Multifunctional Magnetocontrollable Superwettable-Microcilia Surface for Directional Droplet Manipulation

Adaptive Surfaces with Stimuli-Responsive Wettability: From Tailoring to Applications

Adaptive surfaces with tunable wettability have attracted considerable attention due to their increasing importance in adapting to real applications. By incorporation of stimuli-sensitive materials that enable control over surface chemistry or topographical features, or both, a variety of adaptive surfaces are engineered to exhibit reversible tailoring in wettability. This Review provides a comprehensive review of the development of adaptive surfaces with stimuli-responsive wettability. It begins by outlining the background and significance of the adaptive surfaces. Then, this Review delves into the fundamental theories that govern surface wettability, focusing on the influence of external stimuli on wetting behavior. The discussion then shifts to highlighting various triggers, such as magnetism, photo, temperature, pH, electricity, and gas stimuli, that drive response in wettability, as well as surfaces that respond to dual or multiple stimuli. This Review further explores the primary and leading applications in droplet manipulation, oil-water separation, and water harvesting. To conclude, we encapsulate the challenges, potential solutions, and future directions for improving tunable wettability on these surfaces.

Dipolar Brush Polymers: A Numerical Study of the Force Exerted onto a Penetrating Colloidal Particle Under an External Field

Langevin Dynamics numerical simulations have been used to compute the force profiles that dipolar polymer brushes exert onto a penetrating colloidal particle. It has been observed that force profiles are strongly influenced by externally applied fields: at large distances from the grafting surface, a force barrier appears, and at shorter distances a region with lower repulsive forces develops. Furthermore, with the right combination of polymer grafting density, polymer chain length and strength of the external field, it is possible to observe in this intermediate region both the existence of net attractive forces onto the penetrating particle and the emergence of a stationary point. The existence of these regions of low repulsive or net attractive forces inside the dipolar brushes, as well as their dependence on the different parameters of the system can be qualitatively reasoned in terms of a competition between steric repulsion forces and Kelvin forces arising from the dipolar mismatch between different regions of the system. The possibility to tune force profile features such as force barriers and stationary points via an external field paves the way for many potential surface-particle-related applications.

Preparation of Aluminum-Based Dual-Gradient Surfaces for Directional Droplet Transport by Bioinspired Sarracenia Microstructures

Inspired by the ultrafast directional water transport structure of Sarracenia trichomes, hierarchical textured surfaces with specific microgrooves were prepared based on laser processing combined with dip modification, in response to the growing problem of freshwater scarcity. The prepared surfaces were tested for droplet transport behavior to investigate the relationship between the surface structure and the driving force of directional water transport and their effects on the water transport distance and water transport velocity. The results showed that surfaces with a superhydrophobic background associated channels of multirib structures, and a dual-gradient surface of gradient hydrophobic background associated channels with gradient structure performed the best in terms of water transport efficiency. In addition, the water transport process of different samples under Mode II was simulated by CFD, and the dynamic evolution of water film mode formation was obtained to be divided into four phases, including film formation, transition state, mode formation, and stable water transport. This study provides a good reference for the development and preparation of surfaces for directional water transport.

Acoustofluidic Tweezers Integrated with Droplet Sensing Enable Multifunctional Closed-Loop Droplet Manipulation

Droplet manipulation technologies with surface acoustic waves attract significant attention for applications in fluid handling and bioanalysis. However, existing technologies face challenges in automation, precision, and functional integration, limiting broader applications. In this work, a highly integrated droplet-sensing acoustofluidic tweezer is developed, incorporating orthogonally arranged slanted finger interdigital transducers and a custom-designed control and detection circuit system. Using a single acoustic device, this tweezer enables switchable acoustic droplet manipulation and detection, providing multifunctional closed-loop manipulation of on-chip microliter-scale droplets. The platform takes advantage of the wideband frequency response characteristics of the transducers, along with an automated droplet detection algorithm, enabling high-precision detection of central positions, edge positions, contact diameters, and the number of droplets. With this feedback, automated closed-loop control of various droplet manipulation functions, including transportation, merging, mixing, splitting, and internal particle enrichment, is achieved for the first time on a single acoustic platform. This significantly enhances the precision, efficiency, and fault tolerance of the manipulation process. This droplet-sensing acoustofluidic tweezer provides an innovative acoustic solution for droplet manipulation technologies in fields such as fluid processing and biosensing, demonstrating significant application potential.

Superhydrophobic Magnetic-Driven Reactor for Microliter Droplet Reaction Interface Visualization

The development of an efficient, convenient, and cost-effective droplet-driven reactor to observe the reaction microphenomenon is crucial for investigating the chemical reaction and synthesis mechanisms. Herein, an efficient and economical strategy by combining micro-extrusion compression molding (μ-ECM) and surface modification was proposed to fabricate a superhydrophobic magnetic-driven reactor (SMDR) for microliter droplet reaction interface visualization. The wall-like array microstructures with favorable geometric uniformity and the nano-SiO2 coating with uniform dispersion endow the SMDR with robust superhydrophobicity, featuring a contact angle of 159.5 ± 1.0° and a rolling angle of 5.1 ± 0.5°. Due to the uniform dispersion of Fe3O4 in thermoplastic elastomer (TPE), the SMDR possesses sensitive magnetic responsiveness, which can drive droplets to move rapidly, continuously, and losslessly on horizontal and inclined planes, even on a plane with an inclination angle of up to 15°. Interestingly, the SMDR was successfully used to visualize the interface formation and evolution of three simple mixing/reaction processes, which provides a convenient, efficient, and low-cost method for the study of the droplet mixing reaction process and interface visualization.

