Electrostatic tweezer for droplet manipulation

Individual and collective manipulation of multifunctional bimodal droplets in three dimensions

Spatiotemporally controllable droplet manipulation is vital across numerous applications, particularly in miniature droplet robots known for their exceptional deformability. Despite notable advancements, current droplet control methods are predominantly limited to two-dimensional (2D) deformation and motion of an individual droplet, with minimal exploration of 3D manipulation and collective droplet behaviors. Here, we introduce a bimodal actuation strategy, merging magnetic and optical fields, for remote and programmable 3D guidance of individual ferrofluidic droplets and droplet collectives. The magnetic field induces a magnetic dipole force, prompting the formation of droplet collectives. Simultaneously, the optical field triggers isothermal changes in interfacial tension through Marangoni flows, enhancing buoyancy and facilitating 3D movements of individual and collective droplets. Moreover, these droplets can function autonomously as soft robots, capable of transporting objects. Alternatively, when combined with a hydrogel shell, they assemble into jellyfish-like robots, driven by sunlight. These findings present an efficient strategy for droplet manipulation, broadening the capabilities of droplet-based robotics.

Electrically mediated self-assembly and manipulation of drops at an interface

The fluid-fluid interface is a complex environment for a floating object where the statics and dynamics may be governed by capillarity, gravity, inertia, and other external body forces. Yet, the alignment of these forces in intricate ways may result in beautiful pattern formation and self-assembly of these objects, as in the case of crystalline order observed with bubble rafts or colloidal particles. While interfacial self-assembly has been explored widely, controlled manipulation of floating objects, e.g. drops, at the fluid-fluid interface still remains a challenge largely unexplored. In this work, we reveal the self-assembly and manipulation of water drops floating at an oil-air interface. We show that the assembly occurs due to electrostatic interactions between the drops and their environment. We highlight the role of the boundary surrounding the system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Using experiments and theory, we show that the depth of the oil bath plays an important role in setting the distance between the self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops actively and passively at the interface.

Noncontact Microfluidics of Highly Viscous Liquids for Accurate Self-Splitting and Pipetting

Accurate dosing for various liquids, especially for highly viscous liquids, is fundamental in wide-ranging from molecular crosslinking to material processing. Despite droppers or pipettes being widely used as pipetting devices, they are powerless for quantificationally splitting and dosing highly viscous liquids (>100 mPa s) like polymer liquids due to the intertwined macromolecular chains and strong cohesion energy. Here, a highly transparent photopyroelectric slippery (PS) platform is provided to achieve noncontact self-splitting for liquids with viscosity as high as 15 000 mPa s, just with the assistance of sunlight and a cooling source to provide a local temperature difference (ΔT). Moreover, to guarantee the accuracy for pipetting liquids (>80%), the ultrathin MXene film (within a thickness of 20 nm) is self-assembled as the photo-thermal layers, overcoming the trade-off between transparency and photothermal property. Compared with traditional pipetting strategies (≈1.3% accuracy for pipetting polymer liquids), this accurate microfluidic chip shows great potential in adhesive systems (bonding strength, twice than using the droppers or pipettes).

Portable Triboelectric Electrostatic Tweezer for External Manipulation of Droplets within a Closed Femtosecond Laser-Treated Superhydrophobic System

Controllable droplet manipulation has diverse applications; however, limited methods exist for externally manipulating droplets in confined spaces. Herein, we propose a portable triboelectric electrostatic tweezer (TET) by integrating electrostatic forces with a superhydrophobic surface that can even manipulate droplets in an enclosed space. Electrostatic induction causes the droplet to be subjected to an electrostatic force in an electrostatic field so that the droplet can be moved freely with the TET on a superhydrophobic platform. Characterized by its high precision, flexibility, and robust binding strength, TET can manipulate droplets under various conditions and achieve a wide range of representative fluid applications such as droplet microreactors, precise self-cleaning, cargo transportation, the targeted delivery of chemicals, liquid sorting, soft droplet robotics, and cell labeling. Specifically, TET demonstrated the ability to manipulate internal droplets from the outside of a closed system, such as performing cell labeling experiments within a sealed Petri dish without opening the culture system.

