Droplet Merging Research Articles

Microfluidic Encapsulation of DNAs in Liquid Beads for Digital Loop‐Mediated Isothermal Amplification

Digital nucleic acid analysis has emerged as a prominent tool for the detection and absolute quantification of diverse pathogens. Digital loop‐mediated isothermal amplification (dLAMP) offers highly sensitive, specific, time‐efficient, and cost‐effective nucleic acid amplification. However, existing dLAMP techniques face challenges such as droplet merging, reliance on surfactants, restricted partition capacities, and the potential for sample loss during heating. Herein, these issues are addressed by introducing liquid beads for sample partitioning. Compared to microwells, our approach overcomes the limitations of chamber dimensions, enabling the analysis of an unlimited number of digitized targets. Furthermore, our novel approach effectively addresses sample loss and merging during thermal processing and eliminates the need for surfactants. Accurate and reproducible the quantitative detection of the gene cluster XALB1 of leaf scald disease is conducted using dLAMP based on liquid beads to verify its availability. The results demonstrate a high correlation between target concentration and positive signals, indicating the robust performance of our technique. A comparative analysis is then performed between dLAMP using liquid beads and using single droplets. Benchmarking these two techniques highlights the effectiveness of our innovative technique in overcoming existing challenges in dLAMP.

Micrometer Groove Array Structures Inspired by Desert Grasses on Curved Substrates for Enhanced Surface Condensation Efficiency.

Drawing inspiration from the microstructures on biological surfaces to create highly efficient water-collecting surfaces is an effective way to address water scarcity. Inspired by the role of the convex and concave grooves on the surface of Namib desert grass in promoting condensation, we show that optimizing the curvature radius improves the condensation rate of droplets. This convex-concave geometry, combined with nanoneedle structures on the groove ridges, facilitates droplet merging and self-removal through jumping, refreshing the condensation site and further enhancing condensation efficiency. Meanwhile, reducing the adhesive resistance in the groove valleys accelerates droplet migration and removal. We believe this design strategy can be applied to a wide range of water collection and phase change heat transfer applications.

Elimination of satellite droplets in droplet streams by superposing harmonic perturbations

In tin-droplet laser-produced plasma sources, uniform droplet streams with large droplet spacing are desired to minimize the interference of explosion on neighboring droplets. Such droplet streams can be generated in low wavenumber regimes. However, satellite droplets easily appear among main droplets in those regimes, resulting in plenty of undesirable debris. Herein, a novel odd harmonic superposition perturbation method is proposed to eliminate satellite droplets and enhance droplet spacing of uniform droplet streams. The superposition number (N) and the phase difference (θ) of odd harmonic perturbations are adjusted to facilitate the coalescence of satellite droplets with main droplets. First, the superposed odd-order harmonic components could induce additional disturbance growth in jet surfaces, and finally lead to the asymmetric necking on filaments formed between two adjacent main droplets, featured as various carrot-shaped configurations. This asymmetric necking will cause unbalanced surface tension forces at the two sides of filaments, resulting in a velocity difference between satellite and main droplets. Based on this principle, by setting N and k to 3 and 0.2, respectively, satellite droplets positioned above main droplets accelerate, while those below decelerate, achieving complete coalescence between main and satellite droplets. Furthermore, the phase difference is found to determine the jet breakup location and satellite droplet merging characteristics. As θ varies from 0° to 90°, the droplet size significantly decreases while the droplet spacing remains constant since the perturbation energy increases. The merge direction of satellite droplets reverses from upward to downward due to enhanced unbalanced surface tension forces and velocity differences. As θ continuously increases to 270°, the droplet size further decreases along with a slight decrease in droplet spacing. Finally, by setting N = 3 and θ = 0°, mono-disperse tin droplet streams with a mean diameter of 31.5 μm and a maximum droplet spacing-to-diameter ratio of 17.7 are successfully formed. This work presents a novel approach for eliminating satellite droplets to achieve uniform tin droplet streams with large droplet spacing without increasing the droplet diameter.

High-Efficiency Interdigitated Electrode-Based Droplet Merger for Enabling Error-Free Droplet Microfluidic Systems.