Reconfigurability-Encoded Hierarchical Rectifiers for Versatile 3D Liquid Manipulation

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

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.

An Active Strategy Based on Different Droplet Removal Modes on Polydimethylsiloxane Magnetic Microstructures

The efficient removal of droplets on solid surfaces holds significant importance in the field of fog collection, condensation heat transfer, and so on. However, on current typical surfaces, droplets are characterized by a passive and single removal mode, contingent on the traction force (e.g., capillary force, Laplace pressure, etc.) generated by the surface's physics and chemistry design, posing challenges for enhancing the efficiency of droplet removal. In this paper, an effective active strategy based on different removal modes is demonstrated on magnetic responsive polydimethylsiloxane (PDMS) superhydrophobic microplates (RM-MPSM). By regulating the parameters of microplates and droplet volume, different effective departure modes (top jumping and side departure) can be induced to facilitate the removal of droplets. Moreover, the removal volume of droplets through the side departure mode exhibits a significant reduction compared to that observed in the top jumping mode. The exceptional removal ability of RM-MPSM demonstrates adaptability to diverse functional applications: efficient fog collection, removal of condensation droplets and micro-particles. The efficient modes of droplet removal demonstrated in this work hold significant implications for broadening its application in many fields, such as droplet collection, heat transfer, and anti-icing.

Enhanced and controlled droplet ejection on magnetic responsive polydimethylsiloxane microarrays

Efficient removal of droplets from solid surfaces is significant in various fields, including fog collection and condensation heat transfer. However, droplets removal on common surfaces with static structures often occurs passively, which limits the possibility of increasing removal efficiency and lacks intelligent controllability. In this paper, an active strategy based on extrusion ejection is proposed and demonstrated on the magnetic responsive polydimethylsiloxane (PDMS) superhydrophobic microplates (MPSM). The MPSM can reversibly transit between the upright and tilted state as the external magnetic field is alternately applied and removed. Under the magnetic field, the direction and trajectories of droplets departure can be intelligently controlled, demonstrating excellent controllability. More importantly, compared with the static structure where the droplet must reach a certain size before departure, droplets can be ejected at smaller sizes as the MPSM is tilted. These advantages are of great significance in many fields, such as a highly efficient fog harvesting system. This strategy of extrusion ejection based on dynamic surface structure control reported in this work may provide fresh ideas for efficient droplet manipulation.

Controllable Manipulation of Large-Volume Droplet on Non-Slippery Surfaces Based on Triboelectric Contactless Charge Injection

Controllable droplet manipulation is crucial in diverse scientific and engineering fields. Traditional electric-based methods usually rely on commercial high-voltage (HV) power sources, which are typically bulky, expensive, and potentially hazardous. The triboelectric nanogenerator (TENG) is a highly studied device that can generate HV output with limited current, showing great potential in droplet manipulation applications. However, current TENG-based approaches usually utilize traditional free-standing TENGs that produce short-pulsed alternating-current signals. This limitation hinders continuous electrostatic forces necessary for precise droplet control, leading to complex circuitry and suboptimal droplet motion control in terms of volume, distance, direction, and momentum. Here, a triboelectric contactless charge injection (TCCI) method employing a novel dual-functional triboelectric nanogenerator (DF-TENG), is proposed. The DF-TENG can produce both high voltage and constant current during unidirectional motion, enabling continuous corona discharges for contactless charge injection into the droplets. Using this method, a large-volume droplet (3000 µL) can be controlled with momentum up to 115.2 g mm s-1, quintupling the highest value recorded by the traditional methods. Moreover, the TCCI method is adaptable for a variety of non-slippery substrates and droplets of different compositions and viscosities, which makes it an ideal manipulation strategy for droplet transport, chemical reactions, and even driving solids.

Towards high performance and durable soft tactile actuators

Soft actuators are gaining significant attention due to their ability to provide realistic tactile sensations in various applications. However, their soft nature makes them vulnerable to damage from external factors, limiting actuation stability and device lifespan. The susceptibility to damage becomes higher with these actuators often in direct contact with their surroundings to generate tactile feedback. Upon onset of damage, the stability or repeatability of the device will be undermined. Eventually, when complete failure occurs, these actuators are disposed of, accumulating waste and driving the consumption of natural resources. This emphasizes the need to enhance the durability of soft tactile actuators for continued operation. This review presents the principles of tactile feedback of actuators, followed by a discussion of the mechanisms, advancements, and challenges faced by soft tactile actuators to realize high actuation performance, categorized by their driving stimuli. Diverse approaches to achieve durability are evaluated, including self-healing, damage resistance, self-cleaning, and temperature stability for soft actuators. In these sections, current challenges and potential material designs are identified, paving the way for developing durable soft tactile actuators.

Metachronal Motion of Biological and Artificial Cilia

Cilia are slender, hair-like cell protrusions that are present ubiquitously in the natural world. They perform essential functions, such as generating fluid flow, propulsion, and feeding, in organisms ranging from protozoa to the human body. The coordinated beating of cilia, which results in wavelike motions known as metachrony, has fascinated researchers for decades for its role in functions such as flow generation and mucus transport. Inspired by nature, researchers have explored diverse materials for the fabrication of artificial cilia and developed several methods to mimic the metachronal motion observed in their biological counterparts. In this review, we will introduce the different types of metachronal motion generated by both biological and artificial cilia, the latter including pneumatically, photonically, electrically, and magnetically driven artificial cilia. Furthermore, we review the possible applications of metachronal motion by artificial cilia, focusing on flow generation, transport of mucus, particles, and droplets, and microrobotic locomotion. The overall aim of this review is to offer a comprehensive overview of the metachronal motions exhibited by diverse artificial cilia and the corresponding practical implementations. Additionally, we identify the potential future directions within this field. These insights present an exciting opportunity for further advancements in this domain.