High-throughput single biomarker identification using droplet nanopore

Biomarkers are present in various metabolism processes, demanding precise and meticulous analysis at the single-molecule level for accurate clinical diagnosis. Given the need for high sensitivity, biological nanopore have been applied for single biomarker sensing. However, the detection of low-volume biomarkers poses challenges due to their low concentrations in dilute buffer solutions, as well as difficulty in parallel detection. Here, a droplet nanopore technique is developed for low-volume and high-throughput single biomarker detection at the sub-microliter scale, which shows a 2000-fold volume reduction compared to conventional setups. To prove the concept, this nanopore sensing platform not only enables multichannel recording but also significantly lowers the detection limit for various types of biomarkers such as angiotensin II, to 42 pg. This advancement enables direct biomarker detection at the picogram level. Such a leap forward in detection capability positions this nanopore sensing platform as a promising candidate for point-of-care testing of biomarker at single-molecule level, while substantially minimizing the need for sample dilution.

Single-Electrode Electrostatic Repulsion Phenomenon for Remote Actuation and Manipulation

One of the fundamental principles of electrostatics is that an uncharged object will be attracted to a charged object through electrostatic induction as the two approaches one another. We refer to the charged object as a single electrode and examine the scenario where a positive voltage is applied. Because of electrostatic induction phenomenon, single-electrode electrostatics only generates electrostatic attraction forces. Here, we discover that single-electrode electrostatics can generate electrostatic repulsion forces and define this new phenomenon as single-electrode electrostatic repulsion phenomenon. We investigate the fundamental electrostatic phenomena, giving a curve of electrostatic force versus voltage and then defining 3 regions. Remote actuation and manipulation are essential technologies that are of enormous concern, with tweezers playing an important role. Various tweezers designed on the basis of external fields of optics, acoustics, and magnetism can be used for remote actuation and manipulation, but some inherent drawbacks still exist. Tweezers would benefit greatly from our discovery in electrostatics. On the basis of this discovery, we propose the concept of electrostatic tweezers, which can achieve noncontact and remote actuation and manipulation. Experimental characterizations and successful applications in metamaterials, robots, and manipulating objects demonstrated that electrostatic tweezers can produce large deformation rates (>6,000%), fast actuation (>100 Hz), and remote manipulating distance (~15 cm) and have the advantages of simple device structure, easy control, lightweight, no dielectric breakdown, and low cost. Our work may deepen people's understanding of single-electrode electrostatics and opens new opportunities for remote actuation and manipulation.

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.

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.

Smart materials for light control of droplets

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

Multifunctional Electrostatic Droplet Manipulation on the Femtosecond Laser-Prepared Slippery Surfaces

A strategy to manipulate droplets on the lubricated slippery surfaces using tribostatic electricity is proposed. By employing femtosecond laser-induced porous microstructures, we prepared a slippery surface with ultralow adhesion to various liquids. Electrostatic induction causes the charges within the droplet to be redistributed; thus, the droplet on the as-prepared slippery surfaces can be guided by electrostatic force under the electrostatic field, with controllable sliding direction and unlimited transport distance. The combination of electrostatic interaction and slippery surfaces allows us to manipulate droplets with a wide volume range (from 100 nL to 0.5 mL), charged droplets (including electrostatic attraction and repulsion), corrosive droplets, and even organic droplets with ultralow surface tension. In addition, droplets on tilted surfaces, curved surfaces, and inverted slippery surfaces can also be manipulated. Especially, the slippery surfaces can even allow the electrostatic interaction to manipulate alcohol with surface tension as low as 22.3 mN/m and liquid droplets suspended on a downward surface, which is not possible with reported superhydrophobic substrates. The features of slippery surfaces make the electrostatic manipulation successfully applied in versatile droplet manipulation, droplet patterning, chemical microreaction, transport of solid cargo, targeted delivery of chemicals, and liquid sorting.

