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Liquid–liquid phase separation plays a prominent role in the physics of life, providing the cells with various membrane-less compartments. These structures exhibit a range of material properties that, in many cases, change over time. Inspired by this, we investigate here an aqueous two-phase system formed by mixing polyethylene glycol with dextran. We modulate the material properties of the resulting dextran droplets by adding DNA that readily enters the droplets. We find a non-monotonic dependence of the physical properties of the droplets under the imposed ionic conditions. [ABSTRACT FROM AUTHOR]

We show that the reflection interference fringe (RIF) is formed on a screen far away from the microdroplets placed on a prism-based substrate, which have low contact angles and thin droplet heights, caused by the dual convex–concave profile of the droplet, not a pure convex profile. The geometric formulation shows that the interference fringes are caused by the optical path difference when the reflected rays from the upper convex profile at the droplet–air interface interfere with reflection from the lower concave profile at oblique angles lower than the critical angle. Analytic solutions are obtained for the droplet height and the contact angle out of the fringe number and the fringe radius in RIF from the geometric formulation. Furthermore, the ray tracing simulation is conducted using the custom-designed code. The geometric formulation and the ray tracing show excellent agreement with the experimental observation in the relation between the droplet height and the fringe number and the relation between the contact angle and the fringe radius. This study is remarkable as the droplet's dual profile cannot be easily observed with the existing techniques. However, the RIF technique can effectively verify the existence of a dual profile of the microdroplets in a simple setup. In this work, the RIF technique is successfully developed as a new optical diagnostic technique to determine the microdroplet features, such as the dual profile, the height, the contact angle, the inflection point, and the precursor film thickness, by simply measuring the RIF patterns on the far-field screen. [ABSTRACT FROM AUTHOR]

Experimental findings by others suggest that OH radicals are formed in unexpected abundance on or near surfaces of 1–50 µm microdroplets comprised of pure water, but the mechanism by which these radicals are generated is not yet fully resolved. In this work, we examine two possibilities using ab initio electronic structure methods: (1) electron transfer (ET) from a microdroplet surface-bound OH− anion to a nearby H3O+ cation and (2) proton transfer (PT) from such a H3O+ cation to a nearby OH− anion. Our findings suggest that both processes are possible but only if the droplet's underlying water molecules comprising the microdroplet provide little screening of the Coulomb interaction between the anion and cation once they reach ∼10 Å of one another. In the ET event, an OH radical is formed directly; for PT, the OH formation occurs because the new O–H bond formed by the transferred proton is created at a bond length sufficiently elongated to permit homolytic cleavage. Both the ET and PT pathways predict that H atoms will also be formed. Finally, we discuss the roles played by strong local electric fields in mechanisms that have previously been proposed and that occur in our two mechanisms. [ABSTRACT FROM AUTHOR]

Temperature monitoring has immediate relevance to many areas of research, from atmospheric environmental studies to biological sample and food preservation to chemical reactions. Here, we use a triplebarrel electrode to provide temperature readouts in bulk solution and microdroplets, as well as electrochemically monitor freezing events in a microdroplet. Using this method, we are able to identify distinct characteristics of a freezing aqueous droplet (supercooling, ice formation beginning and end, temperature change, and thawing) with greater temporal resolution than a standard thermocouple and without the use of microscopy. By correlating the amperometric signal change caused by alterations in the diffusion coefficient of the electrochemical system in response to temperature changes, we can calculate the instantaneous temperature at our electrode, as well as the physical behavior of ice formation and expansion. Our results suggest that these electrochemical techniques can provide real-time monitoring of the physical processes involved in aqueous temperature change and ice nucleation events. Here, we employ a novel technique using triple-barrel electrodes to provide temperature readouts in bulk solution and microdroplets, as well as electrochemically monitor freezing events in a microdroplet. Because ice nucleation spans many research fields, it is important to have a variety of tools that can be used to better understand these frozen systems. Our data shows that electrochemistry can provide real-time information on the thermal properties of aqueous environments, and these types of measurements can be extended to microdroplets. The electrochemical signal details all the significant moments in a droplet freezing event, allowing us to use electrochemistry as a stand-alone tool for monitoring freezing events with excellent temporal and spatial resolution. [ABSTRACT FROM AUTHOR]

Controlling the volume and geometrical ratio of microdroplets offers numerous advantages, particularly in fields like pharmaceutics, photonics, soft electronics, and robotics. For microdroplets generated via microfluidics, precise control of their length-to-width ( L w ) ratio enhances their overall performance and contributes to a comprehensive understanding of the results. Advanced experimental tools, including droplet generation systems, visualization techniques, digital processing algorithms, and analysis, are crucial, especially for in situ control of microdroplet size during generation. This study investigates the variation in microdroplet L w ratio and their generation frequencies using two novel approaches: the probability density function (derived both from the Gaussian assumption and directly from experimental data distribution) and the fast Fourier transform. The configuration utilizing one syringe pump (1mp) and syringes (made entirely of plastic) demonstrated the best monodispersity in microdroplet generation with volume differences ranged from 40 ± 2.2 nl to 65 ± 6.7 nl, corresponding to coefficient of variation values of 5.5 % and 11 % , respectively. The analysis of ∼7 million L w data points revealed different types of oscillation frequencies. Specifically, a frequency of around 1 mHz originated from the syringe pump, while frequencies between ∼0.5 and 3 Hz were attributed to the syringe materials and the interaction of a combination of syringe pumps. These results contribute to the phenomenological understanding and classification of droplet generation within the T-junction microfluidic system. Furthermore, they present an enhanced method for visualizing the geometrical variations of the generated microdroplets. [ABSTRACT FROM AUTHOR]

