s12274-022-5318-y.

Magnetocontrollable droplet mobility on liquid crystal-infused porous surfaces

Magnetocontrollable droplet mobility on surfaces of both solids and simple fluids have been widely used in a wide range of applications. However, little is understood about the effect of the magnetic field on the wettability and mobility of droplets on structured fluids. Here, we report the manipulation of the dynamic behaviors of water droplets on a film of thermotropic liquid crystals (LCs). We find that the static wetting behavior and static friction of water droplets on a 4'-octyl-4-biphenylcarbonitrile (8CB) film strongly depend on the LC mesophases, and that a magnetic field caused no measurable change to these properties. However, we find that the droplet dynamics can be affected by a magnetic field as it slides on a nematic 8CB film, but not on isotropic 8CB, and is dependent on both the direction and strength of the magnetic field. By measuring the dynamic friction of a droplet sliding on a nematic 8CB film, we find that a magnetic field alters the internal orientational ordering of the 8CB which in turn affects its viscosity. We support this interpretation with a scaling argument using the LC magnetic coherence length that includes (i) the elastic energy from the long-range orientational ordering of 8CB and (ii) the free energy from the interaction between 8CB and a magnetic field. Overall, these results advance our understanding of droplet mobility on LC films and enable new designs for responsive surfaces that can manipulate the mobility of water droplets.

Electronic Supplementary Material

Supplementary material (further details of the stability of LCIPS against water-induced dewetting, the interfacial tension and contact angle measurement using a goniometer, the estimation of the thickness of LC wrapping layer at air-water interface on droplets, SEM measurements, the average sliding velocity of a water droplet on 5CB, E7, silicone oil, and mineral oil films with and without a magnetic field, representative force diagram (Fd versus time) of a 3-µL water droplet moving at a speed of 0.1 mm/s on a nematic 8CB film, Fdynamic acting on 3 µL water droplets moving at speeds of 0.1-1 mm/s on an isotropic 8CB film, the calculated magnetic coherence length as a function of the magnitude of the magnetic field applied to the nematic LCIPS, and the apparent advancing and receding contact angles of a moving water droplet on nematic LCIPS as a function of time, and polarized light micrographs (top view) of a nematic 8CB film between two DMOAP-functionalized glass slides before and after applying a horizontal magnetic field) is available in the online version of this article at 10.1007/s12274-022-5318-y.

Survival in desert: Extreme water adaptations and bioinspired structural designs

Deserts are the driest places in the world, desert creatures have evolved special adaptations to survive in this extreme water shortage environment. The collection and transport of condensed water have been of particular interest regarding the potential transfer of the underlying mechanisms to technical applications. In this review, the mechanisms of water capture and transport were first summarized. Secondly, an introduction of four typical desert creatures including cactus, desert beetles, lizards, and snakes which have special adaptations to manage water was elaborated. Thirdly, the recent progress of biomimetic water-collecting structures including cactus, desert beetles, and lizards inspired designs and the influence of overflow on water collection was demonstrated. Finally, the conclusions were drawn, and future issues were pointed out. The present study will further promote research on bioinspired water management strategies.

Meniscus-Induced Directional Self-Transport of Submerged Bubbles on a Slippery Oil-Infused Pillar Array with Height-Gradient

Directional manipulation of submerged bubbles is fundamental for both theoretical research and industrial production. However, most current strategies are limited to the upward motion direction, complex surface topography, and additional apparatuses. Here, we report a meniscus-induced self-transport platform, namely, a slippery oil-infused pillar array with height-gradient (SOPAH) by combining femtosecond laser drilling and replica mold technology. Owing to the unbalanced capillary force and Laplace pressure difference, bubbles on SOPAH tend to spontaneously transport along the meniscus gradient toward a higher elevation. The self-transport performances of bubbles near the pillars depend on the complex meniscus shape. Significantly, to understand the underlying transport mechanism, the 3D meniscus profile is simulated by solving the Young-Laplace equation. It is found that the concave valleys formed between the adjacent pillars can change the gradient direction of the meniscus and lead to the varied transport performances. Finally, by taking advantage of a water electrolysis system, the assembled SOPAH serving as a bubble-collecting device is successfully deployed. This work should not only bring new insights into the meniscus-induced self-transport dynamics but also benefit potential applications in the field of intelligent bubble manipulation.

