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Microfluidic devices and systems have entered many areas of chemical engineering, and the rate of their adoption is only increasing. As we approach and adapt to the critical global challenges we face in the near future, it is important to consider the capabilities of flow chemistry and its applications in next-generation technologies for sustainability, energy production, and tailor-made specialty chemicals. We present the introduction of microfluidics into the fundamental unit operations of chemical engineering. We discuss the traits and advantages of microfluidic approaches to different reactive systems, both well-established and emerging, with a focus on the integration of modular microfluidic devices into high-efficiency experimental platforms for accelerated process optimization and intensified continuous manufacturing. Finally, we discuss the current state and new horizons in self-driven experimentation in flow chemistry for both intelligent exploration through the chemical universe and distributed manufacturing.

We study microreactors with internal fields of posts as typical examples of structured microreactors to elucidate flow fields and their implications for mass transfer. Laser-induced fluorescence (LIF) visualization combined with image analysis is used to systematically quantify key features such as interfacial area, phase holdup and the characteristics of the post-wetting layer. The subsequent mass transport analysis yields insight into how the posts contribute to the overall enhanced mass transfer performance compared to open channels, and provides predictions of mass transfer performance under varying operating conditions. Computational fluid dynamic (CFD) simulations of multiphase flow using the volume-of-fluid (VOF) method are in good agreement with experimentally observed multiphase flows.

Increase in the ionic strength of water that is mediated by the reaction of carbon dioxide (CO2) with nitrogenous bases is a promising approach toward phase separation in mixtures of water with organic solvents and potentially water purification. Conventional macroscale studies of this complicated process are challenging, due to its occurrence via several consecutive and concurrent steps, mass transfer limitation, and lack of control over gas-liquid interfaces. We report a new microfluidic strategy for fundamental studies of liquid-liquid phase separation mediated by CO2 as well as screening of the efficiency of nitrogenous agents. A single set of microfluidic experiments provided qualitative and quantitative information on the kinetics and completeness of water-tetrahydrofuran phase separation, the minimum amount of CO2 required to complete phase separation, the total CO2 uptake, and the rate of CO2 consumption by the liquid mixture. The efficiency of tertiary diamines with different lengths of alkyl chain was examined in a time- and labor-efficient manner and characterized with the proposed efficiency parameter. A wealth of information obtained using the MF methodology can facilitate the development of new additives for switchable solvents in green chemistry applications.

Carbon dioxide (CO2) sequestration, storage and recycling will greatly benefit from comprehensive studies of physical and chemical gas-liquid processes involving CO2. Over the past five years, microfluidics emerged as a valuable tool in CO2-related research, due to superior mass and heat transfer, reduced axial dispersion, well-defined gas-liquid interfacial areas and the ability to vary reagent concentrations in a high-throughput manner. This Minireview highlights recent progress in microfluidic studies of CO2-related processes, including dissolution of CO2 in physical solvents, CO2 reactions, the utilization of CO2 in materials science, and the use of supercritical CO2 as a "green" solvent., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

To develop an understanding of the nature of complex, spatiotemporal interactions between cells and the extracellular matrix (ECM), artificial ECMs formed from hydrogels with a particular spectrum of properties are being developed at a rapid pace. We report the microfluidic generation of small, monodisperse composite agarose-gelatin hydrogel modules (microgel particles) that can be used for cell encapsulation and can serve as instructive cellular microenvironments. The agarose component of the microgels gelled under reduced temperature, while gelatin modified with phenolic hydroxyl groups underwent peroxidase-catalyzed gelation. Microgel composition, structure, morphology, and rigidity were tuned in a high-throughput manner. The results of this work are important for the generation of libraries of cell-laden polymer microgels for single-cell analysis, tissue engineering, and fundamental studies of the role of local microenvironments in cell fate.