At present, a variety of active and passive methods for generating microdroplets with different morphologies are available. Microcapsules with multi-core or compartment structures not only exhibit characteristics such as encapsulation, isolation, and leak prevention, but also possess specific functions, including enhanced buffering performance and superior heat transfer characteristics. Nevertheless, the high-throughput manufacturing of controllable multi-core droplets remains a significant challenge, constrained by the complexity of the equipment, the inconvenience of control, and the high cost. This study introduces a novel flow focusing method that integrates biphasic excitation to produce uniformly distributed double-emulsion droplets with a controlled number of cores at high throughput. The breakup of coaxial jets has been studied under different excitation frequencies, amplitudes, and flow rates of inner and outer liquids, with a particular focus on the change of the droplet morphology as the controllable parameter varies. By applying excitation to both the inner and outer jets in the weak coupling mode, our technique exhibits promising outcomes in achieving uniformity and controllability in the number of cores of the generated droplets. The scaling laws of the compound droplet size have been obtained, providing theoretical guidance for practical applications. The proposed biphasic excitation approach enhances the precision and efficiency of droplet generation processes in a range of applications, including pharmaceuticals, biotechnology, and materials science. [ABSTRACT FROM AUTHOR]

Fluorocarbon oils are uniquely suited for many biomedical applications due to their inert, bioorthogonal properties. In order to interface fluorocarbon oils with biological systems, non‐ionic fluorosurfactants are necessary. However, there is a paucity of non‐ionic fluorosurfactants with low interfacial tension (IFT) to stabilize fluorocarbon phases in aqueous environments (such as oil‐in‐water emulsions). We developed non‐ionic fluorosurfactants composed of a polyethylene glycol (PEG) segment covalently bonded to a flexible perfluoropolyether (PFPE) segment that confer low IFTs between a fluorocarbon oil (HFE‐7700) and water. The synthesis of a panel of surfactants spanning a molecular weight range of 0.64–66 kDa with various hydrophilic‐lipophilic balances allowed for identification of minimal IFTs, ranging from 1.4 to 17.8 mN m−1. The majority of these custom fluorosurfactants display poor solubility in water, allowing their co‐introduction with fluorocarbon oils and minimal leaching. We applied the PEG5PFPE1 surfactant for mechanical force measurements in zebrafish, enabling exceptional sensitivity. [ABSTRACT FROM AUTHOR]

Polymer microparticle synthesis based on the surface‐templated method is a simple and environmentally friendly method to produce various microparticles. Unique particles with different compositions can be fabricated by simply annealing a polymer on a liquid‐repellent surface. However, there are hurdles to producing particles of homogeneous sizes with large quantities and varying the shape of particles. Here, a new approach to synthesizing multiple polymer microparticles using micropatterns with wettability contrast is presented. Polymer microparticles are formed in two steps. First, a layer of poly(sodium‐4‐styrenesulfonate) is deposited on the hydrophilic regions by dipping and withdrawing this micropattern from a polymer solution, and an array of microdroplets is formed. A dewetting‐inducing layer on the pattern is introduced, and then target polymer patches are sequentially generated on it. By annealing over Tg, the contact line of the target polymer patch is freely receded, creating a particle form. The size and shape of the microparticle can be controlled by varying the micropatterns. In addition, it is demonstrated that microparticles made of polymer blends or polymer/nanoparticle composite are easily produced. This versatile method offers the potential of surface‐templated synthesis to tailor polymer microparticles with different sizes, shapes, and functionalities in various research and applications. [ABSTRACT FROM AUTHOR]

Traditional circuit board fabrication schemes are not directly applicable to the production of flexible, multi‐material circuits. This article reports a technique, microfluidic directed material patterning, which combines soft microfluidic stamps and low‐temperature solution‐phase deposition to generate multi‐material circuits on flexible, non‐planar polymeric supports. Specifically, metallic and semiconductive traces are combined on commodity plastic films to yield functional photosensitive circuits that can be used in the spectrophotometric detection and concentration measurement of microdroplets on 3D "e‐plates." The photoresistive material cadmium sulfide is used in these circuits because it is suitable for visible light detection and it can be deposited directly from aqueous solutions following established bath deposition procedures. This method can produce colorimetric devices capable of quantifying micromolar concentrations of Allura Red in microdroplets of Kool‐Aid. This technique presents the opportunity for producing single‐use or low‐use disposable/recyclable devices for flexible 3D sensors and detectors following a convenient, low‐waste fabrication scheme. The general capabilities of this approach, in terms of substrate geometry and device layout (e.g., the number, area, and pattern of photoresistive elements), can be applied to the design and manufacture of more intricate, multiplexed devices supportive of advanced and/or specialized functions that go beyond those reported in this initial demonstration. [ABSTRACT FROM AUTHOR]

The curious chemistry observed in microdroplets has captivated chemists in recent years and has led to an investigation into their ability to drive seemingly impossible chemistries. One particularly interesting capability of these microdroplets is their ability to accelerate reactions by several orders of magnitude. While there have been many investigations into which reactions can be accelerated by confinement within microdroplets, no study has directly compared reaction acceleration at the liquid|liquid and gas|liquid interfaces. Here, we confine glucose oxidase, one of life's most important enzymes, to microdroplets and monitor the turnover rate of glucose by the electroactive cofactor, hexacyanoferrate (III). We use stochastic electrochemistry to monitor the collision of single femtoliter water droplets on an ultramicroelectrode. We also develop a measurement modality to robustly quantify reaction rates for femtoliter liquid aerosol droplets, where the majority of the interface is gas|liquid. We demonstrate that the gas|liquid interface accelerates enzyme turnover by over an order of magnitude over the liquid|liquid interface. This is the first apples-to-apples comparison of reaction acceleration at two distinct interfaces that indicates that the gas|liquid interface plays a central role in driving curious chemistry. [ABSTRACT FROM AUTHOR]

Seed amplification assays (SAAs) are a promising avenue for the early diagnosis of neurodegenerative diseases. However, when amplifying fibrils from patient‐derived samples in multiwell plates, it is currently highly challenging to accurately quantify the aggregates. It is therefore desirable to transfer such assays into a digital format in microemulsion droplets to enable direct quantification of aggregate numbers. To achieve transfer from conventional plate‐based to the microfluidic digital format, effective seed amplification needs to be achieved inside the microdroplets. Therefore, we establish a new set of assay conditions that enable highly efficient seed amplification in plates without any shaking. However, the same set of conditions displayed a very different behavior upon transfer to a microfluidic platform where no amplification was observed. We demonstrate that this is caused by the suppression of all secondary processes that could amplify the seeds in the complete absence of mechanical perturbations inside the microdroplets. We further show that the amplification inside droplets can be achieved by subjecting the microemulsions to high‐frequency vibrations using a piezo device. Taken together, our results provide novel insights into the physical requirements of alpha‐synuclein seed amplification and demonstrate a pathway towards the development of effective digital SAAs. [ABSTRACT FROM AUTHOR]