Merging two droplets into a droplet to add and mix two contents is one of the common droplet microfluidic functions with droplet generation and sorting, performing broad ranges of biological and chemical assays in droplets. However, traditional droplet-merging techniques often encounter unsynchronized droplets, causing overmerging or mis-merging, and unwanted merging outside of the desired zone. This is more severe when the incoming droplets to be merged are polydisperse in their sizes, often observed in assays that require long-term incubation, elevated-temperature, and/or multiple droplet processing steps. Here, we developed an interdigitated electrode (IDE)-based droplet merger consisting of a droplet autosynchronizing channel and a merging channel. The autosynchronizing channel provides >95% merging efficiency even when 20% polydispersity in the droplet size exists. The highly localized and enhanced dielectrophoretic force generated by the IDEs on the channel bottom allows droplet merging at an extremely low voltage (4.5 V) and only locally at the IDE region. A systematic evaluation of how various design and operation parameters of the IDE merger, such as IDE finger dimensions, dielectric coating layer thickness, droplet size, and droplet flow speed impact the performance was conducted. The optimized device showed consistent performance even when operating for up to 100 h consecutively at high throughput (100 droplets/s). The presented technology has been integrated into a droplet microfluidics workflow to test the lytic activities of bacteriophage on bacterial host cells with 100% merging efficiency. We expect this function to be integrated into droplet microfluidic systems performing broad ranges of high-throughput chemical and biological assays.

Inkjet-Assisted Electroforming of Magnetically Guidable Devices for Interfacial Microfluidic Manipulation

In the last few decades, inkjet printing (IJP) evolved into a versatile manufacturing technique able to find application in a great number of industrial fields (e.g. electronics, display production, flexible devices). Thanks to its scalability and efficiency, IJP proved to be one of the most promising technologies to pattern materials on a wide variety of rigid and flexible substrates. Recently, it also found application in the field of microfabrication. It was employed, for example, to manufacture microelectromechanical systems (MEMS) [1] or microelectronic components and sensors [2]. Another possible application of IJP, still not developed in the existing literature, is the production of magnetically guidable microrobots [3]. These can find application in a wide variety of fields, including for example micromanipulation, cell transport and drug delivery applications.Many applications of IJP in microfabrication, and microrobots production in particular, require the presence of conductive or magnetic layers to allow actuation or sensing. By directly employing IJP, however, it is difficult to obtain highly conductive and mechanically stable layers. From this point of view, the variety and the performances of IJP printed conductive and magnetic materials are in many cases limited. To expand IJP applicability to micromanufacturing, we developed a novel approach to indirectly pattern bulk metallic microstructures by combining IJP printing and electroforming. Initially, a layer of SU-8 photoresist is IJP printed [4] on a conductive substrate to form a negative of the final pattern. The positive pattern is then growth by means of electrodeposition inside this negative SU-8 pattern. SU-8 is finally removed, leaving an indirectly printed metal pattern. It is also possible to dissolve the substrate in a suitable aggressive solution, leaving thus freestanding electroformed planar structures.In the present work, the technique described is applied to the microfabrication of untethered functional microdevices inspired to the shape and behavior of water-striders (Gerridae) [5]. These insects exploit water surface tension to walk on the surface of shallow lakes and ponds. Taking inspiration from Gerridae, micromachined devices able to move on the air-water interface were developed. A stack of three metallic layers (Cu, NiFe, Cu) was electrodeposited inside a SU-8 pattern. The substrate was subsequently dissolved, releasing thus the electroformed microdevices. These were actuated by applying a controlled magnetic field gradient and they proved able to manipulate droplets of fluids floating at the air-water interface. Moreover, the possibility to carry out microreactions by merging droplets in a controlled way was demonstrated as well.[1] R. Bernasconi et al., Micromachines 14, 2082 (2023)[2] N. K. Mani et al., “Bioelectrochemical Interface Engineering”, chapter 19, John Wiley & Sons (2020)[3] S. Palagi et al., Nat. Rev. Mater. 3(6), 113 (2018)[4] R. Bernasconi et al., Polymer 185, 121933 (2019)[5] R. Bernasconi et al., ACS Appl. Mater. Interfaces 15, 2396-2408 (2023)

Droplet Microfluidics for High-Throughput Screening and Directed Evolution of Biomolecules.