Microfluidics (MFs) offers a promising method for the preparation of polymer microgels with exquisite control over their dimensions, shapes and morphologies. A challenging task in this process is the generation of droplets (precursors for microgels) from highly viscous polymer solutions. Spatial separation of MF emulsification and gelation of the precursor droplets on chip can address this challenge. In the present work, we explored the application of the "direct injection" method for the preparation of microgels by adding a highly concentrated polymer solution or a gelling agent directly into the precursor droplets. In the first system, primary droplets were generated from a dilute aqueous solution of agarose, followed by the injection of the concentrated agarose solution directly in the primary droplets. The secondary droplets served as precursors for microgels. In the second system, primary droplets were generated from the low-viscous solution of methyl-β-cyclodextrin and poly(ethylene glycol) end-terminated with octadecyl hydrophobic groups. Addition of surfactant directly into the primary droplets led to the binding of methyl-β-cyclodextrin to the surfactant, thereby releasing hydrophobized poly(ethylene glycol) to form polymer microgels. Our results show that, when optimized, the direct injection method can be used for microgel preparation from highly viscous liquids and thus this method expands the range of polymers used for MF generation of microgels.

Photostability of colloidal quantum dots (QDs) is one of the major criteria determining their long‐term applicability in energy and chemical technologies. Yet, photostability studies of QDs are extremely sensitive to experimental conditions, lack a detailed mechanistic understanding, and are time‐, material‐, and labor‐intensive. Herein, an automated microfluidic platform for accelerated photostability studies of colloidal QDs is introduced, 3.5× faster and 100× more material efficient than the conventional flask‐based studies. The developed microfluidic strategy provides real‐time in situ access to the optical properties of QDs throughout the photostability experiments. Specifically, the material‐efficient microfluidic platform is used to study the mechanism and kinetics of CdSe QDs' photodegradation. The studies suggest that a generation of singlet oxygen via triplet energy transfer from colloidal CdSe QDs can initiate photo‐oxidation of CdSe QDs. Furthermore, the presence of at least one additional photodegradation pathway of CdSe QDs parallel to the photo‐oxidation pathway is unveiled. The systematic photostability experiments reveal how the incident photon flux and the starting average diameters of CdSe QDs affect their photodegradation rates. This work sheds light on the complex and multifaceted photodegradation phenomena of colloidal CdSe QDs and illustrates the unique characteristics of microfluidic strategies to improve and accelerate photostability studies of QDs. [ABSTRACT FROM AUTHOR]

A generalizable and versatile microfluidic approach for facile synthesis of a wide range of metal oxide microparticles using atypical metal‐organic precursors is reported. Microparticles of three single oxide materials, zinc(II) oxide, tin(IV) oxide, and cerium(IV) oxide, as well as a binary rare earth mixed oxide, lanthanum(III) praseodymium(III) oxide, are synthesized in flow. The tin(IV) oxide is shown to vary in composition from 14.2 % to 0 % orthorhombic phase at annealing temperatures ranging from 500 °C to 900 °C, while the lanthanum(III) praseodymium(III) oxide forms at a relatively low temperature of ∼700 °C. [ABSTRACT FROM AUTHOR]

Lead halide perovskite (LHP) nanocrystals (NCs) are considered an emerging class of advanced functional materials with numerous outstanding optoelectronic characteristics. Despite their success in the field, their precision synthesis and fundamental mechanistic studies remain a challenge. The vast colloidal synthesis and processing parameters of LHP NCs in combination with the batch‐to‐batch and lab‐to‐lab variation problems further complicate their progress. In response, a self‐driving fluidic micro‐processor is presented for accelerated navigation through the complex synthesis and processing parameter space of NCs with multistage chemistries. The capability of the developed autonomous experimentation strategy is demonstrated for a time‐, material‐, and labor‐efficient search through the sequential halide exchange and cation doping reactions of LHP NCs. Next, a machine learning model of the modular fluidic micro‐processors is autonomously built for accelerated fundamental studies of the in‐flow metal cation doping of LHP NCs. The surrogate model of the sequential halide exchange and cation doping reactions of LHP NCs is then utilized for five closed‐loop synthesis campaigns with different target NC doping levels. The precise and intelligent NC synthesis and processing strategy, presented herein, can be further applied toward the autonomous discovery and development of novel impurity‐doped NCs with applications in next‐generation energy technologies. [ABSTRACT FROM AUTHOR]