Surface-enhanced Raman spectroscopy (SERS) is a widely used analytical technique known for its high sensitivity and broad applicability. Despite its potential, SERS faces challenges related to detection sensitivity and reproducibility. This study proposes an innovative method to enhance SERS performance by employing water microdroplets as optical lenses on localized silver nanoparticle-decorated porous silicon (LocAg-PS) substrates. The hydrophobic nature of the LocAg-PS substrate not only ensures precise positioning of the microdroplet lenses on the silver nanoparticle grafted pad (AgNP pad) but also forms a plano-convex-like microdroplet lens for the focusing of the excitation laser and the collection of scattered light. Experimental results demonstrate that using microdroplet lenses enhances the SERS signal intensity and reproducibility, providing a rapid and cost-effective solution for advanced SERS analysis. [ABSTRACT FROM AUTHOR]

The effect of temperature profile of the surface of a water layer on the formation and geometrical structure of a cluster of levitating droplets is studied in a series of laboratory experiments. The experiments show that a local temperature maximum of the water surface is a necessary condition for the droplet cluster formation. A quantitative criterion for the transformation of a monolayer of randomly placed microdroplets into a self-assembled cluster of relatively large droplets is obtained. A qualitative physical description of the formation of a flat levitating droplet cluster of an axisymmetric hexagonal structure is given, based on the experimentally verified concept of the decisive role of aerodynamic forces acting on water droplets from an upward vapor-air flow. Unusual droplet clusters resulting from high surfactant concentrations and rapidly changing or more intense local heating of the underlying water layer were observed. These are elegant ring clusters, small clusters of a controlled number of large droplets, and chain clusters with branching chains of droplets in their central part. The use of a recently developed experimental procedure based on the injection of initial microdroplets with a piezoelectric dispenser makes it possible to generate hierarchical clusters, which contain continuously transforming aggregates of several droplets held in contact by electrostatic interaction. An overview of various types of droplet clusters including relatively stable hexagonal clusters, emerging and rapidly breaking up ring-shaped clusters, small clusters of a desirable number of nearly identical droplets, chain clusters containing growing branched chains of water droplets, and hierarchical clusters with rearranged small groups of nearly merging droplets provides a complete picture not only of the transition from chaotically moving droplets to self-arranged clusters, but also of the diversity of droplet clusters. [ABSTRACT FROM AUTHOR]

Accelerated synthesis of gold nanoparticles (AuNPs) in charged microdroplets produced by electrospray ionization (ESI) was exploited to modify the surface of graphite screen-printed electrodes (GSPEs). The deposited AuNPs were then functionalized by the charged microdroplets deposition of 6-ferrocenyl-hexanethiol (6Fc-ht) solutions that act as reducing and stabilizing agents and provide electrochemical properties for the modified electrodes. The morphology and composition of the AuNPs were characterized by scanning electron microscopy (SEM). Cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of the modified electrodes. The results showed that the ESI microdroplets deposition technique produces uniform and well-dispersed AuNPs on GSPE, and optimal conditions for deposition were identified, enhancing GSPE electrocatalytic performance. Further functionalization by ESI microdroplets of AuNPs with 6Fc-ht demonstrated improved redox properties compared with the conventional self-assembled monolayer (SAM) method, highlighting the technique's potential for the easy and fast functionalization of electrochemical sensors. [ABSTRACT FROM AUTHOR]

Microdroplet impact icing is a common occurrence in high altitudes or cloud environments. There is a lack of in-depth understanding regarding the icing behaviors of microdroplets interacting with anti-icing microstructures. While the impact of microstructure pitch on delaying icing is extensively studied, the microstructure height, as a similarly technology-controllable structural parameter, has received comparatively little attention in research. This study investigates the impact dynamics of microdroplets on supercooled superhydrophobic microgrooves, utilizing the coupled level-set and volume of fluid and enthalpy-porosity technologies for precise simulation. The behaviors of microdroplets impact on supercooled surface manifest distinctive characteristics compared with ambient surface, particularly evident when We < 170 (βmax (maximum spreading diameter)/βMod (predicted spreading diameter) ∼ 0.8). Unlike flat surfaces, microgrooves have been shown to reduce βmax by 52%. Additionally, an increase in H/Do (ratio of microgroove height H and droplet diameter Do) leads to a diminishing trend in βmax, showing a reduction of 13%. The outcomes of microdroplet impact progress from complete bouncing, partial bouncing, bouncing with satellite droplet breakup, to deposition, sequentially as the We and surface supercooling degree escalate. Heightening the microstructure alters the transition point between partial bouncing and bouncing with breakup, thereby expanding the range of droplet bouncing (with thresholds being doubled). This effect is attributed to satellite droplet breakup, supported by simulation results suggesting that droplet breakup reduces interface curvature, thus facilitating bouncing. Moreover, microgroove structures have the potential to decrease the dimensionless contact time of droplets by 33% when comparing H/Do = 0.4 with H/Do = 0.1. [ABSTRACT FROM AUTHOR]

Effect mechanisms of the undercooling degree and the surface configuration on the ice growth characteristics were revealed under micro-droplets icing conditions. Preferential ice crystals appear firstly on the surfaces due to the randomness of icing, and obtain growth advantages to form protruding structures. Protruding structures block the incoming droplets from contacting the substrates, causing voids around the structures. The undercooling degree mainly affects the density and the growth rate of preferential ice crystals. With the increase of undercooling degree, the preferential ice crystals have higher density and growth rate, resulting in stronger growth advantage and higher porosity. The surface configuration affects the growth mode, and the ice layer grows with uniform mode, spreading mode and structure-induced mode on the aluminum, smooth Polytetrafluoroethylene (PTFE) and rough PTFE surface respectively, causing the needle-like, ridge-like and cluster-like ice crystals. The rough structures effectively improve the porosity of the ice layer, which is beneficial for optimizing the icephobic property of the materials. This paper provides important theoretical guidance for the design of subsequent icephobic materials. [ABSTRACT FROM AUTHOR]

