Functional microfluidics:theory,microfabrication,and applications

Microfluidic devices are composed of microchannels with a diameter ranging from ten to a few hundred micrometers.Thus,quite a small(10-9–10-18l)amount of liquid can be manipulated by such a precise system.In the past three decades,significant progress in materials science,microfabrication,and various applications has boosted the development of promising functional microfluidic devices.In this review,the recent progress on novel microfluidic devices with various functions and applications is presented.First,the theory and numerical methods for studying the performance of microfluidic devices are briefly introduced.Then,materials and fabrication methods of functional microfluidic devices are summarized.Next,the recent significant advances in applications of microfluidic devices are highlighted,including heat sinks,clean water production,chemical reactions,sensors,biomedicine,capillaric circuits,wearable electronic devices,and microrobotics.Finally,perspectives on the challenges and future developments of functional microfluidic devices are presented.This review aims to inspire researchers from various fields engineering,materials,chemistry,mathematics,physics,and more—to collaborate and drive forward the development and applications of functional microfluidic devices,specifically for achieving carbon neutrality.

Selective liquid directional steering enabled by dual-scale reentrant ratchets

Achieving well-controlled directional steering of liquids is of great significance for both fundamental study and practical applications, such as microfluidics, biomedicine, and heat management. Recent advances allow liquids with different surface tensions to select their spreading directions on a same surface composed of macro ratchets with dual reentrant curvatures. Nevertheless, such intriguing directional steering function relies on 3D printed sophisticated structures and additional polishing process to eliminate the inevitable microgrooves-like surface deficiency generated from printing process, which increases the manufacturing complexity and severally hinders practical applications. Herein, we developed a simplified dual-scale structure that enables directional liquid steering via a straightforward 3D printing process without the need of any physical and chemical post-treatment. The dual-scale structure consists of macroscale tilt ratchet equipped with a reentrant tip and microscale grooves that decorated on the whole surface along a specific orientation. Distinct from conventional design requiring the elimination of microgrooves-like surface deficiency, we demonstrated that the microgrooves of dual-scale structure play a key role in delaying or promoting the local flow of liquids, tuning of which could even enable liquids select different spreading pathways. This study provides a new insight for developing surfaces with tunable multi-scale structures, and also advances our fundamental understanding of the interaction between liquid spreading dynamics and surface topography.

Generating micro/nanostructures on magnesium alloy surface using ultraprecision diamond surface texturing process

The lightness and high strength-to-weight ratio of the magnesium alloy have attracted more interest in various applications.However,micro/nanostructure generation on their surfaces remains a challenge due to the flammability and ignition.Motivated by this,this study proposed a machining process,named the ultraprecision diamond surface texturing process,to machine the micro/nanostructures on magnesium alloy surfaces.Experimental results showed the various microstructures and sawtooth-shaped nanostructures were successfully generated on the AZ31B magnesium alloy surfaces,demonstrating the effectiveness of this proposed machining process.Furthermore,sawtooth-shaped nanostructures had the function of inducing the optical effect and generating different colors on workpiece surfaces.The colorful letter and colorful flower image were clearly viewed on magnesium alloy surfaces.The corresponding cutting force,chip morphology,and tool wear were systematically investigated to understand the machining mechanism of micro/nanostructures on magnesium alloy surfaces.The proposed machining process can further improve the performances of the magnesium alloy and extend its functions to other fields,such as optics.

Photothermal superhydrophobic copper nanowire assemblies: fabrication and deicing/defrosting applications

