Droplet Reactor Research Articles

Dual Droplet Functionality: Phototaxis and Photopolymerization.

The use of phototaxis to move droplets in liquids offers the opportunity to emulate natural processes such as the controlled transport of materials in fluidic environments and to undertake chemistry at specific locations. We have developed a photoactive organic droplet, whose movement in aqueous solution is driven by a photoinitiator, as a result of a light-induced reaction within the droplet generating a Marangoni flow. The photoinitiator not only drives the droplet motion but can also be used to initiate polymerization following transfer of the droplet to a specific location and its merging with a monomer-containing droplet. The same light is used to control the transport of the droplet and the polymerization. The efficacy of this droplet transport and reactor system has been demonstrated by the site-specific underwater polymerization of N-isopropylacrylamide to repair a leaking vessel and the adhesion of two materials together.

Kinetic study of TBD catalyzed δ-valerolactone polymerization using a gas-driven droplet flow reactor

The reaction kinetics of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyzed δ-valerolactone polymerization was determined using a gas-driven droplet reactor.

Doping Graphene into Monodispersed Fe3O4 Microspheres with Droplet Microfluidics for Enhanced Electrochemical Performance in Lithium‐Ion Batteries

AbstractGraphene‐doped Fe3O4 monodispersed microspheres are fabricated utilizing a droplet microfluidic technology. The electrochemical performance of the Fe3O4 microspheres as anode material for Li‐ion batteries is enhanced by doping trace graphene in the droplet reactor. The discharge capacity remains 550 mAh/g at 0.1 C after 190 cycles, which is 2 times higher than that of the graphene doped Fe3O4 samples prepared by the traditional precipitation method, and 5 times higher than that of pure Fe3O4 after 100 cycles. The uniform droplet reactor in form of a droplet microfluidic chip might provide a homogeneous microscopic condition for in‐situ electrostatic co‐assembly of the positive‐charged Fe colloids and the negative‐charged graphene oxide sheets to shape a homogeneous solid framework, resulting in homogeneously doping trace graphene into the Fe3O4 microspheres.

Droplet-based synthesis of homogeneous magnetic iron oxide nanoparticles.

Nanoparticles have gained large interest in a number of different fields due to their unique properties. In medical applications, for example, magnetic nanoparticles can be used for targeting, imaging, magnetically induced thermotherapy, or for any combination of the three. However, it is still a challenge to obtain narrowly dispersed, reproducible particles through a typical lab-scale synthesis when researching these materials. Here, we present a droplet capillary reactor that can be used for the synthesis of magnetic iron oxide nanoparticles. Compared to conventional batch synthesis, the particles synthesized in our droplet reactor have a narrower size distribution and a higher reproducibility. Furthermore, we demonstrate how the particle size can be changed from 5.2 ± 0.9 nm to 11.8 ± 1.7 nm by changing the reaction temperature and droplet residence time in the droplet capillary reactor.

Microfluidic synthesis of metal oxide nanoparticles via the nonaqueous method

Microfluidic synthesis allows for a good control of the particle formation conditions while minimizing the consumption of material. In this study, we exploited these advantages for the nonaqueous synthesis of TiO2, ZnO and CeO2 nanoparticles in a closed micro droplet reactor which resulted in well-defined particle structures. Monodisperse droplets are generated in amicrofluidic flow-focusing area and subsequently transported to a reaction zone allowing synthesis temperatures of up to 200 °C. In addition, we showed that a continuous particle generation process using solid precursor species can be realized without channel clogging. We present different microfluidic designs with reaction zones consisting either of a long meandering channel or a short expansion channel to realize the different reaction times required for the respective model systems. Interestingly, we found that at elevated temperatures, the reaction mixture diffuses into the surrounding continuous phase fluid which leads to a shrinkage of the droplets, forcing eventually the assembly of highly spherical aggregate structures. Depending on the different model systems, we obtained nanospheres of about 100 nm from the CeO2 synthesis and microspheres up to 17 µm in size from the TiO2 synthesis. In contrast, ZnO syntheses at milder temperatures were conducted without any droplet shrinkage which resulted in randomly shaped aggregates of 150 nm that self-assembled in the droplet micro reactor. Thus, a versatile synthesis technique is presented that can be applied for various metal oxides over a broad range of reaction parameters, as a promising approach for the controlled production of high quality nanoparticles.