At the air–water interface, many reactions are accelerated, sometimes by several orders of magnitude. This phenomenon has proved to be particularly important in water microdroplets, where the spontaneous oxidation of many species stable in bulk has been experimentally demonstrated. Different theories have been proposed to explain this finding, but it is currently believed that the role of interfacial electric fields is key. In this work, we have carried out a quantum chemistry study aimed at shedding some light on this question. We have studied two prototypical processes in which a hydroxide anion transfers its excess electron to either the water environment or a dioxygen molecule. To model the interface, we use a cluster of 21 water molecules immersed in an electric field, and we examine the energetics of the studied reactions as a function of field magnitude. Our results reveal that electric fields close to those estimated for the neat air–water interface (∼0.15 V ⋅ Å−1) have a moderate effect on the reaction energetics and that much stronger fields (>1 V ⋅ Å−1) are required to get spontaneous electron transfer. Therefore, the study suggests that additional factors such as an excess charge in microdroplets need to be considered for explaining the experimental observations. [ABSTRACT FROM AUTHOR]

A catalyst-free method was developed for the ring opening of azlactones (also known as oxazolones) in water microdroplets. Azlactone was dissolved in a water : acetonitrile (1 : 1) mixture, and the solution is sprayed by using nitrogen gas at a pressure of 120 psi to generate microdroplets. This method promoted selective cleavage of the lactone bond to afford the corresponding N-benzoyl derivatives in up to 94% isolated yield with no epimerization. Our method produces the ring-opening products in milliseconds (up to 94 μmol for 33.3 minutes), and may have utility for high-throughput synthesis applications. [ABSTRACT FROM AUTHOR]

The extraordinary chemistry of microdroplets has reshaped how we as a community think about reactivity near multiphase boundaries. Even though interesting physico-chemical properties of microdroplets have been reported, "sessile" droplets' inherent mobility, which has been implicated as a driving force for curious chemistry, has not been well established. This paper seeks to answer the question: Can adsorbed microdroplets be mobile at the nanoscale? This is a tantalizing question, as almost no measurement technique has the spatiotemporal resolution to answer it. Here, we demonstrate a highly sensitive technique to detect nanometric motions of insulating bodies adsorbed to electrified microinterfaces. We place an organic droplet atop a microelectrode and track its dissolution by driving a heterogeneous reaction in the aqueous continuous phase. As the droplet's contact radius approaches the size of the microelectrode, the current versus time curve remarkably displays abrupt changes in current. We used finite element modeling to demonstrate these abrupt steps are due to nanometric movements of the three-phase boundary, where the nonaqueous droplet meets the aqueous phase and the electrode. Furthermore, the velocity with which the liquid interface moves can be estimated to tens-to-hundreds of nanometers per second. Our results indicate that processes that are driven by contact electrification and the frictional movement of bodies on a surface may be at play even when a droplet seems quiescent. [ABSTRACT FROM AUTHOR]

Droplet generation is crucial in various scientific and industrial fields, such as drug delivery, diagnostics, and inkjet printing. While microfluidic platforms enable precise droplet formation, traditional methods often require costly and complex setups, limiting their accessibility. This study introduces a simple, low-cost approach using an off-the-shelf unit and a 3D-printed reservoir. The device, equipped with a driver board, piezo-ring transducer, and a metal sheet with holes, generates oil-in-water (O/W) droplets with an average diameter of 4.62 ± 0.67 µm without external fluid pumps. Its simplicity, cost-effectiveness, and scalability make it highly suitable for both lab-on-chip and industrial applications, demonstrating the feasibility of large-scale uniform droplet production. [ABSTRACT FROM AUTHOR]

Investigating the self‐sorting behaviour of assemblies with subtle structural differences is a captivating yet challenging endeavour. Herein, we elucidate the unusual self‐sorting behaviour of metallacages with subtle structural differences in batch reactors and microdroplets. Narcissistic self‐sorting of metallacages has been observed for two ligands with identical sizes, shapes, and symmetries, with only minor differences in the substituted groups. In particular, the self‐sorting process in microdroplets occurs within 1 min at room temperature, in stark contrast to batch reactors, which require equilibration for 30 min. To reveal the mechanism of self‐sorting and the role of microdroplets, we conducted a series of experiments and theoretical calculations, including competitive self‐assembly, cage‐to‐cage transformation, control experiments involving model metallacages with larger cavities, noncovalent interaction analysis, and root mean square deviation (RMSD) analysis. This research demonstrates an unusual case of self‐sorting of very similar assemblies and provides a new strategy for facilitating the self‐sorting efficiency of supramolecular systems. [ABSTRACT FROM AUTHOR]

Droplet microarrays (DMAs) leveraging wettability differences are instrumental in digital immunoassays, single-cell analysis, and high-throughput screening. This study introduces an enhanced Teflon lift-off process to fabricate hydrophilic–hydrophobic patterns on a digital microfluidic (DMF) chip, thereby integrating DMAs with DMF technology. By employing DMF for droplet manipulation and utilizing wettability differences, the automated generation of high-throughput DMAs was achieved. The volume of the microdroplets ranged from picoliters to nanoliters. For droplets with a diameter of 150 μm, the array density reached up to 1282 cm−2. We systematically investigated the influence of various DMF parameters on the formation of DMAs and applied this technique to particle distribution, achieving a single-cell isolation rate of approximately 30%. We believe that this method will be a potent tool to enhance the capabilities of DMAs and DMF technology and extend their applicability across more fields. [ABSTRACT FROM AUTHOR]

Multi‐level anticounterfeiting tags have been developed using a combination of different materials. Polyvinyl alcohol (PVA) mixed with titanium dioxide (TiO2) is used to produce flexible substrates. Fluorescent Opuntia Ficus‐indica (OFI) extract dissolved with polymethyl methacrylate (PMMA) is then sprayed over the substrate to create a random, yet unique deposition of droplets. Photographs of the tags are taken under UV illumination at different angles and analyzed through the scale‐invariant feature transform (SIFT) algorithm to extract their unique features. The SIFT analysis reveals hundreds to thousands of matched features when a given tag is compared with itself, whereas this number drops to tens for different tags. To enhance the security of the tags, ITO is sputtered onto one of them in the form of a pattern formed by a patch array exhibiting a specific fingerprint at terahertz (THz) frequencies. The evaluation of ITO reflectance shows that each patch array has a unique and unpredictable response stemming from its distinct electro‐optical characteristics. The non‐deterministic response of sprayed dye droplets and ITO patches enables the realization of two‐level authentication, which is difficult to replicate at a reasonable cost. The simple manufacturing process and inexpensive materials involved make the proposed tags easily integrable into packaging. [ABSTRACT FROM AUTHOR]