Ice and frost buildup continuously pose significant challenges to multiple fields.As a promising de-icing/defrosting alternative,designing photothermal coatings that leverage on the abundant sunlight source on the earth to facilitate ice/frost melting has attracted tremendous attention recently.However,previous designs suffered from either localized surface heating owing to the limited thermal conductivity or unsatisfied meltwater removal rate due to strong water/substrate interaction.Herein,we developed a facile approach to fabricate surfaces that combine photothermal,heat-conducting,and superhydrophobic properties into one to achieve efficient de-icing and defrosting.Featuring copper nanowire assemblies,such surfaces were fabricated via the simple template-assisted electrodeposition method,allowing us to tune the nanowire assembly geometry by adjusting the template dimensions and electrodeposition time.The highly ordered copper nanowire assemblies facilitated efficient sunlight absorption and lateral heat spreading,resulting in a fast overall temperature rise to enable the thawing of ice and frost.Further promoted by the excellent water repellency of the surface,the thawed ice and frost could be spontaneously and promptly removed.In this way,the all-in-one design enabled highly enhanced de-icing and defrosting performance compared to other nanostructured surfaces merely with superhydrophobicity,photothermal effect,or the combination of both.In particular,the defrosting efficiency could approach∼100%,which was the highest compared to previous studies.Overall,our approach demonstrates a promising path toward designing highly effective artificial deicing/defrosting surfaces.

Bioinspired integrated triboelectric electronic tongue

An electronic tongue(E-tongue)comprises a series of sensors that simulate human perception of taste and embedded artificial intelligence(AI)for data analysis and recognition.Traditional E-tongues based on electrochemical methods suffer from a bulky size and require larger sample volumes and extra power sources,limiting their applications in in vivo medical diagnosis and analytical chemistry.Inspired by the mechanics of the human tongue,triboelectric components have been incorporated into E-tongue platforms to overcome these limitations.In this study,an integrated multichannel triboelectric bioinspired E-tongue(TBIET)device was developed on a single glass slide chip to improve the device’s taste classification accuracy by utilizing numerous sensory signals.The detection capability of the TBIET was further validated using various test samples,including representative human body,environmental,and beverage samples.The TBIET achieved a remarkably high classification accuracy.For instance,chemical solutions showed 100%identification accuracy,environmental samples reached 98.3%accuracy,and four typical teas demonstrated 97.0%accuracy.Additionally,the classification accuracy of NaCl solutions with five different concentrations reached 96.9%.The innovative TBIET exhibits a remarkable capacity to detect and analyze droplets with ultrahigh sensitivity to their electrical properties.Moreover,it offers a high degree of reliability in accurately detecting and analyzing various liquid samples within a short timeframe.The development of a self-powered portable triboelectric E-tongue prototype is a notable advancement in the field and is one that can greatly enhance the feasibility of rapid on-site detection of liquid samples in various settings.

Aerodynamic Super-Repellent Surfaces

Repelling liquid drops from engineering surfaces has attracted great attention in a variety of applications.To achieve efficient liquid shedding,delicate surface textures are often introduced to sustain air pockets at the liquid-solid interface.However,those surfaces are prone to suffer from mechanical failure,which may bring reliability issues and thus limits their applications.Here,inspired by the aerodynamic Leidenfrost effect,we present that impacting drops are directionally repelled from smooth surfaces supplied with an exogenous air layer.Our theoretical analysis reveals that the synchronized nonwetting and oblique bouncing behavior is attributed to the aerodynamic force arising from the air layer.The versatility and practicability of our approach allow for drop repellency without the aid of any surface wettability treatment and also avoid the consideration of mechanical stability issues,which thereby provides a promising candidate for the applications that necessitate liquid shedding,e.g.,resolve the problem of tiny raindrop adhesion on the automobile side window during driving.

Interfacial friction at action: Interactions, regulation, and applications

Friction is a fundamental force that impacts almost all interface-related applications.Over the past decade,there is a revival in our basic understanding and practical applications of the friction.In this review,we discuss the recent progress on solid–liquid interfacial friction from the perspective of interfaces.We first discuss the fundamentals and theoretical evolution of solid–liquid interfacial friction based on both bulk interactions and molecular interactions.Then,we summarize the interfacial friction regulation strategies manifested in both natural surfaces and artificial systems,focusing on how liquid,solid,gas,and hydrodynamic coupling actions mediate interfacial friction.Next,we discuss some practical applications that are inhibited or reinforced by interfacial friction.At last,we present the challenges to further understand and regulate interfacial friction.