Rewritable printing of ionic liquid nanofilm utilizing focused ion beam induced film wetting

Manipulating liquid flow over open solid substrate at nanoscale is important for printing, sensing, and energy devices. The predominant methods of liquid maneuvering usually involve complicated surface fabrications, while recent attempts employing external stimuli face difficulties in attaining nanoscale flow control. Here we report a largely unexplored ion beam induced film wetting (IBFW) technology for open surface nanofluidics. Local electrostatic forces, which are generated by the unique charging effect of Helium focused ion beam (HFIB), induce precursor film of ionic liquid and the disjoining pressure propels and stabilizes the nanofilm with desired patterns. The IBFW technique eliminates the complicated surface fabrication procedures to achieve nanoscale flow in a controllable and rewritable manner. By combining with electrochemical deposition, various solid materials with desired patterns can be produced.

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.

On-Demand, Contact-Less and Loss-Less Droplet Manipulation via Contact Electrification

While there are many droplet manipulation techniques, all of them suffer from at least one of the following drawbacks - complex fabrication or complex equipment or liquid loss. In this work, a simple and portable technique is demonstrated that enables on-demand, contact-less and loss-less manipulation of liquid droplets through a combination of contact electrification and slipperiness. In conjunction with numerical simulations, a quantitative analysis is presented to explain the onset of droplet motion. Utilizing the contact electrification technique, contact-less and loss-less manipulation of polar and non-polar liquid droplets on different surface chemistries and geometries is demonstrated. It is envisioned that the technique can pave the way to simple, inexpensive, and portable lab on a chip and point of care devices.

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

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.

Mechano-Driven Tribo-Electrophoresis Enabled Human-Droplet Interaction in 3D Space

Electronically controlled droplet manipulation has widespread applications in biochemistry, life sciences, and industry. However, current technologies such as electrowetting, electrostatics, and surface charge printing rely on complex electrode arrays and external power supplies, leading to inefficient manipulation. In light of these limitations, a novel method is proposed, which leverages tribo-electrophoresis (TEP) to pipette in an oil medium, thereby enabling human-droplet interactions to be constructed with greater efficiency. The approach involves the rational design of a triboelectric nanogenerator-electrostatic tweezer that generates an electric field to charge the droplet and improves the maneuverability of the charged droplet, including aligned/non-aligned pipetting and stable transport in the clamped state, which can be accomplished solely by hand motion. The TEP method not only provides droplets with freedom to move in three dimensions but also offers a feasibility case for chemical reactions in the liquid phase and non-invasive sample extraction. This breakthrough establishes a cornerstone for human-droplet interactions capitalized on triboelectric nanogenerators, opening new avenues for research in droplet manipulation.

Antidurotaxis Droplet Motion onto Gradient Brush Substrates

Durotaxis motion is a spectacular phenomenon manifesting itself by the autonomous motion of a nano-object between parts of a substrate with different stiffness. This motion usually takes place along a stiffness gradient from softer to stiffer parts of the substrate. Here, we propose a new design of a polymer brush substrate that demonstrates antidurotaxis droplet motion, that is, droplet motion from stiffer to softer parts of the substrate. By carrying out extensive molecular dynamics simulation of a coarse-grained model, we find that antidurotaxis is solely controlled by the gradient in the grafting density of the brush and is favorable for fluids with a strong attraction to the substrate (low surface energy). The driving force of the antidurotaxial motion is the minimization of the droplet-substrate interfacial energy, which is attributed to the penetration of the droplet into the brush. Thus, we anticipate that the proposed substrate design offers a new understanding and possibilities in the area of autonomous motion of droplets for applications in microfluidics, energy conservation, and biology.