Here we developed a microdroplet-based strategy for the rapid synthesis of uniform polydopamine nano- and microspheres. Polydopamine spheres with controllable sizes were generated within hundreds of microseconds by simply spraying water solutions of dopamine into microdroplets. Mass spectrometry revealed that dopamine was primarily oxidized into aminochrome, acting as the major building block for polydopamine. We anticipate that microdroplet chemistry will be rich in opportunities for the synthesis of functional nano- and micromaterials. [ABSTRACT FROM AUTHOR]

We developed a rapid polymorphic screening approach based on contracting sessile microdroplets, which offers several advantages: (1) achieves very high supersaturation to facilitate formation of metastable forms; (2) allows systematic labeling of samples; (3) gives access to the statistical distribution of polymorphic selectivity as a function of experimental conditions; (4) ensures the formation of crystal for each droplet, addressing the problem of uncrystallized droplets in traditional microfluidics. We studied the competitive nucleation of D -mannitol polymorphs and investigated the effect of droplet volume on polymorphic selectivity. We showed that our observed polymorph distributions at different volumes are qualitatively consistent with the predictions of classical nucleation theory except for very small volumes where thermodynamic confinement or surface effects could play a substantial role. Overall, our microfluidic approach can be a promising tool not only for routine screening of pharmaceutical polymorphs in the industrial context but also in the fundamental understanding of the mechanisms underlying the competitive nucleation of polymorphs. [ABSTRACT FROM AUTHOR]

Droplet microfluidic-based technology is a powerful tool for biotechnology, and it is also expected that it will be applied to culturing and screening methods. Using this technology, a new high-throughput screening method for lactic acid bacteria was developed. In this study, the conventional culture of lactic acid bacteria that form clear zones on an agar medium was reproduced in water-in-oil droplets, and only the droplets in which lactic acid bacteria grew were collected one by one. Using this method, the specific recovery of Lactiplantibacillus plantarum from a mixture of L. plantarum and Escherichia coli and the acquirement of lactic acid bacteria from an environmental sample were successful. This method could be applied to various conventional screening methods using the clear zone as a microbial growth indicator. This has expanded the possibilities of applying droplet microfluidic-based technology to microbial cultivations. [ABSTRACT FROM AUTHOR]

Interaction between tumor‐associated macrophages and tumor cells is crucial for tumor development, metastasis, and the related immune process. However, the macrophages are highly heterogeneous spanning from anti‐tumorigenic to pro‐tumorigenic, which needs to be understood at the single‐cell level. Herein, a sessile microdroplet system designed for monitoring cellular behavior and analyzing intercellular interaction, demonstrated with macrophage‐tumor cell pairs is presented. An automatic procedure based on the inkjet printing method is utilized for the precise pairing and co‐encapsulation of heterotypic cells within picoliter droplets. The sessile nature of microdroplets ensures controlled fusion and provides stable environments conducive to adherent cell culture. The nitric oxide generation and morphological changes over incubation are explored to reveal the complicated interactions from a single‐cell perspective. The immune response of macrophages under distinct cellular microenvironments is recorded. The results demonstrate that the tumor microenvironment displays a modulating role in polarizing macrophages from anti‐tumorigenic into pro‐tumorigenic phenotype. The approach provides a versatile and compatible platform to investigate intercellular interaction at the single‐cell level, showing promising potential for advancing single‐cell behavior studies. [ABSTRACT FROM AUTHOR]

Organic contaminants have increasingly become a main threat to the water environment, necessitating novel methods for removing from polluted water. In this study, a kind of magnetic ethyl acetate microdrops (Fe3O4@KCC-1@EA) is fabricated for this purpose The KCC-1 shell of Fe3O4@KCC-1 nanospheres is a layer of silica with a dendritic fibrous structure. The ethyl acetate shell of the Fe3O4@KCC-1@EA microdrops provides them with the properties of an organic solvent. While the magnetic core makes them magnetically manipulable. Adding Fe3O4@KCC-1@EA microdrops to bisphenol A-polluted water allows the contaminants to be extracted into the ethyl acetate shell. These microdrops, saturated with bisphenol A, are then easily separated from the water phase with an external magnetic field, achieving a removal rate of over 98%. Besides bisphenol A, the Fe3O4@KCC-1@EA microdrops could also be employed to remove other organic contaminants. This method could provide a new pathway for water purification from organic contaminants. [ABSTRACT FROM AUTHOR]

We present a smart roof that makes fragmented droplets from the impact of raindrops on superhydrophobic meshes and utilizes the droplets for agricultural spraying. This facile method transforms raindrops or waterdrops into uniform microdroplets, which can both reduce crop lodging induced by heavy rainfall, and realize uniform spraying of pesticides. [ABSTRACT FROM AUTHOR]

Biomolecular condensates are membraneless organelles that orchestrate various metabolic pathways in living cells. Understanding how these crowded structures regulate enzyme reactions remains yet challenging due to their dynamic and intricate nature. Coacervate microdroplets formed by associative liquid‐liquid phase separation of oppositely charged polyions have emerged as relevant condensate models to study enzyme catalysis. Enzyme reactions within these droplets show altered kinetics, influenced by factors such as enzyme and substrate partitioning, crowding, and interactions with coacervate components; it is often challenging to disentangle the contributions of each. Here, we investigate the peroxidase activity of a de novo enzyme within polysaccharide‐based coacervates. By comparing the reaction kinetics in buffer, in a suspension of coacervates and in the bulk coacervate phase collected after centrifugation of the droplets, we show that the coacervate phase significantly increases the enzyme catalytic efficiency. We demonstrate that the main origin of this enhanced activity lies in macromolecular crowding coupled to changes in the conformational dynamics of the enzyme within the coacervate environment. Altogether, these findings underline the crucial role of the coacervate matrix in enzyme catalysis, beyond simple partitioning effects. The observed boost in enzyme activity within the coacervate phase provides insights for designing biocatalytically active synthetic organelles. [ABSTRACT FROM AUTHOR]