Bio-inspired water-driven electricity generators:From fundamental mechanisms to practical applications

Harvesting water energy in various forms of water motion,such as evaporation,raindrops,river flows,ocean waves,and other,is promising to relieve the global energy crisis and reach the aim of carbon neutrality.However,this highly decentralized and distributed water energy poses a challenge on conventional electromagnetic hydropower technologies that feature centralization and scalization.Recently,this problem has been gradually addressed by the emergence of a myriad of electricity generators that take inspiration from natural living organisms,which have the capability to efficiently process and manage water and energy for survival in the natural competition.Imitating the liquid-solid behaviors manifested in ubiquitous biological processes,these generators allow for the efficient energy conversion from water-solid interaction into the charge transfer or electrical output under natural driving,such as gravity and solar power.However,in spite of the rapid development of the field,a fundamental understanding of these generators and their ability to bridge the gap between the fundamentals and the practical applications remains elusive.In this review,we first introduce the latest progress in the fundamental understanding in bio-inspired electricity generators that allow for efficient harvesting water energy in various forms,ranging from water evaporation,droplet to wave or flow,and then summarize the development of the engineering design of the various bio-inspired electricity generator in the practical applications,including self-powered sensor and wearable electronics.Finally,the prospects and urgent problems,such as how to achieve large-scale electricity generation,are presented.

Stretchable magnesium-air battery based on dual ions conducting hydrogel for intelligent biomedical applications

Flexible and bio-integrated electronics have attracted great attention due to their enormous contributions to personalized medical devices.Power sources,serving as one of the most important components,have been suffering from many problems,including deficient biocompatibility,poor stretchability,and unstable electrical outputs under deformed conditions,which limits the practical applications in flexible and bio-integrated electronics.Here,we reported a fully stretchable magnesium(Mg)–air battery based on dual-ions-conducting hydrogels(SDICH).The high-performance battery enables long-term operation with lighting 120 lighting emitting diodes(LEDs)for over 5 h.Benefiting from the advanced materials and mechanical designs,the battery exhibits stability electrical outputs under stretching,which allows to operate ordinarily under various mechanical deformations without performance decay.Furthermore,the great biocompatibility of the battery offers great opportunity for biomedical applications,which is demonstrated by a self-adaption wound dressing system.The in vitro and in vivo results prove that the self-adaption wound dressing can effectively prevent wound inflammation and promote wound healing.By exploiting thermal feedback mechanics,the system can adjust antibiotic release rate and dosage spontaneously according to the real-time wound conditions.The proposed fully stretchable Mg-air battery and self-adaption wound dressing display great potential in skin-integrated electronics and personalized medicine.

Low-frequency blue energy harvesting for sustainable and active anticorrosion

Engineering materials serving in marine surroundings are inevitably corroded.The corrosive marine conditions can also be utilized to harvest kinetic ocean wave energy to solve this problem.Leveraging water–solid triboelectrification to harvest lowfrequency wave energy for active anticorrosion is promising.Existing techniques are efficient in harnessing environmental energy with frequencies higher than 3 Hz,whereas the dominated ocean waves with optimal wave spectral density fluctuate from 0.45 to 1.5 Hz.Herein,we proposed a highly efficient and sustainable blue energy-powered cathodic protection(BECP)strategy by fusing water–solid triboelectric nanogenerators and cathodic protection technology.Leveraging the highly efficient triboelectrification between the moving water and hydrophobic fluorinated ethylene propylene tube,we developed the built-in power module,enabling the harvest of ocean wave energy lower than 1.5 Hz.The generated volumetric current density is 28.9 mA·m^(-3),5–20 times higher than the values of the reported devices.Moreover,the proposed BECP performs comparably to conventional cathodic protection in corrosion inhibition.Significantly,the proposed approach can be easily applied to ships,buoys,and other offshore platforms to simultaneously realize blue energy harvesting and engineering material protection,providing an alternative to traditional active protection technology.

Rotating Surfaces Promote the Shedding of Droplets

Achieving rapid shedding of droplets from solid surfaces has received substantial attention because of its diverse applications.Previous studies have focused on minimizing contact times of liquid droplets interacting with stationary surfaces,yet little consideration has been given to that of moving surfaces.Here,we report a different scenario:A water droplet rapidly detaches from micro/nanotextured rotating surfaces in an intriguing doughnut shape,contributing to about 40%contact time reduction compared with that on stationary surfaces.The doughnut-shaped bouncing droplet fragments into satellites and spontaneously scatters,thus avoiding further collision with the substrate.In particular,the contact time is highly dependent on impact velocities of droplets,beyond previous descriptions of classical inertialcapillary scaling law.Our results not only deepen the fundamental understanding of droplet dynamics on moving surfaces but also suggest a synergistic mechanism to actively regulate the contact time by coupling the kinematics of droplet impingement and surface rotation.