Directed evolution is a powerful technique for creating biomolecules such as proteins and nucleic acids with tailor-made properties for therapeutic and industrial applications by mimicking the natural evolution processes in the laboratory. Droplet microfluidics improved classical directed evolution by enabling time-consuming and laborious steps in this iterative process to be performed within monodispersed droplets in a highly controlled and automated manner. Droplet microfluidic chips can generate, manipulate, and sort individual droplets at kilohertz rates in a user-defined microchannel geometry, allowing new strategies for high-throughput screening and evolution of biomolecules. In this review, we discuss directed evolution studies in which droplet-based microfluidic systems were used to screen and improve the functional properties of biomolecules. We provide a systematic overview of basic on-chip fluidic operations, including reagent mixing by merging continuous fluid streams and droplet pairs, reagent addition by picoinjection, droplet generation, droplet incubation in delay lines, chambers and hydrodynamic traps, and droplet sorting techniques. Various microfluidic strategies for directed evolution using single and multiple emulsions and biomimetic materials (giant lipid vesicles, microgels, and microcapsules) are highlighted. Completely cell-free microfluidic-assisted in vitro compartmentalization methods that eliminate the need to clone DNA into cells after each round of mutagenesis are also presented.

Automated droplet manipulation enabled by a machine-vision-assisted acoustic tweezer

Automated droplet manipulation holds great significance for various biochemical applications, including but limited to heavy metal ions detection. Despite notable progress, contactless-acoustic-tweezer-based automated droplet manipulation on superhydrophobic surfaces is rarely reported. In this work, we introduce a machine-vision-assisted acoustic tweezer (MVAAT) for automated and contactless manipulation of droplets on a superhydrophobic surface. The MVAAT generates ultrasound standing waves between an ultrasonic transducer (UST) and a superhydrophobic surface, inducing acoustic radiation force to facilitate contactless droplet manipulation on the superhydrophobic surface. An industrial-camera-based machine vision system is employed for real-time detection and tracking of droplets, providing precise droplet positions for automated droplet transportation and merging via a UST equipped on a Cartesian robot. Experimental results demonstrate that the proposed MVAAT can transport droplets of various volumes on a superhydrophobic surface along planned paths at considerably high velocities exceeding one centimeter per second. Furthermore, with the vision feedback, the MVAAT can reliably and precisely transport a droplet to a target position, even if the droplet fails to follow the UST during its movement. Moreover, automated merging and patterning of multiple droplets are achieved, showcasing the versatility of the MVAAT. Last, the MVAAT is applied for fluorescence-based Cu2+ detection to showcase its potential for practical biochemical analyses. The proposed MVAAT significantly expands the capability of ultrasound for droplet manipulation on superhydrophobic surfaces, promising numerous practical applications in biology and chemistry.

Airborne Acoustic Vortex End Effector‐Based Contactless, Multi‐Mode, Programmable Control of Object Surfing

AbstractTweezers based on optical, electric, magnetic, and acoustic fields have shown great potential for contactless object manipulation. However, current tweezers designed for manipulating millimeter‐sized objects such as droplets, particles, and small animals exhibit limitations in translation resolution, range, and path complexity. Here, a novel acoustic vortex tweezers system is introduced, which leverages a unique airborne acoustic vortex end effector integrated with a three‐degree‐of‐freedom (DoF) linear motion stage, for enabling contactless, multi‐mode, programmable manipulation of millimeter‐sized objects. The acoustic vortex end effector utilizes a cascaded circular acoustic array, which is portable and battery‐powered, to generate an acoustic vortex with a ring‐shaped energy pattern. The vortex applies acoustic radiation forces to trap and spin an object at its center, simultaneously protecting this object by repelling other materials away with its high‐energy ring. Moreover, The vortex tweezers system facilitates contactless, multi‐mode, programmable object surfing, as demonstrated in experiments involving trapping, repelling, and spinning particles, translating particles along complex paths, guiding particles around barriers, translating and rotating droplets containing zebrafish larvae, and merging droplets. With these capabilities, It is anticipated that the tweezers system will become a valuable tool for the automated, contactless handling of droplets, particles, and bio‐samples in biomedical and biochemical research.