Wetting Ridge-Guided Directional Water Self-Transport

Directional water self-transport plays a crucial role in diverse applications such as biosensing and water harvesting. Despite extensive progress, current strategies for directional water self-transport are restricted to a short self-driving distance, single function, and complicated fabrication methods. Here, a lubricant-infused heterogeneous superwettability surface (LIHSS) for directional water self-transport is proposed on polyimide (PI) film through femtosecond laser direct writing and lubricant infusion. By tuning the parameters of the femtosecond laser, the wettability of PI film can be transformed into superhydrophobic or superhydrophilic. After trapping water droplets on the superhydrophilic surface and depositing excess lubricant, the asymmetrical wetting ridge drives water droplets by an attractive capillary force on the LIHSS. Notably, the maximum droplet self-driving distance can approach ≈3 mm, which is nearly twice as long as the previously reported strategies for direction water self-transport. Significantly, it is demonstrated that this strategy makes it possible to achieve water self-transport, anti-gravity pumping, and chemical microreaction on a tilted LIHSS. This work provides an efficient method to fabricate a promising platform for realizing directional water self-transport.

Judicious Design and Rapid Manufacturing of a Flexible, Mechanically Resistant Liquid-Like Coating with Strong Bonding and Antifouling Abilities

Fluorine-free liquid-repellent coatings have been highly demanded for a variety of applications. However, rapid formation of coatings possessing outstanding oil repellency and strong bonding ability as well as good mechanical strength (e.g., bendability, impact resistance, and scratch resistance) remains a grand challenge. Herein, a robust strategy to rapidly create fluorine-free oil-repellent coatings in only 30 s via rational design of a semi-interpenetrating polymer network structure is reported. The resulting coating manifests strong bonding capability both in air and underwater. More importantly, it not only provides unprecedented oil repellency, even to high-viscosity crude oil, but also achieves both excellent bendability and hardness. This simple yet effective design strategy opens up a new avenue to manufacture multifunctional materials and devices with desirable features and structural complexities for applications in sustainable antifouling, drag reduction, nondestructive transportation, liquid collection, and biomedicine, among other areas.

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.

Universal and Stable Slippery Coatings: Chemical Combination Induced Adhesive-Lubricant Cooperation

Liquid lubricant of low affinity makes slippery coatings widely used in lubricating, anti-biofouling, anti-icing, fluid guiding, and drag reduction. Two critical challenges, however, remain in the practical application of slippery coatings consisting of liquid lubricants: (1) universality regardless of roughness and chemical composition of substrates, (2) stability of surface lubricity against evaporation. Herein, a chemical method is reported to create a universal and stable slippery lubricant-adhesive cooperated coating (SLACC) through acid catalyzed dehydration reaction between the phenolic hydroxyl of polydopamine (PDA), with universal (for challenge-1) and strong (for challenge-2) adhesion properties, and liquid-like polydimethylsiloxane (PDMS), with lubricant properties. Through overlying PDMS on PDA, a spatial gradient interpenetration of chemical combined PDA and PDMS leaving lubricant PDMS at the outermost of coating is achieved. This structure contributes to the following performances of SLACC: nearly universality suitable for 100 different abiotic or biotic substrates and stability sustainable for long-term usages, UV radiating, refrigerating, hot air drying, freeze drying, knife scratch and abrasion. This proposed strategy is envisioned anti-fouling from plane to tube and exhibits drag reduction in confined space.

Light-induced charged slippery surfaces

Slippery lubricant-infused porous (SLIPS) and superhydrophobic surfaces have emerged as promising interfacial materials for various applications such as self-cleaning, anti-icing, and antifouling. Paradoxically, the coverage/screening of lubricant layer on underlying rough matrix endows functionalities impossible on superhydrophobic surfaces; however, the inherent flexibility in programming droplet manipulation through tailoring structure or surface charge gradient in underlying matrix is compromised. Here, we develop a class of slippery material that harnesses the dual advantages of both solid and lubricant. This is achieved by rationally constructing a photothermal-responsive composite matrix with real-time light-induced surface charge regeneration capability, enabling photocontrol of droplets in various working scenarios. We demonstrate that this light-induced charged slippery surface (LICS) exerts photocontrol of droplets with fast speed, long distance, antigravity motion, and directionally collective motion. We further extend the LICS to biomedical domains, ranging from specific morphological hydrogel bead formation in an open environment to biological diagnosis and analysis in closed-channel microfluidics.