Programmable droplet manipulation enabled by charged-surface pattern reconfiguration

Programmable droplet manipulation based on external stimulation is in high demand in various modern technologies.Despite notable progress,current manipulation strategies still suffer from a common drawback such as single control means of modulating the external stimulation input,which leads to huge challenges in sophisticated and large scale-up droplet handling.Herein,a unique patternreconfiguration-driven droplet manipulation method is developed on conductive/nonconductive pattern surfaces under charge deposition.Contactless charge deposition induces the“edge barrier”phenomenon at the boundaries of conductive/nonconductive patterns,analogous to an invisible and tunable wall guiding droplet behaviors.The edge barrier effect can be flexibly tuned by the nonconductive surface pattern.Thus,with charge deposition,surfaces are endowed with protean control functionality.The design of conductive/nonconductive patterns can effectively enable multifunction droplet manipulations,including track-guided sliding,sorting,merging,and mixing.Moreover,dynamical pattern reconfiguration drives programmable fluidics with sophisticated and large scale-up droplet handling capabilities in a low-cost and simple approach.

Nature-inspired surface topography: design and function

Learning from nature has traditionally and continuously provided important insights to drive a paradigm shift in technology.In particular,recent studies show that many biological organisms exhibit spectacular surface topography such as shape,size,spatial organization,periodicity,interconnectivity,and hierarchy to endow them with the capability to adapt dynamically and responsively to a wide range of environments.More excitingly,in a broader perspective,these normally neglected topological features have the potential to fundamentally change the way of how engineering surface works,such as how fluid flows,how heat is transported,and how energy is generated,saved,and converted,to name a few.Thus,the design of nature-inspired surface topography for unique functions will spur new thinking and provide paradigm shift in the development of the new engineering surfaces.In this review,we first present a brief introduction to some insights extracted from nature.Then,we highlight recent progress in designing new surface topographies and demonstrate their applications in emerging areas including thermal-fluid transport,anti-icing,water harvesting,power generation,adhesive control,and soft robotics.Finally,we offer our perspectives on this emerging field,with the aim to stimulate new thinking on the development of next-generation of new materials and devices,and dramatically extend the boundaries of traditional engineering.

Electronic Skin from High-Throughput Fabrication of Intrinsically Stretchable Lead Zirconate Titanate Elastomer

Electronic skin made of thin,soft,stretchable devices that can mimic the human skin and reconstruct the tactile sensation and perception offers great opportunities for prosthesis sensing,robotics controlling,and human-machine interfaces.Advanced materials and mechanics engineering of thin film devices has proven to be an efficient route to enable and enhance flexibility and stretchability of various electronic skins;however,the density of devices is still low owing to the limitation in existing fabrication techniques.Here,we report a high-throughput one-step process to fabricate large tactile sensing arrays with a sensor density of 25 sensors/cm^(2) for electronic skin,where the sensors are based on intrinsically stretchable piezoelectric lead zirconate titanate(PZT)elastomer.The PZT elastomer sensor arrays with great uniformity and passive-driven manner enable highresolution tactile sensing,simplify the data acquisition process,and lower the manufacturing cost.The high-throughput fabrication process provides a general platform for integrating intrinsically stretchable materials into large area,high device density soft electronics for the next-generation electronic skin.