Go with the flow

Controlling shape and size of noble-metal nanocrystals in an automated fashion is highly desirable for large-scale production of nanomaterials. Younan Xia and co-workers propose the design of a device for the automated synthesis of uniform nanocrystals, featuring a droplet reactor, online separation and purification capabilities.

Kinetically controlled synthesis of bimetallic nanostructures by flowrate manipulation in a continuous flow droplet reactor

We show that different Au–Pd nanoparticles, ranging from sharp-branched octopods to core@shell octahedra, can be achieved by inline manipulation of reagent flowrates in a microreactor for seeded growth.

Encoding Microreactors with Droplet Chains in Microfluidics.

Droplet-based high throughput biomolecular screening and combinatorial synthesis entail a viable indexing strategy to be developed for the identification of each microreactor. Here, we propose a novel indexing scheme based on the generation of droplet sequences on demand to form unique encoding droplet chains in fluidic networks. These codes are represented by multiunit and multilevel droplets packages, with each code unit possessing several distinct signal levels, potentially allowing large encoding capacity. For proof of concept, we use magnetic nanoparticles as the encoding material and a giant magnetoresistance (GMR) sensor-based active sorting system supplemented with an optical detector to generate and decode the sequence of one exemplar sample droplet reactor and a 4-unit quaternary magnetic code. The indexing capacity offered by 4-unit multilevel codes with this indexing strategy is estimated to exceed 104, which holds great promise for large-scale droplet-based screening and synthesis.

Synthesis of Core@Shell Nanostructures in a Continuous Flow Droplet Reactor: Controlling Structure through Relative Flow Rates.

Bimetallic nanostructures are primarily synthesized in small volume batches. However, droplet-based reactors are receiving attention due to their ability to maintain thermal and compositional equilibrium within and between droplets, enabling flow operations for inline analyses and the scale-up of nanomaterial syntheses. Here, the syntheses of shape-controlled core@shell Au@Pd nanostructures with variable shell thicknesses are reported through control of the relative flow rates of reagents within the microreactor. Specifically, Pd shells were grown on cubic or octahedral Au seeds, selected as a model system. In batch reactions, shell thickness is determined by precursor concentration; however, as shown here, precursor feedstock concentration can be held constant, with the precursor concentration within the droplets being controlled through relative flow rates. This approach allows process conditions to be modified inline rather than from batch to batch to achieve particles with different shell thicknesses, and this procedure should be applicable to other multicomponent systems.

Reducing-Agent-Free Instant Synthesis of Carbon-Supported Pd Catalysts in a Green Leidenfrost Droplet Reactor and Catalytic Activity in Formic Acid Dehydrogenation.

The development of green synthesis methods for supported noble metal catalysts remains important challenges to improve their sustainability. Here we first synthesized carbon-supported Pd catalysts in a green Leidenfrost droplet reactor without reducing agents, high-temperature calcination and reduction procedures. When the aqueous solution containing Pd nitrate precursor, carbon support, and water is dripped on a hot plate, vapor layer is formed between a solution droplet and hot surface, which allow the solution droplet to be levitated on the hot surface (Leidenfrost phenomena). Subsequently, Pd nanoparticles can be prepared without reducing agents in a weakly basic droplet reactor created by the Leidenfrost phenomena, and then the as-prepared Pd nanoparticles are loaded on carbon supports during boiling down the droplet on hot surface. Compared to conventional incipient wetness and chemical synthetic methods, the Leidenfrost droplet reactor does not need energy-consuming, time-consuming, and environmentally unfriendly procedures, which leads to much shorter synthesis time, lower carbon dioxide emission, and more ecofriendly process in comparison with conventional synthesis methods. Moreover, the catalysts synthesized in the Leidenfrost droplet reactor provided much better catalytic activity for room-temperature formic acid decomposition than those prepared by the incipient wetness method.

Analyzing Benzene and Cyclohexane Emulsion Droplet Collisions on Ultramicroelectrodes.

We report the collisions of single emulsion oil droplets with extremely low dielectric constants (e.g., benzene, ε of 2.27, or cyclohexane, ε of 2.02) as studied via emulsion droplet reactor (EDR) on an ultramicroelectrode (UME). By applying appropriate potentials to the UME, we observed the electrochemical effects of single-collision signals from the bulk electrolysis of single emulsion droplets. Different hydrophobic redox species (ferrocene, decamethyl-ferrocene, or metalloporphyrin) were trapped in a mixed benzene (or cyclohexane) oil-in-water emulsion using an ionic liquid as the supporting electrolyte and emulsifier. The emulsions were prepared using ultrasonic processing. Spike-like responses were observed in each i-t response due to the complete electrolysis of all of the above-mentioned redox species within the droplet. On the basis of these single-particle collision results, the collision frequency, size distribution, i-t decay behavior of the emulsion droplets, and possible mechanisms are analyzed and discussed. This work demonstrated that bulk electrolysis can be achieved in a few seconds in these attoliter reactors, suggesting many applications, such as analysis and electrosynthesis in low dielectric constant solvents, which have a much broader potential window.