Self-Driven Droplet Motions Below their Icing Points

Liquid fluidity is a most key prerequisite for a broad range of technologies, from energy, fluid machineries, microfluidic devices, water, and oil transportation to bio-deliveries. While from thermodynamics, the liquid fluidity gradually diminishes as temperature decreases until completely solidified below icing points. Here, self-driven droplet motions are discovered and demonstrated occurring in icing environments and accelerating with both moving distances and droplet volumes. The self-driven motions, including self-depinning and continuous wriggling, require no surface pre-preparation or energy input but are triggered by the overpressure spontaneously established during icing and then continuously accelerated by capillary pulling of frosts. Such self-driven motions are generic to a broad class of liquid types, volumes, and numbers on various micro-nanostructured surfaces and can be facilely manipulated by introducing pressure gradients spontaneously or externally. The discovery and control of self-driven motions below icing points can greatly broaden liquid-related applications in icing environments.

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.

Laser-guided programmable construction of cell-laden hydrogel microstructures forin vitrodrug evaluation

Cell-laden hydrogel microstructures have been used in broad applications in tissue engineering, translational medicine, and cell-based assays for pharmaceutical research. However, the construction of cell-laden hydrogel microstructuresin vitroremains challenging. The technologies permitting generation of multicellular structures with different cellular compositions and spatial distributions are needed. Herein, we propose a laser-guided programmable hydrogel-microstructures-construction platform, allowing controllable and heterogeneous assembly of multiple cellular spheroids into spatially organized multicellular structures with good bioactivity. And the cell-laden hydrogel microstructures could be further leveraged forin vitrodrug evaluation. We demonstrate that cells within hydrogels exhibit significantly higher half-maximal inhibitory concentration values against doxorubicin compared with traditional 2D plate culture. Moreover, we reveal the differences in drug responses between heterogeneous and homogeneous cell-laden hydrogel microstructures, providing valuable insight intoin vitrodrug evaluation.

Microfluidic platform using focused ultrasound passing through hydrophobic meshes with jump availability

Applications in chemistry, biology, medicine, and engineering require the large-scale manipulation of a wide range of chemicals, samples, and specimens. To achieve maximum efficiency, parallel control of microlitre droplets using automated techniques is essential. Electrowetting-on-dielectric (EWOD), which manipulates droplets using the imbalance of wetting on a substrate, is the most widely employed method. However, EWOD is limited in its capability to make droplets detach from the substrate (jumping), which hinders throughput and device integration. Here, we propose a novel microfluidic system based on focused ultrasound passing through a hydrophobic mesh with droplets resting on top. A phased array dynamically creates foci to manipulate droplets of up to 300 μL. This platform offers a jump height of up to 10 cm, a 27-fold improvement over conventional EWOD systems. In addition, droplets can be merged or split by pushing them against a hydrophobic knife. We demonstrate Suzuki-Miyaura cross-coupling using our platform, showing its potential for a wide range of chemical experiments. Biofouling in our system was lower than in conventional EWOD, demonstrating its high suitability for biological experiments. Focused ultrasound allows the manipulation of both solid and liquid targets. Our platform provides a foundation for the advancement of micro-robotics, additive manufacturing, and laboratory automation.

Small-Scale Robotics with Tailored Wettability

Small-scale robots (SSRs) have emerged as promising and versatile tools in various biomedical, sensing, decontamination, and manipulation applications, as they are uniquely capable of performing tasks at small length scales. With the miniaturization of robots from the macroscale to millimeter-, micrometer-, and nanometer-scales, the viscous and surface forces, namely adhesive forces and surface tension have become dominant. These forces significantly impact motion efficiency. Surface engineering of robots with both hydrophilic and hydrophobic functionalization presents a brand-new pathway to overcome motion resistance and enhance the ability to target and regulate robots for various tasks. This review focuses on the current progress and future perspectives of SSRs with hydrophilic and hydrophobic modifications (including both tethered and untethered robots). The study emphasizes the distinct advantages of SSRs, such as improved maneuverability and reduced drag forces, and outlines their potential applications. With continued innovation, rational surface engineering is expected to endow SSRs with exceptional mobility and functionality, which can broaden their applications, enhance their penetration depth, reduce surface fouling, and inhibit bacterial adhesion.