Targeted drug delivery system (DDS) holds exciting prospects in biomedical clinical research and drug development. However, it faces a significant challenge in achieving stable and safe targeted transportation without drug leakage while obtaining precise and controllable drug dosing with a high drug utilization rate and low side effects. Herein, an optical tweezers‐based light‐driven technique is proposed to transport and construct multi‐core/shell microdroplets enveloping drug carriers and target cells as a novel, safe, and efficient targeted DDS. Drug‐loaded microdroplets with core/shell structures are first fabricated by a new and simple injection extrusion method and then it is transported to a target cell by a dynamic optical trap with a low optical power, which ensures no drug leakage, exogenous materials, and biothermal damage are introduced during the drug delivery process. Next, precise drug dosing is realized by pushing the target cell into the drug‐loaded microdroplet under the actions of optical forces, which constructs a multi‐core/shell microdroplet structure with a pure drug environment to improve the drug action efficiency. Moreover, quantitative drug dosing with a high drug utilization rate can also achieved by controlling the sizes/number of drug droplets inside the microdroplets. This light‐driven DDS is of great interest to frontier medical research and disease treatment fields. [ABSTRACT FROM AUTHOR]

The investigation of the interplay between complex coacervate microdroplets and amphiphilic molecules offers valuable insights into the processes of prebiotic compartmentalization on the early Earth and presents a promising avenue for future advancements in biotechnology. Herein, the interaction between complex coacervate microdroplets and amphiphilic molecule (decanoic acid) is systematically investigated by varying charge strengths of negatively charged polyelectrolytes (DNA and PAA) and positively charged polyelectrolytes (PDDA and DEAE‐Dextran). It is found that the interaction between amphiphilic molecule and complex coacervate microdroplets depended on the delicate balance between the interaction between decanoic acid and polyelectrolyte and the interaction between two polyelectrolytes. The different spatial distribution of amphiphilic molecule can result in differences in the internal microenvironment, which can further alter the uptake or exclusion of small molecules and biomolecules with different charges and polarities and functional biological process. [ABSTRACT FROM AUTHOR]

To examine if 12.5 μl timolol maleate 0.5% microdrops dispensed with the Nanodropper Adaptor provide noninferior intraocular pressure (IOP) reduction compared with conventional 28 μl drops in patients with open-angle glaucoma (OAG) and ocular hypertension (OHT). Prospective, noninferiority, parallel, multicenter, single-masked, active-controlled, randomized trial. Treatment-naïve subjects who were recently diagnosed with OAG and OHT at the Aravind Eye Care System. Both eyes of subjects received 1 commercially available drop or both eyes of subjects received 1 microdrop of timolol maleate 0.5%. We measured IOP, resting heart rate (HR), and blood pressure (BP) at baseline and 1, 2, 5, and 8 hours after timolol administration. The IOP was the primary outcome measure. Secondary outcomes were resting HR, systolic BP (sBP), and diastolic BP (dBP). Adaptor-mediated microdrops and conventional drops of timolol significantly decreased IOP compared with baseline at all timepoints. Noninferiority was established at 3 of 4 timepoints. Heart rate decreases with Nanodropper were approximately 3 beats per minute (bpm) less than with conventional drops. Timolol microdrops appear to be as effective in ocular hypotensive action as conventional drops with a slightly attenuated effect on resting HR and BP. Proprietary or commercial disclosure may be found after the references. [ABSTRACT FROM AUTHOR]

Background: Recently, microbotulinum, a new technique that involves injecting botulinum toxin type A (BoNTA) microdroplets into superficial cutaneous tissue, has gained popularity. The precise distribution of BoNTA in the targeted area profoundly affects outcomes. Many factors may influence the effective area of BoNTA in the dermis. This study aimed to determine the dermal distribution properties of BoNTA to guide microbotulinum injection. Methods: Ten healthy males aged 18--65 years without BoNTA treatment in the previous year were recruited to receive intradermal injections in the chest and back. Ultrasound was used to ensure the intradermal delivery of injections and measure the dermal thickness. The minor iodine starch test was performed at baseline and 3 days, 7 days, 21 days, 1 month, and 2 months after treatment. Results: All participants received intradermal injections. The dermis was thinner on the chest (thickness, 0.20±0.03 cm) than on the back (thickness, 0.39 ± 0.07 cm) (P<0.05). An injection in the thicker dermis had a significantly smaller effective area at every follow-up visit. The drug concentration did not affect the effective area except at 3 days after treatment. Injection speed did not influence the effective area at any follow-up visits. Conclusion: An injection in a thicker dermis leads to a smaller effective area for intradermal injections. When the BoNTA dose is the same, the drug concentration and injection speed do not matter. [ABSTRACT FROM AUTHOR]

In order to study Bacillus subtilis biofilm formation in microdroplets, we use microfluidics technology to make the droplets and confocal microscopy to capture bacterial movement and biofilm formation in the droplets. We develop a multi-target tracking methodology, using a YOLOv5 detector to identify cells and a DeepSORT algorithm to track cell movements. We find that Bacillus subtilis bacteria with autonomous migration and biofilm-forming ability prefer to cluster and swarm near the microdroplet surface, rather than in the droplet interior. Bacterial mobility depends on phenotype and spatial location within the droplet. The motile cells move about 3.5 times faster than the matrix-producing cells. When the cells are near the wall of the droplet, the direction of the motion of motile cells is along that wall. When the cells are inside the droplet, the direction of the motion of motile cells is disordered, i.e., there is no clear directional or goal-oriented movement. This contrast increases the cell contact probability and facilitates the formation of a Bacillus subtilis biofilm in the droplet. Furthermore, we develop a mathematical model to describe the motion behavior of Bacillus subtilis in microdroplets, which is useful for exploring the influence of motility on biofilm formation. [ABSTRACT FROM AUTHOR]