Despite the groundbreaking advancements in the synthesis of inorganic lead halide perovskite (LHP) nanocrystals (NCs), stimulated from their intriguing size‐, composition‐, and morphology‐dependent optical and optoelectronic properties, their formation mechanism through the hot‐injection (HI) synthetic route is not well‐understood. In this work, for the first time, in‐flow HI synthesis of cesium lead iodide (CsPbI3) NCs is introduced and a comprehensive understanding of the interdependent competing reaction parameters controlling the NC morphology (nanocube vs nanoplatelet) and properties is provided. Utilizing the developed flow synthesis strategy, a change in the CsPbI3 NC formation mechanism at temperatures higher than 150 °C, resulting in different CsPbI3 morphologies is revealed. Through comparison of the flow‐ versus flask‐based synthesis, deficiencies of batch reactors in reproducible and scalable synthesis of CsPbI3 NCs with fast formation kinetics are demonstrated. The developed modular flow chemistry route provides a new frontier for high‐temperature studies of solution‐processed LHP NCs and enables their consistent and reliable continuous nanomanufacturing for next‐generation energy technologies. [ABSTRACT FROM AUTHOR]

Identifying the optimal formulation of emerging inorganic lead halide perovskite quantum dots (LHP QDs) with their vast colloidal synthesis universe and multiple synthesis/postsynthesis processing parameters is a challenging undertaking for material‐ and time‐intensive, batch synthesis strategies. Herein, a modular microfluidic synthesis strategy, integrated with an artificial intelligence (AI)‐guided decision‐making agent for intelligent navigation through the complex colloidal synthesis universe of LHP QDs with 10 individually controlled synthesis parameters and an accessible parameter space exceeding 2 × 107, is introduced. Utilizing the developed autonomous microfluidic experimentation strategy within a global learning framework, the optimal formulation of LHP QDs is rapidly identified through a two‐step colloidal synthesis and postsynthesis halide exchange reaction, for 10 different emission colors in less than 40 min per desired peak emission energy. Using two in‐series microfluidic reactors enables continuous bandgap engineering of LHP QDs via in‐line halide exchange reactions without the need for an intermediate washing step. Using an inert gas within a three‐phase flow format enables successful, self‐synchronized continuous delivery of halide salt precursor into moving droplets containing LHP QDs, resulting in accelerated closed‐loop formulation optimization and end‐to‐end continuous manufacturing of LHP QDs with desired optoelectronic properties. [ABSTRACT FROM AUTHOR]

Opportunities for accessible microfluidic device integration have sharply grown with the rise of readily available lab-in-a-tube strategies. Herein, we present a facile, non-invasive, plug-and-play phase velocity and length measuring strategy for rapid deployment onto tube-based microfluidic systems, enabling quick and accurate residence (reaction) time measurement and tuning. Our approach utilizes inexpensive off-the-shelf optical phase sensors and requires no prior knowledge of the fluid composition or physical properties. Compared to camera-based measurements in fluoropolymer tubing, the optical phase sensor-based technique shows mean absolute percentage errors of 1.3% for velocity and 3.3% for length. Utilizing the developed multiphase flow monitoring technique, we screen the accessible parameter space of gas–liquid segmented flows. To further demonstrate the functionality of this process monitoring strategy, we implement two feedback controllers to establish simultaneous setpoint control for phase velocity and length. Next, to showcase the effectiveness and versatility of the developed multiphase flow process controller, we apply it to systematic studies of the effect of liquid slug velocity (controlling precursor mixing timescale) on the colloidal synthesis of cesium lead tribromide nanocrystals. By varying the liquid slug velocity and maintaining constant precursor composition, liquid slug length, and residence time, we observe a bandgap tunability from 2.43 eV (510 nm) to 2.52 eV (494 nm). [ABSTRACT FROM AUTHOR]