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.

Magnetic Bead Manipulation in Microfluidic Chips for Biological Application

Magnetic beads manipulation in microfluidic chips is a promising research field for biological application, especially in the detection of biological targets. In this review, we intend to present a thorough and in-depth overview of recent magnetic beads manipulation in microfluidic chips and its biological application. First, we introduce the mechanism of magnetic manipulation in microfluidic chip, including force analysis, particle properties, and surface modification. Then, we compare some existing methods of magnetic manipulation in microfluidic chip and list their biological application. Besides, the suggestions and outlook for future developments in the magnetic manipulation system are also discussed and summarized.

Spreading dynamics of microdroplets on nanostructured surfaces

HYPOTHESIS

Droplet spreading governs various daily phenomena and industrial processes. Insights about microdroplet spreading are limited due to experimental difficulties arising from microdroplet manipulation and substrate wettability control. For droplet sizes approaching the capillary length scale, the gravitational force plays an important role in spreading. In contrast, capillary and viscous forces dominate as the droplet size reduces to smaller length scales. We hypothesize that the dynamic spreading behavior of microdroplets whose radius is far lower than the capillary length differs substantially from established and well understood dynamics.

EXPERIMENTS

To systematically investigate the spreading dynamics of microdroplets, we develop contact-initiated wetting techniques combined with structuring-independent wettability control to achieve microdroplet (<500 μm) spreading on arbitrary surfaces while eliminating parasitic pinning effects (pining force ∼ 0) and initial impact momentum effects (Weber number ∼ 0).

FINDINGS

Our experiments reveal that the capillary-driven initial spreading of microdroplets is shorter, with significantly reduced oscillation dampening, when compared to millimeter-scale droplets. Furthermore, spreading along with capillary wave propagation results in coupling between the spreading velocity and dynamic contact angle at the contact line. These findings, along with our proposed microdroplet manipulation platform, may find application in microscale heat transfer, advanced manufacturing, and aerosol transmission studies.

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.

Unidirectional Droplet Propulsion onto Gradient Brushes without External Energy Supply

Using extensive molecular dynamics simulation of a coarse-grained model, we demonstrate the possibility of sustained unidirectional motion (durotaxis) of droplets without external energy supply when placed on a polymer brush substrate with stiffness gradient in a certain direction. The governing key parameters for the specific substrate design studied, which determine the durotaxis efficiency, are found to be the grafting density of the brush and the droplet adhesion to the brush surface, whereas the strength of the stiffness gradient, the viscosity of the droplet, or the length of the polymer chains of the brush have only a minor effect on the process. It is shown that this durotaxial motion is driven by the steady increase of the interfacial energy between droplet and brush as the droplet moves from softer to stiffer parts of the substrate whereby the mean driving force gradually declines with decreasing roughness of the brush surface. We anticipate that our findings indicate further possibilities in the area of nanoscale motion without external energy supply.

Light-Programmed Bistate Colloidal Actuation Based on Photothermal Active Plasmonic Substrate

Active particles have been regarded as the key models to mimic and understand the complex systems of nature. Although chemical and field-powered active particles have received wide attentions, light-programmed actuation with long-range interaction and high throughput remains elusive. Here, we utilize photothermal active plasmonic substrate made of porous anodic aluminum oxide filled with Au nanoparticles and poly( N -isopropylacrylamide) (PNIPAM) to optically oscillate silica beads with robust reversibility. The thermal gradient generated by the laser beam incurs the phase change of PNIPAM, producing gradient of surface forces and large volume changes within the complex system. The dynamic evolution of phase change and water diffusion in PNIPAM films result in bistate locomotion of silica beads, which can be programmed by modulating the laser beam. This light-programmed bistate colloidal actuation provides promising opportunity to control and mimic the natural complex systems.