An interfacial floating tumbler with a penetrable structure and Janus wettability inspired by Pistia stratiotes

Designing advanced interfacial materials is decisive to the improvement of multiphase systems. Inspired by the superior floatability of Pistia stratiotes, here we present a superhydrophobic/hydrophilic 3D Janus floater with a water managing ability. Its self-regulated floatation mechanism, as well as its water removal logic, should provide insight into the development of multifunctional interfacial carriers in the fields of micro-devices, solar evaporation, etc.

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.

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.

Directional passive transport of nanodroplets on general axisymmetric surfaces

Rapid removal of small-sized droplets passively using fixed structures is a key challenge for various applications including anti-icing, rapid cooling, and water harvesting. In this work, we investigate the directional motion of nanodroplets on axisymmetric surfaces with curvature gradient through molecular dynamics (MD) simulations. It is found that as the shape of the axisymmetric surface is changed from a dome to a trumpet, the droplet velocity is greatly enhanced, by a factor of ∼14. Such an increase is mainly caused by the increment in the driving force. The droplet velocity changes nonlinearly as the surface wettability is varied and assumes the maximum at the contact angle of ∼75°. We derive a formula for the driving force of nanodroplets on general axisymmetric surfaces by evaluating the pressure gradient inside the droplet induced by the curvature gradient. Molecular dynamics simulations are performed to directly measure the driving force and confirm that the theoretical formula works well. By illustrating the reduced initial velocity of droplets as a function of a dimensionless number, which represents the ratio of the driving force to the retentive force due to contact angle hysteresis, we show that the onset of droplet motion on axisymmetric surfaces occurs when the dimensionless number is above a critical value. The dimensionless number reveals the effects of surface geometry, surface wettability, and droplet size on the droplet motion.

Circadian humidity fluctuation induced capillary flow for sustainable mobile energy

Circadian humidity fluctuation is an important factor that affects human life all over the world. Here we show that spherical cap-shaped ionic liquid drops sitting on nanowire array are able to continuously output electricity when exposed to outdoor air, which we attribute to the daily humidity fluctuation induced directional capillary flow. Specifically, ionic liquid drops could absorb/desorb water around the liquid/vapor interface and swell/shrink depending on air humidity fluctuation. While pinning of the drop by nanowire array suppresses advancing/receding of triple-phase contact line. To maintain the surface tension-regulated spherical cap profile, inward/outward flow arises for removing excess fluid from the edge or filling the perimeter with fluid from center. This moisture absorption/desorption-caused capillary flow is confirmed by in-situ microscope imaging. We conduct further research to reveal how environmental humidity affects flow rate and power generation performance. To further illustrate feasibility of our strategy, we combine the generators to light up a red diode and LCD screen. All these results present the great potential of tiny humidity fluctuation as an easily accessible anytime-and-anywhere small-scale green energy resource.

Droplet generators convert mechanical movements of droplets into small-scale electricity. Here, Tang et al. report a humidity-driven power generator by utilizing daily humidity fluctuation in atmosphere enabling continuous generation of electricity upon moisture absorption and desorption cycles.

Manipulation and control of droplets on surfaces in a homogeneous electric field

A method to manipulate and control droplets on a surface is presented. The method is based on inducing electric dipoles inside the droplets using a homogeneous external electric field. It is shown that the repulsive dipole force efficiently suppresses the coalescence of droplets moving on a liquid-infused surface (LIS). Using a combination of experiments, numerical computations and semi-analytical models, the dependence of the repulsion force on the droplet volumes, the distance between the droplets and the electric field strength is revealed. The method allows to suppress coalescence in complex multi-droplet flows and is real-time adaptive. When the electric field strength exceeds a critical value, tip streaming from the droplets sets in. Based on that, it becomes possible to withdraw minute samples from an array of droplets in a parallel process.

Control of droplet coalescence is a major challenge of droplet microfluidics. Here, the authors show that homogenous external electric field can induce dipoles inside droplets, which can be used to withdraw samples from an array of droplets.