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.

Adhesion performance of magnetically responsive surfaces under wet conditions

Adhesion is the key functionality to pick-and-place objects in wet environments. Recently, various micropillars and external stimuli have been proposed to achieve reversible wet adhesion. However, their underlying mechanisms of liquid/solid regulations have not been sufficiently revealed. Herein, two kinds of magnetically responsive micropillar arrays with different terminals (pointed and flat) are developed using a spray self-assembly method. The coupling effect of geometric structures and external stimuli on the wet adhesion performance between a solid substrate and the developed surface is discussed. In situ observation and analysis of theoretical models demonstrate that changes in adhesive forces are mainly caused by the length of the liquid bridge and the apparent contact angle of the developed surface. The adhesion conversion efficiency in the presence of an on/off magnetic field can achieve a highest value of 72% for the micropillar arrays with flat terminals, which exceeds 3 times that of the micropillar arrays with pointed terminals. In addition, wet adhesion measurements during the process of repeatedly switching the magnetic field demonstrate the durability and cyclic reversibility of the magnetically responsive surface. Furthermore, the transportation of microcomponents verifies the application potential of the magnetically responsive surface, which may provide inspiration for transfer printing systems and wet climbing robots.

Two-Photon Polymerization of Butterfly Wing Scale Inspired Surfaces with Anisotropic Wettability

Wings of Morph aega butterflies are natural surfaces that exhibit anisotropic liquid wettability. The direction-dependent arrangement of the wing scales creates orientation-turnable microstructures with two distinct contact modes for liquid droplets. Enabled by recent developments in additive manufacturing, such natural surface designs coupled with hydrophobicity play a crucial role in applications such as self-cleaning, anti-icing, and fluidic manipulation. However, the interplay among resolution, architecture, and performance of bioinspired structures is barely achieved. Herein, inspired by the wing scales of the Morpho aega butterfly, full-scale synthetic surfaces with anisotropic wettability fabricated by two-photon polymerization are reported. The quality of the artificial butterfly scale is improved by optimizing the laser scanning strategy and the objective lens movement path. The corresponding contact angles of water on the fabricated architecture with various design parameters are measured, and the anisotropic fluidic wettability is investigated. Results demonstrate that tuning the geometrical parameters and spatial arrangement of the artificial wing scales enables anisotropic behaviors of the droplet's motion. The measured results also indicate a reverse phenomenon of the fabricated surfaces in contrast to their natural counterparts, possibly attributed to the significant difference in equilibrium wettability between the fabricated microstructures and the natural Morpho aega surface. These findings are utilized to design next-generation fluid-controllable interfaces for manipulating liquid mobility on synthetic surfaces.

High-Efficient Microdroplet Harvesting and Detaching Inspired from Sarracenia Lid Trichome

Fog harvesting plays a pivotal role in harnessing atmospheric water resources and holds significant promise for alleviating global water scarcity. Nonetheless, enhancing harvesting efficiency remains a persistent challenge, especially concerning the rapid detachment of droplets from surfaces. In this study, we discovered that the trichomes of Sarracenia not only efficiently harvest and transport liquid but also quickly drain harvested liquid. We have elucidated the augmentation mechanism behind effective fog harvesting and drainage within the lid of Sarracenia. The trichomes facing the counterflow can enhance fog harvesting efficiency by 80% through air-flow-assisted spreading of liquid film. The wedge corner generated by the interface between hydrophilic and hydrophobic surfaces, coupled with the reduction of cross-sectional angles, diminishes the adhesive force of liquid droplets, fosters droplet spheroidization, and substantially facilitates droplet detachment. In addition, the quantitative detachment of droplets can be achieved by adjusting the cross-sectional angle and wetting gradient. This integrated structure combining efficient condensation and detachment has diverse applications in cooling towers and seawater desalination.

Bioinspired magnetic cilia: from materials to applications

Microscale and nanoscale cilia are ubiquitous in natural systems where they serve diverse biological functions. Bioinspired artificial magnetic cilia have emerged as a highly promising technology with vast potential applications, ranging from soft robotics to highly precise sensors. In this review, we comprehensively discuss the roles of cilia in nature and the various types of magnetic particles utilized in magnetic cilia; additionally, we explore the top-down and bottom-up fabrication techniques employed for their production. Furthermore, we examine the various applications of magnetic cilia, including their use in soft robotics, droplet and particle control systems, fluidics, optical devices, and sensors. Finally, we present our conclusions and the future outlook for magnetic cilia research and development, including the challenges that need to be overcome and the potential for further integration with emerging technologies.

Advances in the Research of Photo, Electrical, and Magnetic Responsive Smart Superhydrophobic Materials: Synthesis and Potential Applications

With the rapid advancement of technology, the wettability of conventional superhydrophobic materials no longer suffice to meet the demands of practical applications. Intelligent responsive superhydrophobic materials have emerged as a highly sought-after material in various fields. The exceptional superhydrophobicity, reversible wetting, and intelligently controllable characteristics of these materials have led to extensive applications across industries, including industry, agriculture, defense, and medicine. Therefore, the development of intelligent superhydrophobic materials with superior performance, economic practicality, enhanced sensitivity, and controllability assumes utmost importance in advancing technology worldwide. This article provides a summary of the wettability principles of superhydrophobic surfaces and the mechanisms behind intelligent responsive superhydrophobicity. Furthermore, it reviews and analyzes the recent research progress on light, electric, and magnetic responsive superhydrophobic materials, encompassing aspects such as material synthesis, modification, performance, and responses under diverse external stimuli. The article also explores the challenges associated with different types of responsive superhydrophobic materials and the unique application prospects of light, electric, and magnetic responsive superhydrophobic materials. Additionally, it outlines the future directions for the development of intelligent responsive superhydrophobic materials.