Triboelectric wetting for continuous droplet transport

Manipulating liquid is of great significance in fields from life sciences to industrial applications. Owing to its advantages in manipulating liquids with high precision and flexibility, electrowetting on dielectric (EWOD) has been widely used in various applications. Despite this, its efficient operation generally needs electrode arrays and sophisticated circuit control. Here, we develop a largely unexplored triboelectric wetting (TEW) phenomenon that can directly exploit the triboelectric charges to achieve the programmed and precise water droplet control. This key feature lies in the rational design of a chemical molecular layer that can generate and store triboelectric charges through agile triboelectrification. The TEW eliminates the requirement of the electric circuit design and additional source input and allows for manipulating liquids of various compositions, volumes, and arrays on various substrates in a controllable manner. This previously unexplored wetting mechanism and control strategy will find diverse applications ranging from controllable chemical reactions to surface defogging.

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.

Chopstick-Like Structure for the Free Transfer of Microdroplets in Robot Chemistry Laboratory

As we all know, chopsticks can hold food, so can we use this method to carry Newtonian fluids such as droplets? This paper studies the process of this transfer and uses this method to realize the manipulation of open microfluidics by robots. To realize this transfer operation, we first analyzed the force of droplets in this chopstick-like structure and found that the bidirectional movement of droplets in this structure can be achieved by changing the structural parameters. Afterward, the whole process of the transfer of droplets using the chopstick-like structure was analyzed, and the parameter requirements for realizing this transfer were determined. The research in this paper provides a theoretical basis for the controllable manipulation of droplets which can be widely used in unmanned laboratories.

Janus Charged Droplet Manipulation Mediated by Invisible Charge Walls

The ability to control the mobility and function of droplets is fundamental to developing open surface microfluidics. Despite notable progress in the manipulation of droplets, the existing strategies are still limited in functionalizing droplets. Herein, the coupling of droplet motion and functionalization elicited by an invisible charge wall is reported. The charged superamphiphobic surface is overlapped with a conductor to induce free charge, creating the invisible charge wall at the overlapping boundary. The charge wall can trap droplets and polarize them into Janus charged state. It is found that the trapping degree and the charge distribution in the Janus charged droplet depend on the original surface charge on the superamphiphobic surface. The invisible charge wall can also be established at diverse boundary curvatures, allowing to design pathways for droplet manipulations. Furthermore, the enrichment of protein and nanomaterial in the manipulated Janus charged droplet is demonstrated. The strategy provides a potential microfluidic platform with orthogonal functionalities.

Spontaneous Charging of Drops on Lubricant-Infused Surfaces

When a drop of a polar liquid slides over a hydrophobic surface, it acquires a charge. As a result, the surface charges oppositely. For applications such as the generation of electric energy, lubricant-infused surfaces (LIS) may be important because they show a low friction for drops. However, slide electrification on LIS has not been studied yet. Here, slide electrification on lubricant-infused surfaces was studied by measuring the charge generated by series of water drops sliding down inclined surfaces. As LIS, we used PDMS-coated glass with micrometer-thick silicone oil films on top. For PDMS-coated glass without lubricant, the charge for the first drop is highest. Then it decreases and saturates at a steady state charge per drop. With lubricant, the drop charge starts from 0, then it increases and reaches a maximum charge per drop. Afterward, it decreases again before reaching its steady-state value. This dependency is not a unique phenomenon for lubricant-infused PDMS; it also occurs on lubricant-infused micropillar surfaces. We attribute this dependency of charge on drop numbers to a change in surface conductivity and depletion of lubricant. These findings are helpful for understanding the charge process and optimizing solid-liquid nanogenerator devices in applications.