The high-speed impact of liquid microdroplets has emerged as a promising method to eliminate nanoparticle contaminants from semiconductor wafer surfaces. However, existing experimental studies have primarily focused on evaluating their own nozzles for removing particles larger than 50 nm, which does not align with the current research roadmap where the target particles' size decreases down to sub-10 nm. As it is more challenging to remove smaller particles, there is a strong need to better understand droplet's spreading behavior in relation to the detachment of particles from a surface. Nonetheless, there remains a lack of experimental evidence to validate existing models or a scarcity of numerical simulation studies, mainly due to the practical difficulties associated with single droplet experiments. Hence, in this study, we conduct a series of numerical simulations to investigate the time-dependent spreading behavior of the droplet, together with collecting local velocity data at the attached particle position. The local velocity data is then integrated to an existing model to predict the effective cleaning diameter for each impact condition. Starting from the free-fall dropwise impaction, we develop a single-microdroplet cleaning system by minimizing the number of sprayed droplets and capturing their behavior using a high-speed camera (HSC), with aims of validating the model predictions and numerical simulations. Finally, we provide a contour plot for a prior prediction of the effective cleaning diameter from the impaction conditions of microdroplets. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The effects of viscoelasticity on the stability and morphology of the liquid cone in liquid–liquid flow focusing are investigated experimentally and numerically. The particle tracers are utilized in experiments to visualize the flow fields, and the Oldroyd-B model is applied in numerical simulations to describe the viscoelastic characteristics of the liquid cone. Based on the quantitative analyses on the elastic stresses and forces inside the cone, the influence of viscoelasticity on the startup process of the liquid cone is first investigated. The stretching and shrinking stages of the viscoelastic cone are identified, and the startup process of the Newtonian cone is also studied for comparison. By considering the force balance at local jet position, a scaling analysis is proposed to give the criterion for the establishment of the stable cone, which indicates that the axial elastic stress can promote the cone stability. Upon a stable liquid cone, the influences of viscoelasticity on the interface profile and flow field of the cone are further analyzed, indicating that an increase in viscoelasticity leads to more shrinkage of the cone interface. The shrinkage of cone leads to the acceleration of focused liquid and thus the decrease in the recirculation flow size. This fundamental work provides scientific guidance for understanding the influences of viscoelasticity in flow focusing process, contributing to the industrial applications of microdroplets production. [ABSTRACT FROM AUTHOR]

This paper presents a novel design of the device to generate microspheres or micro-droplets based on the membrane emulsification principle. Specifically, the novelty of the device lies in a proposed two-layer or stepwise (by generalization) membrane structure. An important benefit of the stepwise membrane is that it can be fabricated with the low-cost material (SU-8) and using the conventional lithography technology along with a conventional image-based alignment technique. The experiment to examine the effectiveness of the proposed membrane was conducted, and the result shows that microspheres with the size of 2.3 μm and with the size uniformity of 0.8 μm can be achieved, which meets the requirements for most applications in industries. It is noted that the traditional membrane emulsification method can only produce microspheres of around 20 μm. The main contribution of this paper is thus the new design principle of membranes (i.e., stepwise structure), which can be made by the cost-effective fabrication technique, for high performance of droplets production. [ABSTRACT FROM AUTHOR]

Nucleation, the birth of a stable cluster from a disorder, is inherently stochastic. Yet up to date, there are no quantitative studies on NaCl nucleation that accounts for its stochastic nature. Here, we report the first stochastic treatment of NaCl-water nucleation kinetics. Using a recently developed microfluidic system and evaporation model, our measured interfacial energies extracted from a modified Poisson distribution of nucleation time show an excellent agreement with theoretical predictions. Furthermore, analysis of nucleation parameters in 0.5, 1.5, and 5.5 pl microdroplets reveals an interesting interplay between confinement effects and shifting of nucleation mechanisms. Overall, our findings highlight the need to treat nucleation stochastically rather than deterministically to bridge the gap between theory and experiment. [ABSTRACT FROM AUTHOR]

In this study, the mechanisms of jet atomization were analyzed based on a frequency analysis of atomization sounds in the audible range (∼20 kHz). Jet atomization is a two-dimensional, high-speed atomization using a diaphragm, and interesting acoustic signals and atomization phenomena were detected on hydrophobic and hydrophilic diaphragms. The hydrophilic diaphragm strongly interacted and resonated with the surface wave, resulting in symmetrical jet atomization relative to the diaphragm. The resonance between the diaphragm and the surface wave was supported by a calculation of the eigenfrequency of the diaphragm and the coincidence of the droplet diameters as calculated from Lang's equation. Notably, the diaphragm excited by the ultrasonic transducer acted as a new transducer vibrating perpendicular to the transducer. As a result, when the diaphragm and the surface wave were in resonance at 2.4 MHz, a symmetrical two-dimensional high-speed jet atomization was generated in the direction perpendicular to the transducer's vibration direction. This study also revealed that the atomization state can be determined based on the acoustic analysis. This acoustic analysis of atomization sounds can be applied in more advanced atomization control, such as for providing uniform dispersions of droplets containing DNA, drugs, or microplastics. [ABSTRACT FROM AUTHOR]

Membraneless coacervate microdroplets have long been proposed as model protocells as they can grow, divide, and concentrate RNA by natural partitioning. However, the rapid exchange of RNA between these compartments, along with their rapid fusion, both within minutes, means that individual droplets would be unable to maintain their separate genetic identities. Hence, Darwinian evolution would not be possible, and the population would be vulnerable to collapse due to the rapid spread of parasitic RNAs. In this study, we show that distilled water, mimicking rain/freshwater, leads to the formation of electrostatic crosslinks on the interface of coacervate droplets that not only suppress droplet fusion indefinitely but also allow the spatiotemporal compartmentalization of RNA on a timescale of days depending on the length and structure of RNA. We suggest that these nonfusing membraneless droplets could potentially act as protocells with the capacity to evolve compartmentalized ribozymes in prebiotic environments. [ABSTRACT FROM AUTHOR]