Bidirectional Droplet Manipulation on Magnetically Actuated Superhydrophobic Ratchet Surfaces

Droplet manipulation has garnered significant attention in various fields due to its wide range of applications. Among many different methods, magnetic actuation has emerged as a promising approach for remote and instantaneous droplet manipulation. In this study, we present the bidirectional droplet manipulation on a magnetically actuated superhydrophobic ratchet surface. The surface consists of silicon strips anchored on elastomer ridges with superhydrophobic black silicon structures on the top side and magnetic layers on the bottom side. The soft magnetic properties of the strips enable their bidirectional tilting to form a ratchet surface and thus bidirectional droplet manipulation upon varying external magnetic field location and strength. Computational multiphysics models were developed to predict the tilting of the strips, demonstrating the concept of bidirectional tilting along with a tilting angle hysteresis theory. Experimental results confirmed the soft magnetic hysteresis and consequential bidirectional tilting of the strips. The superhydrophobic ratchet surface formed by the tilting strips induced the bidirectional self-propulsion of dispensed droplets through the Laplace pressure gradient, and the horizontal acceleration of the droplets was found to be positively correlated with the tilting angle of the strips. Additionally, a finite element analysis was conducted to identify the critical conditions for dispensed droplet penetration through the gaps between the strips, which hinder the droplet's self-propulsion. The models and findings here provide substantial insights into the design and optimization of magnetically actuated superhydrophobic ratchet surfaces to manipulate droplets in the context of digital microfluidic applications.

Contactless acoustic tweezer for droplet manipulation on superhydrophobic surfaces

Droplet manipulation on superhydrophobic surfaces (DMSS) without conventional pipetting is an emerging liquid handling technology, which can be potentially used for diagnostic, analysis, and synthetic processes. Despite notable progress, controlling droplet motion on superhydrophobic surfaces by contactless acoustic waves is rarely reported. Herein, we report a contactless acoustic tweezer (CAT) for DMSS based on establishing ultrasonic standing wave between an ultrasound transducer (UST) and a superhydrophobic substrate to manipulate droplets without physical contact. The CAT utilizes acoustic radiation forces to trap and move droplets on superhydrophobic surfaces, which allows for precise and controllable movement of droplets by controlling the movement of the UST. Small droplets with volume less than 20 μL can be levitated in mid-air for out-plane manipulation, and large droplets with volume up to 500 μL can be trapped for in-plane manipulation. Experimental results demonstrate the versatility of the CAT for manipulating droplets with various compositions and volumes on various superhydrophobic substrates, offering a versatile and cross-contamination-free liquid handling approach for applications, including but not limited to high-throughput surface-enhanced Raman scattering.

Droplet-Based Microfluidics: Applications in Pharmaceuticals

Droplet-based microfluidics offer great opportunities for applications in various fields, such as diagnostics, food sciences, and drug discovery. A droplet provides an isolated environment for performing a single reaction within a microscale-volume sample, allowing for a fast reaction with a high sensitivity, high throughput, and low risk of cross-contamination. Owing to several remarkable features, droplet-based microfluidic techniques have been intensively studied. In this review, we discuss the impact of droplet microfluidics, particularly focusing on drug screening and development. In addition, we surveyed various methods of device fabrication and droplet generation/manipulation. We further highlight some promising studies covering drug synthesis and delivery that were updated within the last 5 years. This review provides researchers with a quick guide that includes the most up-to-date and relevant information on the latest scientific findings on the development of droplet-based microfluidics in the pharmaceutical field.

An electrothermal platform for active droplet manipulation

The smart control of droplet transport through surface structures and external fields provides exciting opportunities in engineering fields of phase change heat transfer, biomedical chips, and energy harvesting. Here we report the wedge-shaped slippery lubricant-infused porous surface (WS-SLIPS) as an electrothermal platform for active droplet manipulation. WS-SLIPS is fabricated by infusing a wedge-shaped superhydrophobic aluminum plate with phase-changeable paraffin. While the surface wettability of WS-SLIPS can be readily and reversibly switched by the freezing-melting cycle of paraffin, the curvature gradient of the wedge-shaped substrate automatically induces an uneven Laplace pressure inside the droplet, endowing WS-SLIPS the ability to directionally transport droplets without any extra energy input. We demonstrate that WS-SLIPS features spontaneous and controllable droplet transport capability to initiate, brake, lock, and resume the directional motion of various liquid droplets including water, saturated NaCl solution, ethanol solution, and glycerol, under the control of preset DC voltage (∼12 V). In addition, the WS-SLIPS can automatically repair surface scratches or indents when heated and retain the full liquid-manipulating capability afterward. The versatile and robust droplet manipulation platform of WS-SLIPS can be further used in practical scenarios such as laboratory-on-a-chip settings, chemical analysis and microfluidic reactors, paving a new path to develop advanced interface for multifunctional droplet transport.