Controllable and Gradient Wettability of Bilayer Two-Dimensional Materials Regulated by Interlayer Distance

Surfaces with controllable and gradient wettability often require an elaborate design of the microstructure or its response under electrical, thermal, optical, pH, and other stimuli. Generally, the wettability change under these physical or chemical effects relies on a complex mechanism that is difficult to be quantitatively described. In this study, an online controlling strategy for surface wettability and the corresponding theoretical model are put forward based on a bilayer graphene-like atomic structure. Molecular dynamics results indicate that the surface wettability varies toward hydrophilicity after sticking a bottom material regardless of its wettability. But such an influence becomes weak with increasing interlayer distance, and the overall wettability approaches that of the upper layer material gradually. This variation is elucidated by the increase of the work of adhesion, providing new insight into the wetting transparency of graphene. A theoretical model of the governing relationship is established based on the work of adhesion, which correlates the overall surface wettability with the interlayer distance and the wettabilities of individual materials. Moreover, a surface with a uniform wettability gradient is achieved by inclining the bottom material. The spontaneous and steady motion of droplets can be induced by this gradient wettability. The relevant speedup behavior is evaluated through a theoretical model considering the varying interlayer distance, which reveals the critical role of the lower layer. This study proposes a novel strategy for controllable wetting and relevant gradient surfaces using prevailing two-dimensional materials, paving new routes to many applications such as microfluidic chips, virus diagnosis, and intelligent sensors.

Light control of droplets on photo-induced charged surfaces

The manipulation of droplets plays a vital role in fundamental research and practical applications, from chemical reactions to bioanalysis. As an intriguing and active format, light control of droplets, typically induced by photochemistry, photomechanics, light-induced Marangoni effects or light-induced electric fields, enables remote and contactless control with remarkable spatial and temporal accuracy. However, current light control of droplets suffers from poor performance and limited reliability. Here we develop a new superamphiphobic material that integrates the dual merits of light and electric field by rationally preparing liquid metal particles/poly(vinylidene fluoride-trifluoroethylene) polymer composites with photo-induced charge generation capability in real time, enabling light control of droplets on the basis of photo-induced dielectrophoretic force. We demonstrate that this photo-induced charged surface (PICS) imparts a new paradigm for controllable droplet motion, including high average velocity (∼35.9 mm s^-1), unlimited distance, multimode motions (e.g. forward, backward and rotation) and single-to-multiple droplet manipulation, which are otherwise unachievable in conventional strategies. We further extend light control of droplets to robotic and bio-applications, including transporting a solid cargo in a closed tube, crossing a tiny tunnel, avoiding obstacles, sensing the changing environment via naked-eye color shift, preparing hydrogel beads, transporting living cells and reliable biosensing. Our PICS not only provides insight into the development of new smart interface materials and microfluidics, but also brings new possibilities for chemical and biomedical applications.

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.

Reconfigurable Magnetic Liquid Metal Robot for High-Performance Droplet Manipulation

Droplet manipulation is crucial for diverse applications ranging from bioassay to medical diagnosis. Current magnetic-field-driven manipulation strategies are mainly based on fixed or partially tunable structures, which limits their flexibility and versatility. Here, a reconfigurable magnetic liquid metal robot (MLMR) is proposed to address these challenges. Diverse droplet manipulation behaviors including steady transport, oscillatory transport, and release can be achieved by the MLMR, and their underlying physical mechanisms are revealed. Moreover, benefiting from the magnetic-field-induced active deformability and temperature-induced phase transition characteristics, its droplet-loading capacity and shape-locking/unlocking switching can be flexibly adjusted. Because of the fluidity-based adaptive deformability, MLMR can manipulate droplets in challenging confined environments. Significantly, MLMR can accomplish cooperative manipulation of multiple droplets efficiently through on-demand self-splitting and merging. The high-performance droplet manipulation using the reconfigurable and multifunctional MLMR unfolds new potential in microfluidics, biochemistry, and other interdisciplinary fields.

Simple but Efficient Method To Transport Droplets on Arbitrarily Controllable Paths

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