Counterintuitive Ballistic and Directional Liquid Transport on a Flexible Droplet Rectifier

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

Droplet dynamics on slippery surfaces:small droplet,big impact

The dynamic interaction of droplets with slippery surfaces is essential to various industrial applications,such as anti-icing,water-repellency or water-harvesting,anti-bacterial,and phase change heat transfer.With recent progress in materials,manufacturing as well as learning from nature,the physics of droplet dynamics has been greatly enriched owing to the emergence of peculiar wetting states manifested on bio-inspired textured surfaces.This review is devoted to the discussion of the recent progress made in the authors’understanding of the dynamic interaction of small droplets with bio-inspired surfaces.Particular attention is given to droplet impact on slippery surfaces,such as superhydrophobic surfaces characterised with air-solid-liquid triple-phase interface and slippery lubricant infused porous surfaces characterised with the liquid/liquid two-phase interface.Droplet spreading,retraction,contact time,elastodynamics as well as oblique impact are systematically reviewed.Finally,the authors offer their perspectives on this important and highly multidisciplinary research area.

Surface charges as a versatile platform for emerging applications

Surface charges are ubiquitous in nature and their existence is in many forms.For example,at macro-length scale,by contacting and separating two objects,i.e.,contact electrification,one is able to obtain static surface charges.At micro-length scale,electric charges can be found at the air–water interface of a microdroplet because of the triboelectric effect or ionization from high-voltage spray fission.Although static surface charge is a familiar subject,much still remains unknown about how and why such charges form.Contact charge exchange between two metals is known to result from the transfer of electrons.But when at least one of the materials is an insulator or liquid,there is no general understanding of what carries charges from one surface to the other.

In situ Reduction of Silver Nanoparticles on Chitosan Hybrid Copper Phosphate Nanoflowers for Highly Efficient Plasmonic Solar-driven Interfacial Water Evaporation

The development of water purification device using solar energy has received tremendous attention.Despite extensive progress,traditional photothermal conversion usually has a high cost and high environmental impact.To overcome this problem,we develop a low cost,durable and environmentally friendly solar evaporator.This bilayered evaporator is constructed with a thermal insulating polyvinylidene fluoride(PVDF)membrane as a bottom supporting layer and plasmonic silver nanoparticles decorated miero-sized hybrid flower(Ag/MF)as a top light-to-heat conversion layer.Compared with the sample with a flat silver film,the two-tier Ag/MF has a plasmonic enrichment property and high efficiency in converting the solar light to hcat as cach flower can gencrate a microscale hotspot by enriching the absorbed solar light.On the other hand,the PVDF membrane on the bottom with porous structure not only improves the mechanicalstability of the entire structure,but also maintains a stable water supply from the bulk water to the evaporation interface by capillarity and minimizes the thermal conduction.The combination of excellent water evaporation ability simple operation,and low cost of the production process imparts this type of plasmonic enhanced solar-driven interfacial water evaporator with promising prospects for potable water purification for point-of-use applications.

Controlled Cell Patterning on Bioactive Surfaces with Special Wettability

The ability to control cell patterning on artificial substrates with various physicochemical properties is of essence for important implications in cytology and biomedical fields. Despite extensive progress, the ability to control the cell-surface interaction is complicated by the complexity in the physiochemical features of bioactive surfaces. In particular, the manifesta- tion of special wettability rendered by the combination of surface roughness and surface chemistry further enriches the cell-surface interaction. Herein we investigated the cell adhesion behaviors of Circulating Tumor Cells （CTCs） on topog- raphically patterned but chemically homogeneous surfaces. Harnessing the distinctive cell adhesion on surfaces with different topography, we further explored the feasibility of controlled cell patterning using periodic lattices of alternative topographies. We envision that our method provides a designer＇s toolbox to manage the extracellular environment.

Achieving ultra-stable and superior electricity generation by integrating transistor-like design with lubricant armor

Extensive work have been done to harvest untapped water energy in formats of raindrops,flows,waves,and others.However,attaining stable and efficient electricity generation from these low-frequency water kinetic energies at both individual device and large-scale system level remains challenging,partially owing to the difficulty in designing a unit that possesses stable liquid and charge transfer properties,and also can be seamlessly integrated to achieve preferential collective performances without the introduction of tortuous wiring and redundant node connection with external circuit.Here,we report the design of water electricity generators featuring the combination of lubricant layer and transistor-like electrode architecture that endows enhanced electrical performances in different working environments.Such a design is scalable in manufacturing and suitable for facile integration,characterized by significant reduction in the numbers of wiring and nodes and elimination of complex interfacing problems,and represents a significant step toward large-scale,real-life applications.