Ultrasonic tweezer for multifunctional droplet manipulation

Spatiotemporally controllable droplet manipulation is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite considerable advances, droplet manipulation without surface or droplet pretreatment is still challenging in terms of response and functional adaptability. Here, a droplet ultrasonic tweezer (DUT) based on phased array is proposed for versatile droplet manipulation. The DUT can generate a twin trap ultrasonic field at the focal point for trapping and maneuvering the droplet by changing the position of the focal point, which enables a highly flexible and precise programmable control. By leveraging the acoustic radiation force resulting from the twin trap, the droplet can pass through a confined slit 2.5 times smaller than its own size, cross a slope with an inclination up to 80°, and even reciprocate in the vertical direction. These findings provide a satisfactory paradigm for robust contactless droplet manipulation in various practical settings including droplet ballistic ejection, droplet dispensing, and surface cleaning.

Mosaic Patterned Surfaces toward Generating Hardly-Volatile Capsular Droplet Arrays for High-Precision Droplet-Based Storage and Detection

Precise detection involving droplets based on functional surfaces is promising for the parallelization and miniaturization of platforms and is significant in epidemic investigation, analyte recognition, environmental simulation, combinatorial chemistry, etc. However, a challenging and considerable task is obtaining mutually independent droplet arrays without cross-contamination and simultaneously avoiding droplet evaporation-caused quick reagent loss, inaccuracy, and failure. Herein, a strategy to generate mutually independent and hardly-volatile capsular droplet arrays using innovative mosaic patterned surfaces is developed. The evaporation suppression of the capsular droplet arrays is 1712 times higher than the naked droplet. The high evaporation suppression of the capsular droplet arrays on the surfaces is attributed to synergistic blocking of the upper oil and bottom mosaic gasproof layer. The scale-up of the capsular droplet arrays, the flexibility in shape, size, component (including aqueous, colloidal, acid, and alkali solutions), liquid volume, and the high-precision hazardous substance testing proves the concept's high compatibility and practicability. The mutually independent capsular droplet arrays with amazingly high evaporation suppression are essential for the new generation of high-performance open-surface microfluidic chips used in COVID-19 diagnosis and investigation, primary screening, in vitro enzyme reactions, environmental monitoring, nanomaterial synthesis, etc.

Interfacial design for detection of a few molecules

Major advances in molecular detection are being driven by goals associated with the development of methods that are amenable to miniaturization and automation, and that have high sensitivity and low interference. The new detection methods are confronted by many interfacial issues, which when properly addressed can lead to improved performance. One interfacial property, special wettability, can facilitate precise delivery and local enrichment of molecules to sensing elements. This review summarizes applications of unique features of special wettability in molecular detection including (1) chemical and electrochemical reactions in anchored microdroplets on superwetting surfaces, (2) enrichment of analytes and active materials at low contact areas between droplets and superwetting surfaces, (3) complete opposite affinities of superwetting surfaces toward nonpolar/polar solutes and oil/water phases, and (4) directional droplet transportation on asymmetric superwetting surfaces. The challenges and opportunities that exist in design and applications of special wettability in interfacial delivery and enrichment for detection of a few molecules are also discussed.

Fluid Field Modulation in Mass Transfer for Efficient Photocatalysis

Mass transfer is an essential factor determining photocatalytic performance, which can be modulated by fluid field via manipulating the kinetic characteristics of photocatalysts and photocatalytic intermediates. Past decades have witnessed the efforts and achievements made in manipulating mass transfer based on photocatalyst structure and composition design, and thus, a critical survey that scrutinizes the recent progress in this topic is urgently necessitated. This review examines the basic principles of how mass transfer behavior impacts photocatalytic activity accompanying with the discussion on theoretical simulation calculation including fluid flow speed and pattern. Meanwhile, newly emerged viable photocatalytic micro/nanomotors with self-thermophoresis, self-diffusiophoresis, and bubble-propulsion mechanisms as well as magnet-actuated photocatalytic artificial cilia for facilitating mass transfer will be covered. Furthermore, their applications in photocatalytic hydrogen evolution, carbon dioxide reduction, organic pollution degradation, bacteria disinfection and so forth are scrutinized. Finally, a brief summary and future outlook are presented, providing a viable guideline to those working in photocatalysis, mass transfer, and other related fields.

Superhydrophilic–superhydrophobic patterned surfaces: From simplified fabrication to emerging applications

Superhydrophilic–superhydrophobic patterned surfaces constitute a branch of surface chemistry involving the two extreme states of superhydrophilicity and superhydrophobicity combined on the same surface in precise patterns. Such surfaces have many advantages, including controllable wettability, enrichment ability, accessibility, and the ability to manipulate and pattern water droplets, and they offer new functionalities and possibilities for a wide variety of emerging applications, such as microarrays, biomedical assays, microfluidics, and environmental protection. This review presents the basic theory, simplified fabrication, and emerging applications of superhydrophilic–superhydrophobic patterned surfaces. First, the fundamental theories of wettability that explain the spreading of a droplet on a solid surface are described. Then, the fabrication methods for preparing superhydrophilic–superhydrophobic patterned surfaces are introduced, and the emerging applications of such surfaces that are currently being explored are highlighted. Finally, the remaining challenges of constructing such surfaces and future applications that would benefit from their use are discussed.

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.

Microfluidic Applications of Artificial Cilia: Recent Progress, Demonstration, and Future Perspectives

Artificial cilia-based microfluidics is a promising alternative in lab-on-a-chip applications which provides an efficient way to manipulate fluid flow in a microfluidic environment with high precision. Additionally, it can induce favorable local flows toward practical biomedical applications. The endowment of artificial cilia with their anatomy and capabilities such as mixing, pumping, transporting, and sensing lead to advance next-generation applications including precision medicine, digital nanofluidics, and lab-on-chip systems. This review summarizes the importance and significance of the artificial cilia, delineates the recent progress in artificial cilia-based microfluidics toward microfluidic application, and provides future perspectives. The presented knowledge and insights are envisaged to pave the way for innovative advances for the research communities in miniaturization.