Droplet 3D cryobioprinting for fabrication of free‐standing and volumetric structures

AbstractDroplet‐based bioprinting has shown remarkable potential in tissue engineering and regenerative medicine. However, it requires bioinks with low viscosities, which makes it challenging to create complex 3D structures and spatially pattern them with different materials. This study introduces a novel approach to bioprinting sophisticated volumetric objects by merging droplet‐based bioprinting and cryobioprinting techniques. By leveraging the benefits of cryopreservation, we fabricated, for the first time, intricate, self‐supporting cell‐free or cell‐laden structures with single or multiple materials in a simple droplet‐based bioprinting process that is facilitated by depositing the droplets onto a cryoplate followed by crosslinking during revival. The feasibility of this approach is demonstrated by bioprinting several cell types, with cell viability increasing to 80%–90% after up to 2 or 3 weeks of culture. Furthermore, the applicational capabilities of this approach are showcased by bioprinting an endothelialized breast cancer model. The results indicate that merging droplet and cryogenic bioprinting complements current droplet‐based bioprinting techniques and opens new avenues for the fabrication of volumetric objects with enhanced complexity and functionality, presenting exciting potential for biomedical applications.

Treatment of oil sands process-affected water using a semi pilot-scale coalescing filter: Effect of fundamental conditions and water characteristics on filtration performance

The reuse of oil sands process-affected water (OSPW) generated during oil extraction is becoming essential due to oil extraction efficiency and stringent environmental regulations. This study investigates the efficacy of a coalescing filter (CF) in removing oil components from OSPW by exploiting coalescence, which involves merging small oil droplets into larger ones using fibers. Real-time, precise oil measurement systems were employed to evaluate the oil removal efficiency of the CF across synthetic OSPW samples with varying properties. Fundamental conditions and water quality components were systematically adjusted to elucidate the influencing factors on oil removal by the CF. Results showed that despite the diesel having significantly lower viscosity than the oil contents in OSPW, CF achieved a removal efficiency of over 60% within the first 10 min of operation time, regardless of oil concentration. Total suspended solids/turbidity exhibited a decreasing trend as it passed through the filter, indicating fouling occurring within the filter. On the other hands, its increase improved CF performance by 6% compared to baseline conditions, while an increase of alkalinity decreased performance by 10%. When each water quality component exists independently, they do not significantly impact the oil removal efficiency. However, when present together, there is a more decrease in removal efficiency due to the reduced interaction between oil and filter. CF was resistant to changes in water quality. Rather, changes in fundamental factors are more sensitive to CF performance. Filter characteristics and mechanisms make residence time the primary factor affecting oil removal by the CF, with variations exceeding 30% depending on conditions. Therefore, optimizing filter number and flow rate is essential to maximize the filtration performance.

Inkjet printing of triethylene glycol monomethyl ether (TEGMME)-based ink droplet on oxygen plasma treated PDMS substrate for flexible circuit

Circuits fabricated by inkjet printing on polydimethylsiloxane (PDMS) substrate is a promising direction of flexible electronics. Oxygen plasma treatment is a commonly used method to modify PDMS from hydrophobic to hydrophilic, which is prerequisite before inkjet printing. In this study, inkjet printing of triethylene glycol monomethyl ether (TEGMME)-based silver dispersion nanoparticle droplets on oxygen plasma treated PDMS was investigated by molecular dynamics simulations and experiments. It is revealed that the improved hydrophilicity of PDMS surface is contributed to the replacement of hydrophilic groups (-OH and -C=O) from hydrophobic groups (-CH3) on the PDMS molecular chains. Considering the existence of hydroxyl groups in TEGMME molecules, more hydrogen bonds are formed at the interface between the treated PDMS and TEGMME-based droplets during printing process. These significantly optimize the merging of droplets and the distribution of silver nanoparticles. Moreover, too short plasma treatment will lead to poor circuit morphology caused by weak hydrophilicity, while too long treatment will lead to defects caused by coffee ring effect. The suggested process parameter is treating PDMS for 30 s at a radio-frequency power of 45 W. The morphology of printed circuit is greatly improved and the resistivity decreased to 2.5 Ω/mm. A PDMS-based pressure sensor is designed and fabricated by the proposed inkjet printing process, which shows a sensitivity of 1.8 Ω/kPa and R2 of 0.98.

Rupture of thin nematic walls between isotropic droplets of the same mesogenic material

ABSTRACT We prepared isotropic droplets surrounded by nematic fringes in smectic A freely suspended films. During their coalescence, a thin nematic film is formed that acts as a barrier and separates the isotropic cores of these droplets. This film ruptures in the course of the droplet merging process. The observation of the film dynamics provides an exceptional opportunity to study the rupture of a thin liquid barrier embedded in a surrounding viscous liquid. A detailed dynamic characterisation of the nematic walls is performed. Qualitatively similar observations have been reported in 3D for oil drops coalescing in water and vice versa, where the rupture of an intermediate thin fluid film retards the merging process. The peculiarity of the system reported here is that the material of the droplets and the membrane is identical.