Nanoliter-scale protein crystallization and screening with a microfluidic droplet robot.

Large-scale screening of hundreds or even thousands of crystallization conditions while with low sample consumption is in urgent need, in current structural biology research. Here we describe a fully-automated droplet robot for nanoliter-scale crystallization screening that combines the advantages of both automated robotics technique for protein crystallization screening and the droplet-based microfluidic technique. A semi-contact dispensing method was developed to achieve flexible, programmable and reliable liquid-handling operations for nanoliter-scale protein crystallization experiments. We applied the droplet robot in large-scale screening of crystallization conditions of five soluble proteins and one membrane protein with 35–96 different crystallization conditions, study of volume effects on protein crystallization, and determination of phase diagrams of two proteins. The volume for each droplet reactor is only ca. 4–8 nL. The protein consumption significantly reduces 50–500 fold compared with current crystallization stations.

Controlled multistep synthesis in a three-phase droplet reactor.

Channel-fouling is a pervasive problem in continuous flow chemistry, causing poor product control and reactor failure. Droplet chemistry, in which the reaction mixture flows as discrete droplets inside an immiscible carrier liquid, prevents fouling by isolating the reaction from the channel walls. Unfortunately, the difficulty of controllably adding new reagents to an existing droplet stream has largely restricted droplet chemistry to simple reactions in which all reagents are supplied at the time of droplet formation. Here we describe an effective method for repeatedly adding controlled quantities of reagents to droplets. The reagents are injected into a multiphase fluid stream, comprising the carrier liquid, droplets of the reaction mixture and an inert gas that maintains a uniform droplet spacing and suppresses new droplet formation. The method, which is suited to many multistep reactions, is applied to a five-stage quantum dot synthesis wherein particle growth is sustained by repeatedly adding fresh feedstock.

Large-scale synthesis of nanocrystals in a multichannel droplet reactor

We report a multichannel microfluidic droplet reactor for the large-scale, high temperature synthesis of nanocrystals. The reactor was applied here to the production of CdTe, CdSe and alloyed CdSeTe nanocrystals, and found in all cases to provide high quality quantum dots with spectral properties that did not vary between channels or over time. One hour test runs yielded 3.7, 1.5 and 2.1 g of purified CdTe, CdSe and the alloy, respectively, using 0.4 M cadmium precursor solutions and carrier and reagent phase flow rates of 4 and 2 ml min−1. A further nine hour test-run applied to CdTe, utilizing increased carrier and reagent flow rates of 5 and 3 ml min−1, yielded 54.4 g of dry purified material, corresponding to a production rate of 145 g per day. The reactor architecture is inherently scalable and, with only minimal modifications, should allow for straightforward expansion to the kilogram-per-day production levels sought by industry.

Highly efficient capillary polymerase chain reaction using an oscillation droplet microreactor

The current work presents the development of a capillary-based oscillation droplet approach to maximize the potential of a continuous-flow polymerase chain reaction (PCR). Through the full utilization of interfacial chemistry, a water-in-oil (w/o) droplet was generated by allowing an oil–water plug to flow along a polytetrafluoroethylene (PTFE) capillary. The w/o droplet functioned as the reactor for oscillating-flow PCR to provide a stable reaction environment, accelerate reagent mixing, and eliminate surface adsorption. The capillary PCR approach proposed in the current research offers high amplification efficiency, fast reaction speed, and easy system control attributable to the oscillation droplet reactor. Experimental results show that the droplet-based micro-PCR assay requires lower reaction volume (2μL) and shorter reaction time (12min) compared with conventional PCR methods. Taking the amplification of the New Delhi metallo-beta-lactamase (NDM-1) gene as an example, the present work demonstrates that the oscillation droplet PCR assay is capable of achieving high efficiency up to 89.5% and a detection limit of 10 DNA copies. The miniature PCR protocol developed in the current work is fast, robust, and low-cost, thus exhibiting the potential for expansion into various practical applications.