Microscopic artificial cilia - a review

Cilia are microscopic hair-like external cell organelles that are ubiquitously present in nature, also within the human body. They fulfill crucial biological functions: motile cilia provide transportation of fluids and cells, and immotile cilia sense shear stress and concentrations of chemical species. Inspired by nature, scientists have developed artificial cilia mimicking the functions of biological cilia, aiming at application in microfluidic devices like lab-on-chip or organ-on-chip. By actuating the artificial cilia, for example by a magnetic field, an electric field, or pneumatics, microfluidic flow can be generated and particles can be transported. Other functions that have been explored are anti-biofouling and flow sensing. We provide a critical review of the progress in artificial cilia research and development as well as an evaluation of its future potential. We cover all aspects from fabrication approaches, actuation principles, artificial cilia functions - flow generation, particle transport and flow sensing - to applications. In addition to in-depth analyses of the current state of knowledge, we provide classifications of the different approaches and quantitative comparisons of the results obtained. We conclude that artificial cilia research is very much alive, with some concepts close to industrial implementation, and other developments just starting to open novel scientific opportunities.

3D Printing Ultraflexible Magnetic Actuators via Screw Extrusion Method

Soft magnetic actuators with programmable structure design and controllable deformation ability based on 3D printing technology have attracted extensive attention. In this paper, a novel 3D printing strategy is developed to manufacture the ultraflexible magnetic actuator, in which the printed material is composed of magnetic particles and thermoplastic rubber materials. Different from the traditional fused deposition printing, this printing strategy introduces screw extrusion technology to the heating components of the printer to overcome the problem of filament buckling in the flexible material. Thus, the tensile modulus of the printed products can be reduced to as low as ≈2 MPa. Based on the above method, biomimetic magnetic actuators of the sucker and the pump are constructed for adhering and releasing object and pumping liquid. The contraction performance of the magnetic actuator is studied via a series of experiments and the magnetic field-induced deformation is analyzed by the multiphysics-based finite element model. This work proves that ultraflexible magnetic actuators fabricated by this 3D printing strategy show broad prospects in the fields of soft robotics and bionics.

Magneto Twister: Magneto Deformation of the Water-Air Interface by a Superhydrophobic Magnetic Nanoparticle Layer

Remote manipulation of superhydrophobic surfaces provides fascinating features in water interface-related applications. A superhydrophobic magnetic nanoparticle colloid layer is able to float on the water-air interface and form a stable water-solid-air interface due to its inherent water repulsion, buoyancy, and lateral capillarity properties. Moreover, it easily bends downward under an externally applied gradient magnetic field. Thanks to that, the layer creates a stable twister-like structure with a flipped conical shape, under controlled water levels, behaving as a soft and elastic material that proportionally deforms with the applied magnetic field and then goes back to its initial state in the absence of an external force. When the tip of the twister structure touches the bottom of the water container, it provides a stable magneto movable system, which has many applications in the microfluidic field. We introduce, as a proof-of-principle, three possible implementations of this structure in real scenarios, the cargo and transport of water droplets in aqueous media, the generation of magneto controllable plugs in open surface channels, and the removal of floating microplastics from the air-water interface.

Slippery magnetic track inducing droplet and bubble manipulation

It is difficult for traditional droplet manipulation to combine transportation and rapid capture of droplets on an inclined surface. In this work, a slippery magnetic track (SMT) is presented to manipulate droplets and bubbles in a magnetic field. By changing the direction of the magnetic field, the transitions from non-pinning to pinning states on the SMT can be achieved. Through the SMT surface, it is possible to capture and release droplets and bubbles in the vertical direction. The detailed theoretical and experimental studies of droplet and bubble manipulation are discussed. This work demonstrates the versatility of magnetic manipulation, including the transition of droplet trajectory and bubble removal, which will facilitate the research of intelligent interfaces in energy transmission, drug transport and micro engineering.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.

Magneto-Responsive Shutter for On-Demand Droplet Manipulation

Magnetically responsive structured surfaces enabling multifunctional droplet manipulation are of significant interest in both scientific and engineering research. To realize magnetic actuation, current strategies generally employ well-designed microarrays of high-aspect-ratio structure components (e.g., microcilia, micropillars, and microplates) with incorporated magnetism to allow reversible bending deformation driven by magnets. However, such magneto-responsive microarray surfaces suffer from highly restricted deformation range and poor control precision under magnetic field, restraining their droplet manipulation capability. Herein, a novel magneto-responsive shutter (MRS) design composed of arrayed microblades connected to a frame is developed for on-demand droplet manipulation. The microblades can perform two dynamical transformation operations, including reversible swing and rotation, and significantly, the transformation can be precisely controlled over a large rotation range with the highest rotation angle up to 3960°. Functionalized MRSs based on the above design, including Janus-MRS, superhydrophobic MRS (SHP-MRS) and lubricant infused slippery MRS (LIS-MRS), can realize a wide range of droplet manipulations, ranging from switchable wettability, directional droplet bounce, droplet distribution, and droplet merging, to continuous droplet transport along either straight or curved paths. MRS provides a new paradigm of using swing/rotation topographic transformation to replace conventional bending deformation for highly efficient and on-demand multimode droplet manipulation under magnetic actuation.