The ability of analytical strategies to detect and positively identify molecules under extremely dilute conditions is important for the growth and expansion of analytical techniques and instrumentation. At present, few measurement science techniques can robustly approach the measurement of just a few thousand molecules. Here, we present an electrochemical platform for the detection and positive identification of fewer than 1000 molecules of decamethylferrocene ((Cp*)2FeII). We achieve this remarkable detection threshold by trapping (Cp*)2FeII in a 1,2-dichloroethane microdroplet, which is allowed to dissolve into an aqueous continuous phase while on a gold microelectrode (radius ∼6.25 μm). Because electrochemistry is not sensitive enough to observe the charge of less than 1000 molecules, we dissolved μM amounts hexacyanoferrate(III) in the aqueous continuous phase. The biphasic reaction between hexacyanoferrate(III) and Cp2*(Fe)II allows for a feedback loop when the microelectrode is biased sufficiently negative to reduce Cp2*(Fe)III. This feedback loop, a typical EC′ catalytic mechanism, amplifies the electrochemical signal of Cp2*(Fe)II when the droplet is of small enough dimensions for feedback to occur. Our results demonstrate that clever biphasic reactions can be coupled with dissolving microdroplets to access extremely low limits of quantitation in electroanalysis. [ABSTRACT FROM AUTHOR]

We report a novel method to conduct heterogeneous reactions using aqueous-ionic liquid Janus microdroplets as a series of isolated bi-phasic microreactors where AgCl@polyaniline core–shell nanoparticles are successfully synthesized accompanied by polyaniline nano-needles, and enhanced visualization of reaction progression through the color changes in Janus droplets is achieved. [ABSTRACT FROM AUTHOR]

The interaction of perfluorinated molecules, also known as "forever chemicals" due to their pervasiveness, with their environment remains an important yet poorly understood topic. In this work, the self‐assembly of perfluorinated molecules with multivalent hosts, pillar‐[5]‐arenes, is investigated. It is found that perfluoroalkyl diacids and pillar‐[5]‐arenes rapidly and strongly complex with each other at aqueous interfaces, forming solid interfacially templated films. Their complexation is shown to be driven primarily by fluorophilic aggregation and assisted by electrostatic interactions, as supported by the crystal structure of the complexes, and leads to the formation of quasi‐2D phase‐separated films. This self‐assembly process can be further manipulated using aqueous two‐phase system microdroplets, enabling the controlled formation of 3D micro‐scaffolds. [ABSTRACT FROM AUTHOR]

Droplet evaporation and dissolution phenomena are pervasive in both natural and artificial systems, playing crucial roles in various applications. Understanding the intricate processes involved in the evaporation and dissolution of sessile droplets is of paramount importance for applications such as inkjet printing, surface coating, and nanoparticle deposition, etc. In this study, we present a demonstration of electrochemical investigation of the dissolution behaviour in sub-nL droplets down to sub-pL volume. Droplets on an electrode have been studied for decades in the field of electrochemistry to understand the phase transfer of ions at the oil–water interface, accelerated reaction rates in microdroplets, etc. However, the impact of microdroplet dissolution on the redox activity of confined molecules within the droplet has not been explored previously. As a proof-of-principle, we examine the dissolution kinetics of 1,2-dichloroethane droplets (DCE) spiked with 155 μM decamethylferrocene within an aqueous phase on an ultramicroelectrode (r = 6.3 μm). The aqueous phase serves as an infinite sink, enabling the dissolution of DCE droplets while also facilitating convenient electrical contact with the reference/counter electrode (Ag/AgCl 1 M KCl). Through comprehensive voltammetric analysis, we unravel the impact of droplet dissolution on electrochemical response as the droplet reaches minuscule volumes. We validate our experimental findings by finite element modelling, which shows deviations from the experimental results as the droplet accesses negligible volumes, suggesting the presence of complex dissolution modes. [ABSTRACT FROM AUTHOR]

Droplet sorting and enrichment, as a prominent field within microfluidic technology, represent a pivotal stage in the manipulation of droplets and particles. In recent times, droplet sorting methods based on lab-on-disk (LOD) have garnered significant interest among researchers for their inherent merits, including high throughput, ease of operation, seamless device integration, and independence from supplementary driving forces. This study introduces a centrifugal force-driven microfluidic chip comprising spiral microchannels. The chip incorporates microhole arrays along the sidewall of the spiral channels, enabling size-based sorting and enrichment of microdroplets under the influence of multiple forces. Firstly, a comparative analysis was performed to assess the influence of the separation port structure and rotational speed on efficiency, and a mechanical modeling approach was employed to conduct kinetic analyses of droplet behavior during instantaneous separation. Those findings demonstrated a good agreement with the experimental results at ω < 100 rpm. Subsequently, sorting experiments on homogeneous droplets indicated that repetitive sorting could increase the recovery ratios, RT(α), of high-concentration droplets (20.7%) from 35.3% to over 80%. We also conducted a sorting experiment on three-component homogeneous-phase emulsions using a serially connected chip array, and the sorting throughput was 0.58 mL min−1. As a result, the RT(α) for 60 and 160 μm droplets were 99.4% and 88.9%, respectively. Lastly, we conducted elution experiments and dual-sample sorting on a single chip, and the fluorescence results demonstrated that this study provided an efficient and non-cross-contaminating sorting method for non-homogenous phase multi-sample microreactor units. [ABSTRACT FROM AUTHOR]

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air‐water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first‐principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface‐rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed. [ABSTRACT FROM AUTHOR]

Recent advances in composite hydrogels achieve material enhancement or specialized stimuli‐responsive functionalities by pairing with a functional filler. Liquid metals (LM) offer a unique combination of chemical, electrical, and mechanical properties that show great potential in hydrogel composites. Polymerization of hydrogels with LM microdroplets as initiators is a particularly interesting phenomenon that remains in its early stage of development. In this work, an LM‐hydrogel composite is introduced, in which LM microdroplets dispersed inside the hydrogel matrix have dual functions as a polymerization initiator for a polyacrylic acid‐poly vinyl alcohol (PAA/PVA) network and, once polymerized, as passive inclusion to influence its material and stimuli‐responsive characteristics. It is demonstrated that LM microdroplets enable ultra‐fast polymerization in ≈1 min, compared to several hours by conventional polymerization techniques. The results show several mechanical enhancements to the PAA/PVA hydrogels with LM‐initiated polymerization. It is found that LM ratios strongly influence stimuli‐responsive behaviors in the hydrogels, including swelling and ionic bending, where higher LM ratios are found to enhance ionic actuation performance. The dual roles of LM in this composite are analyzed using the experimental characterization results. These LM‐hydrogel composites, which are biocompatible, open up new opportunities in future soft robotics and biomedical applications. [ABSTRACT FROM AUTHOR]