Droplet coalescence in coupled shear and electric fields: A molecular dynamics study

Water droplet coalescence in coupled flow and electric fields is a phenomenon extensively observed in petrochemical, microfluidic, and chemical synthesis processes; however, the existing research in this domain lacks the necessary depth. This paper employs a molecular dynamics approach to elucidate the microscopic mechanisms governing droplet aggregation in water-in-oil emulsions. The investigation spans various shear rates and droplet angles, considering droplet morphology and molecular interactions. The study reveals an interrelation between shear rate and droplet angle effects on droplet coalescence, both seeking to understand the impact of the flow field's shear effect on the electrocoalescence of water droplets. Increasing the shear rate proves advantageous to droplet aggregation up to a certain threshold; however, complete coalescence is hindered beyond the critical shear rate, γ = 7.5 × 109 s−1. Augmenting the droplet angle amplifies the shear force exerted by the flow field on the droplets, while diminishing the radial electric field force. Consequently, an optimal droplet angle of θ = 25° facilitates comprehensive droplet merging. The analysis of the coalescence process, interaction energy, charge density and radial distribution function shows that the approximate process of droplet motion is as follows: the shear effect drives the droplets to approach, and then the directional migration of charge and electrostatic attraction between water molecules drive the coalescence. The existence of a moderate shear effect enables complete droplet coalescence. This study offers theoretical guidance for the exploration of dynamic electrocoalescence technology.

Coalescence of liquid marbles on oil-infused surface

While droplets typically merge instantly in an air medium, alterations to the outer medium can complicate the coalescence process. This study investigates droplet coalescence dynamics when encapsulated by a uniform liquid–solid composite shell, aiming to understand its implications for mechanical stability and merging behavior. The uniform shell around a sessile droplet is produced by using liquid marble on oil-infused surfaces (LMOI). The coalescence dynamics was studied under two different conditions: droplet with LMOI and LMOI with LMOI. In contrast to merging of bare droplets, coalescence involving at least one LMOI reveals a three-step process, including spreading, depletion, and eventual merging phases. Higher oil viscosity influences the merging process, with increased viscosity leading to delayed merging with longer spreading and depletion phases. LMOI exhibits significant resistance to merging with another LMOI, necessitating external triggers like pressure or electric fields for coalescence. These findings provide insights into designing microreactor systems based on LMOI, contributing to the comprehension of their dynamics and functionalities.

Chemical Instability-Induced Wettability Patterns on Superhydrophobic Surfaces.

Chemical instability of liquid-repellent surfaces is one of the nontrivial hurdles that hinders their real-world applications. Although much effort has been made to prepare chemically durable liquid-repellent surfaces, little attention has been paid to exploit the instability for versatile use. Herein, we propose to create hydrophilic patterns on a superhydrophobic surface by taking advantage of its chemical instability induced by acid solution treatment. A superhydrophobic Cu(OH)2 nanoneedle-covered Cu plate that shows poor stability towards HCl solution (1.0 M) is taken as an example. The results show that 2.5 min of HCl solution exposure leads to the etching of Cu(OH)2 nanoneedles and the partial removal of the self-assembled fluoroalkyl silane molecular layer, resulting in the wettability transition from superhydrophobocity to hydrophilicity, and the water contact angle decreases from ~160° to ~30°. Hydrophilic dimples with different diameters are then created on the superhydrophobic surfaces by depositing HCl droplets with different volumes. Afterwards, the hydrophilic dimple-patterned superhydrophobic surfaces are used for water droplet manipulations, including controlled transfer, merging, and nanoliter droplet deposition. The results thereby verify the feasibility of creating wettability patterns on superhydrophobic surfaces by using their chemical instability towards corrosive solutions, which broadens the fabrication methods and applications of functional liquid-repellent surfaces.