Multifunctional Laser-Induced Graphene Papers with Combined Defocusing and Grafting Processes for Patternable and Continuously Tunable Wettability from Superlyophilicity to Superlyophobicity

Functional surfaces with tunable and patternable wettability have attracted significant research interests because of remarkable advantages in biomedicine, environmental, and energy storage applications. Based on combined defocusing and grafting strategy for processing laser-induced graphene papers (LIGPs) with variable surface roughness (58.18-6.08 µm) and F content (0-25.9%), their wettability can be tuned continuously from superlyophilicity (contact angle CA ≈ 0^° ) to superlyophobicity (CA > 150^° ), for various liquids with a wide range of surface tensions from 27.5 to 72.8 mN m^-1 . In addition to reaching multiple wetting characteristics including amphiphilic, amphiphobic, and hydrophobic-oleophilic states, three designable processes are further developed for achieving LIGPs with various wetting patterns, including hydrophilic arrays or channels, hydrophobic-to-hydrophilic gradients, and Janus. Activated by the customly designed structures and properties, multifunctional and multi-scenario applications are successfully attempted, including 2D-/3D- directional cell cultivation, water transportation diode, self-triggered liquid transfer & collection, etc.

Magnetic-actuated "capillary container" for versatile three-dimensional fluid interface manipulation

Fluid interfaces are omnipresent in nature. Engineering the fluid interface is essential to study interfacial processes for basic research and industrial applications. However, it remains challenging to precisely control the fluid interface because of its fluidity and instability. Here, we proposed a magnetic-actuated "capillary container" to realize three-dimensional (3D) fluid interface creation and programmable dynamic manipulation. By wettability modification, 3D fluid interfaces with predesigned sizes and geometries can be constructed in air, water, and oils. Multiple motion modes were realized by adjusting the container's structure and magnetic field. Besides, we demonstrated its feasibility in various fluids by performing selective fluid collection and chemical reaction manipulations. The container can also be encapsulated with an interfacial gelation reaction. Using this process, diverse free-standing 3D membranes were produced, and the dynamic release of riboflavin (vitamin B₂) was studied. This versatile capillary container will provide a promising platform for open microfluidics, interfacial chemistry, and biomedical engineering.

Dynamic Acoustic Levitator Based On Subwavelength Aperture Control

Acoustic levitation provides a means to achieve contactless manipulation of fragile materials and biological samples. Most acoustic levitators rely on complex electronic hardware and software to shape the acoustic field and realize their dynamic operation. Here, the authors introduce a dynamic acoustic levitator that is based on mechanically controlling the opening and (partial) closing of subwavelength apertures. This simple approach relies on the use of a single ultrasonic transducer and is shown to permit the facile and reliable manipulation of a variety targets ranging from solid particles, to fluid and ferrofluidic drops. Experimental observations agree well with numerical simulations of the Gor'kov potential. Remarkably, this system even enables the generation of time-varying potentials and induces oscillatory and rotational motion in the levitated objects via a feedback mechanism between the trapped object and the trapping potential. This is shown to result in long distance translation, in-situ rotation and self-modulated oscillation of the trapped particles. In addition, dense ferrofluidic droplets are levitated and transformed inside the levitator. Controlling subwavelength apertures opens the possibility to realize simple powerful levitators that nevertheless allow for the versatile dynamic manipulation of levitated matter.

Colloidal magnetic brushes: influence of the magnetic content and presence of short-range attractive forces in the micro-structure and field response

The behaviour of supramolecular brushes, whose filaments are composed of sequences of magnetic and non-magnetic colloidal particles, has been studied using Langevin dynamics simulations. Two types of brushes have been considered: sticky or Stockmayer brushes (SB) and non-sticky magnetic brushes (NSB). In both cases, the microstructure and the collective behaviour have been analysed for a wide range of magnetic field strengths including the zero-field case, and negative fields. The results show that, for the same magnetic content, SB placed in a magnetic field present an extensibility up to two times larger than NSB. The analysis of the microstructure of SB at zero field shows that magnetic particles belonging to different filaments in the brush self-organize into ring and chain aggregates, while magnetic colloids in NSB mainly remain in a non-aggregated state. Clustering among magnetic particles belonging to different filaments is observed to gradually fade away as the magnetic content of SB filaments increases towards 100%. Under an external field, SB are observed to form chains, threads and sheets depending on the magnetic content and the applied field strength. The chain-like clusters in SB are observed to decrease in size as the magnetic content in the filaments increases. A non-monotonic field dependence is observed for the average size of these clusters. In spite of the very different microstructure, both NSB and SB are observed to have a very similar magnetization, especially in high strength fields.

Rose Pistil Stigma: Hierarchical Superhydrophobic Surfaces with Hydrophilic Microtips for Microdroplet Manipulation

Finger-like radial hierarchical micropillars with folded tips are observed on the surface of the rose pistil stigma (RPS). Impressively, a water droplet on the surface of the RPS presents a spherical shape and it still hangs on the surface even when the RPS is turned over. Superhydrophobicity and high adhesion to water are demonstrated on the RPS, which is beneficial for the RPS to remain clean and fresh. The special wetting behavior of the RPS is highly related to its hierarchical microstructures and surface chemistry. Finger-like hierarchical micropillars with a high aspect ratio are capable of retaining air to support superhydrophobicity while the microgap between the micropillars and on the hydrophilic tips enables the RPS to retain a high adhesion to water. These findings about the unique wetting behaviors of the RPS may provide inspiration for the design and fabrication of functional wetting surfaces for diverse applications such as microdroplet manipulation, three-dimensional cell culture, and microfluidics.