Direct synthesis of dextran-coated superparamagnetic iron oxide nanoparticles in a capillary-based droplet reactor

We describe the controlled synthesis of dextran-coated superparamagnetic iron oxide nanoparticles (SPIONs) using a stable passively-driven capillary-based droplet reactor. High quality highly crystalline particles were obtained with a narrow size distribution of mean diameter 3.6 nm and standard deviation 0.8 nm. The particles were evaluated for use in MRI, and found to exhibit a large saturation magnetisation of 58 emu/g and a high T2 relaxivity of 66 mM−1s−1 at 4.7 T, signifying good MRI contrast enhancement properties.

A heatable and evaporation-free miniature reactor upon superhydrophobic pedestals

Superhydrophobic material-based miniature reaction systems (MRSs) show great potential in the effective use of valuable reagents and solvents, but suffer from a water evaporation problem. Here we report a novel MRS utilizing superhydrophobic materials to freely manipulate aqueous droplets in oil, which provides a confined space with the capability to perform a heatable (80 °C) and time-consuming (24h) miniature reaction inside the coalesced droplet reactor.

A stable droplet reactor for high temperature nanocrystal synthesis

We report a versatile capillary-based droplet reactor for the controlled synthesis of nanoparticles over a wide range of flow conditions and temperatures. The reactor tolerates large flow-rate differentials between individual reagent streams, and allows droplet composition to be varied independently of residence time and volume. The reactor was successfully applied to the synthesis of metal (Ag), metal-oxide (TiO(2)) and compound semiconductor (CdSe) nanoparticles, and in each case exhibited stable droplet flow over many hours of operation without fouling, even for reactions involving solid intermediates. For CdSe formed by the reaction of Cd oleate and Se, highly controlled growth could be achieved at temperatures of up to 250 °C, with emission spectra varying smoothly and reproducibly with temperature and flow-rate. The droplet reactor showed exceptional stability when operated under constant flow-rate and temperature conditions, yielding particles with well-defined band-edge emission spectra that did not vary over the course of a full day's continuous operation.

A miniature droplet reactor built on nanoparticle-derived superhydrophobic pedestals

The capability to design and modulate materials, shapes, heat transfer, and mass mixing during the process of developing chemical reactors has allowed researchers to explore millions of chemical reactions and assays. However, despite the advantages in engineering array-based microreactors or microfluidic systems, the wetting attachment between solutions of reagents/products and the glass or polymer substrates of containers leads to difficulties in collecting products effectively and preventing channel blockage. Herein we present a miniature droplet reactor which takes advantage of the anti-wetting and low-adhesive properties of nanoparticle-derived superhydrophobic pedestals, allowing aqueous droplets to be manipulated freely but also providing a confined environment for performing a series of aqueous phase chemical reactions on a small scale. Gas- or precipitate-forming reactions can also be performed inside this miniature reactor. Most importantly, reaction products in liquid, solid or gaseous states can be collected effectively, which allows the harvesting of valuable products formed in limited amounts. Such a miniature reactor built on superhydrophobic pedestals provides a new way of performing common chemical reactions and may open the door to the design of next-generation microreaction systems.

Effect of viscoelasticity on drop dynamics in 5:1:5 contraction/expansion microchannel flow

Abstract Recently, we reported how viscoelasticity affects drop dynamics in a microchannel flow using the finite element-front tracking method (FE-FTM). In this work, we investigate drop dynamics for a wider range of parameters: viscosity ratio between droplet and medium ( χ ) , capillary number ( Ca ), droplet size, and fluid elasticity. The Oldroyd-B model is adopted as the constitutive equation for the viscoelastic fluid. We observe that the drop deformation in a microfluidic channel is dependent on Ca , which is more pronounced for smaller χ values. The present work shows that viscoelasticity plays an important role in drop dynamics with increasing χ values for Newtonian droplet in viscoelastic medium, which can be attributed to high normal stress developed in narrow film thickness between droplet and channel for higher χ values. We also study circulation problem inside droplets, which is important in practice, such as in droplet reactor application. The present work shows that circulation intensity is enhanced with decreasing χ values. We find that the relevance of viscoelastic effects on internal circulation is dependent on χ values, and the circulation intensity is distinctively decreased with increasing elasticity for high χ values for Newtonian droplet in viscoelastic medium. We expect that the present work be helpful not only in controlling droplets but also to improve our physical insight on drop dynamics in microchannel flows.