Generalization of numerical simulation results on the electrical coalescence threshold for two conducting droplets based on non-dimensional parameters

Electrocoalescence, the physical process underlying the demulsification of a dielectric dispersion medium containing small conductive droplets (e.g., water), involves droplet merging at low electric fields and splashing at higher voltages. Understanding the physics of electrocoalescence is crucial for optimizing industrial electrocoalescers. However, mathematical modeling of these complex, multiphysics phenomena is challenging, and many published results are questionable. In this study, we utilized a previously developed reliable model for computing the threshold between electrical coalescence and non-coalescence. We investigate the applicability of dimensionless parameters, such as the Ohnesorge number and Weber electric number, to describe the coalescence threshold for uncharged droplets of equal size. Using COMSOL Multiphysics software, we analyze the dependency of the threshold electric field strength on water droplet radius and establish an equivalent dimensionless relationship. Our findings reveal that a universal Weber number quite accurately describes the threshold over a wide range of droplet radii, regardless of changes in liquid viscosity and inter-electrode gap. Direct mathematical simulations using up-to-date numerical models enable us to determine non-dimensional parameter values corresponding to the threshold electric field strength, providing generalizable results.

Effects of catechin on the stability of myofibrillar protein-soybean oil emulsion and the adsorbed properties of myosin at the oil–water interface

The effects of different concentrations of catechin on the stability of myofibrillar protein-soybean oil emulsions and the related mechanisms were investigated. Adding 10 μmol/g catechin had no obvious effects on the emulsion stability and myosin structure, but 50, 100 and 200 μmol/g catechin decreased the emulsion stability. The microstructure observations showed that 10 μmol/g catechin caused a dense and uniform emulsion to form, whereas 50, 100 and 200 μmol/g catechin induced the merging of oil droplets. The addition of 50, 100 and 200 μmol/g catechin caused a decline in both the total sulfhydryl content and surface hydrophobicity, suggesting protein aggregation, which decreased the adsorption capacity of myosin and the elasticity of interfacial film. These results suggested that higher concentrations of catechin were detrimental to the emulsifying properties of myosin and that the dose should be considered when it is used as an antioxidant.

Quantum droplet molecules in Bose-Bose mixtures

We investigate the interaction between two self-bound droplets separated by a finite distance in one-dimensional symmetric Bose-Bose mixtures. The main objective is to identify the regime where such two identical droplets support a stable bound-state known as droplet molecule. Our analysis is based on the generalized Gross-Pitaevskii equation, combining cubic and quadratic nonlinearities, which is solved by both variational method and numerical simulation. We calculate in particular the droplet-droplet interaction potential, the binding energy, the density profiles and the width of the droplet. It is shown that the developed variational approximation properly predicts the stability and the static and dynamic properties of such droplet molecules. Our results reveal that under appropriate adjustment of the system parameters such as the droplet separation and the number of atoms, a stable bound-state is formed in the regime of Gaussian-like shape. Nevertheless, in the flat top regime, the two droplets merge into one, leading to an unstable molecule.

A programmable and automated optical electrowetting-on-dielectric (oEWOD) driven platform for massively parallel and sequential processing of single cell assay operations.

Recently, there has been an increasing emphasis on single cell profiling for high-throughput screening workflows in drug discovery and life sciences research. However, the biology underpinning these screens is often complex and is insufficiently addressed by singleplex assay screens. Traditional single cell screening technologies have created powerful sets of 'omic data that allow users to bioinformatically infer biological function, but have as of yet not empowered direct functional analysis at the level of each individual cell. Consequently, screening campaigns often require multiple secondary screens leading to laborious, time-consuming and expensive workflows in which attrition points may not be queried until late in the process. We describe a platform that harnesses droplet microfluidics and optical electrowetting-on-dielectric (oEWOD) to perform highly-controlled sequential and multiplexed single cell assays in massively parallelised workflows to enable complex cell profiling during screening. Soluble reagents or objects, such as cells or assay beads, are encapsulated into droplets of media in fluorous oil and are actively filtered based on size and optical features ensuring only desirable droplets (e.g. single cell droplets) are retained for analysis, thereby overcoming the Poisson probability distribution. Droplets are stored in an array on a temperature-controlled chip and the history of individual droplets is logged from the point of filter until completion of the workflow. On chip, droplets are subject to an automated and flexible suite of operations including the merging of sample droplets and the fluorescent acquisition of assay readouts to enable complex sequential assay workflows. To demonstrate the broad utility of the platform, we present examples of single-cell functional workflows for various applications such as antibody discovery, infectious disease, and cell and gene therapy.

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.