A Facile and Cost-Effective Method to Prepare a Robust Superhydrophobic RTV Silicone Coating

A facile and cost-effective layer-by-layer assembly method is reported for the fabrication of a superhydrophobic coating using room temperature vulcanized silicone rubber and silica particles of different diameters. The superhydrophobic coating shows a contact angle of 154.6° and a rolling angle of 9°. The coating exhibits excellent self-cleaning properties and corrosion resistance with good softness and mechanical durability. The sample maintains its good superhydrophobicity, even after a series of tests by knife scratching, cutting, and hammering. The fabrication method has great advantages in terms of efficiency, cost, simple procedure and conditions. Its wide application in different substrates and the use of environment-friendly materials are also beneficial to the large-scale production of the coating.

Magnetically Responsive Film Decorated with Microcilia for Robust and Controllable Manipulation of Droplets

Droplet manipulations are critical for applications ranging from biochemical analysis, medical diagnosis to environmental controls. Even though magnetic actuation has exhibited great potential, the capability of high-speed, precise manipulation, and mixing improvement covering a broad droplet volume has not yet been realized. Herein, we demonstrated that the magnetic actuation could be conveniently achieved via decorating the magnetically responsive film with microcilia. Under magnetic field, the film can quickly response with localized deformation, along with the microcilia to realize the surface superhydrophobicity for droplet manipulation with velocity up to ∼173 mm/s covering a broad volume of 2-100 μL. The robust system further allows us to realize rapid and complete droplet mixing within ∼1.6 s. In addition, the microcilia decorated surface can preserve the robust superhydrophobicity after various stability tests, for example, normal pressing, chemical corrosion, and mechanical abrasion, exhibiting the possibility toward the long-term and real applications. With the multifunctional demonstrations such as obvious mixing improvement, parallel manipulation, and serial dilution, we believe that the methodology can open up a magnetic field-based avenue for future applications in digital microfluidics, and biochemical assays, etc.

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

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

Cross-Species Bioinspired Anisotropic Surfaces for Active Droplet Transportation Driven by Unidirectional Microcolumn Waves

Natural evolution has endowed diverse species with distinct geometric micro/nanostructures exhibiting admirable functions. Examples include anisotropic microgrooves/microstripes on the rice leaf surface for passive liquid directional rolling, and motile microcilia widely existed in mammals' body for active matter transportation through in situ oscillation. Till now, bionic studies have been extensively performed by imitating a single specific biologic functional system. However, bionic fabrication of devices integrating multispecies architectures is rarely reported, which may sparkle more fascinating functionalities beyond natural findings. Here, a cross-species design strategy is adopted by combining the anisotropic wettability of the rice leaf surface and the directional transportation characteristics of motile cilia. High-aspect-ratio magnetically responsive microcolumn array (HAR-MRMA) is prepared for active droplet transportation. It is found that just like the motile microcilia, the unidirectional waves are formed by the real-time reconstruction of the microcolumn array under the moving magnetic field, enabling droplet (1-6 μL) to transport along the predetermined anisotropic orbit. Meanwhile, on-demand droplet horizontal transportation on the inclined plane can be realized by the rice leaf-like anisotropic surface, showcasing active nongravity-driven droplet transportation capability of the HAR-MRMA. The directional lossless transportation of droplet holds great potential in the fields of microfluidics, chemical microreaction, and intelligent droplet control system.

Ladderlike Conical Micropillars Facilitating Underwater Gas-Bubble Manipulation in an Aqueous Environment

Underwater gas-bubble manipulation in aqueous environments is of great importance in industry and academia. Although the underwater gas bubble has been proved to be directionally transportable by various structures, transporting gas bubbles in 3D space remains a challenge. In this research, two kinds of tapered pillars, that is, ladderlike and helical ladderlike, were proposed for manipulating gas bubbles. To fabricate such unique structures, an improved alternative coating and etching method was developed. To meet the requirements of underwater gas-bubble transport, a modified gas-bubble slippery technology was also developed to enhance the aerophilic ability. The dynamics of the gas bubble was analyzed using a high-speed camera. The Laplace force that resulted from the geometry gradient was found to play a significant role in tuning the gas-bubble velocity. Through adjustments on the wettability, tilt angle, and geometry of each section of the tapered pillar, tuning the transport velocity from 113.9 ± 10.3 to 309.1 ± 5.8 mm/s becomes possible. On the basis of these findings, the helical ladderlike tapered pillar was fabricated and demonstrated to be able to transport gas bubbles in 3D space. These results may provide a new and systematic way to design and fabricate materials and structures for directional gas-bubble transport in 3D space.

A Facile Method to Prepare a Superhydrophobic Magnesium Alloy Surface

The application of superhydrophobic materials has been handicapped by complex processes and poor environmental friendliness. Magnesium alloys are widely used in daily production due to their low density and good casting properties. A facile and environmentally friendly method was proposed to prepare a superhydrophobic layer with coral-like microstructure on the surface of AZ91D magnesium alloy by high temperature heating. The prepared superhydrophobic surface has a contact angle of 159.1° and a rolling angle of 4.8°. The corrosion current of superhydrophobic surface has been reduced by about two orders of magnitude relative to the magnesium alloy substrate and its inhibition efficiency is 96.94%, which demonstrates its great corrosion resistance. In addition, the superhydrophobic surface has great thermal stability. When the temperature rises to 190 °C, the contact is still above 150°. Excellent self-cleaning and advantages in preparation efficiency, environmental protection and cost-effectiveness will boost its good application prospects.