Palladium (Pd)‐loaded poly‐hydromethylsiloxane (PHMS) microparticles of tunable size and elasticity are prepared in a capillary‐based coaxial flow‐focusing microfluidic device constructed using off‐the‐shelf components. Simultaneous droplet formation and chemical cross‐linking processes are performed by tuning the dilution of the cross‐linking catalyst in the annular flow of the microreactor, resulting in PHMS microparticles synthesized in a single step. The size of the elastomeric microparticles can be tuned by adjusting the flow rate ratio of the polymer and cross‐linker mixture to water, while the elasticity can be tuned by the polymer to cross‐linker ratio as well as the flow rate ratio of the polymer mixture to cross‐linking catalyst mixture. Microparticle elasticity is characterized by the degree of solvent uptake. Application of the synthesized PHMS microparticles in organic synthesis is demonstrated by producing monodispersed Pd‐loaded microparticles and utilizing them as microreaction vessels for continuous Suzuki‐Miyaura cross‐coupling in a Pd‐loaded microparticle‐packed bed reactor. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3188–3197, 2018 [ABSTRACT FROM AUTHOR]

Colloidal organic/inorganic metal-halide perovskite nanocrystals have recently emerged as a potential low-cost replacement for the semiconductor materials in commercial photovoltaics and light emitting diodes. However, unlike III–V and IV–VI semiconductor nanocrystals, studies of colloidal perovskite nanocrystals have yet to develop a fundamental and comprehensive understanding of nucleation and growth kinetics. Here, we introduce a modular and automated microfluidic platform for the systematic studies of room-temperature synthesized cesium–lead halide perovskite nanocrystals. With abundant data collection across the entirety of four orders of magnitude reaction time span, we comprehensively characterize nanocrystal growth within a modular microfluidic reactor. The developed high-throughput screening platform features a custom-designed three-port flow cell with translational capability for in situ spectral characterization of the in-flow synthesized perovskite nanocrystals along a tubular microreactor with an adjustable length, ranging from 3 cm to 196 cm. The translational flow cell allows for sampling of twenty unique residence times at a single equilibrated flow rate. The developed technique requires an average total liquid consumption of 20 μL per spectra and as little as 2 μL at the time of sampling. It may continuously sample up to 30 000 unique spectra per day in both single and multi-phase flow formats. Using the developed plug-and-play microfluidic platform, we study the growth of cesium lead trihalide perovskite nanocrystals through in situ monitoring of their absorption and emission band-gaps at residence times ranging from 100 ms to 17 min. The automated microfluidic platform enables a systematic study of the effect of mixing enhancement on the quality of the synthesized nanocrystals through a direct comparison between single- and multi-phase flow systems at similar reaction time scales. The improved mixing characteristics of the multi-phase flow format results in high-quality perovskite nanocrystals with kinetically tunable emission wavelength, ranging as much as 25 nm at equivalent residence times. Further application of this unique platform would allow rapid parameter optimization in the colloidal synthesis of a wide range of nanomaterials (e.g., metal or semiconductor), that is directly transferable to continuous manufacturing in a numbered-up platform with a similar characteristic length scale. [ABSTRACT FROM AUTHOR]

We present an automated microfluidic platform for in-flow studies of visible-light photoredox catalysis in liquid or gas–liquid reactions at the 15 μL scale. An oscillatory flow strategy enables a flexible residence time while preserving the mixing and heat transfer advantages of flow systems. The adjustable photon flux made possible with the platform is characterized using actinometry. Case studies of oxidative hydroxylation of phenylboronic acids and dimerization of thiophenol demonstrate the capabilities and advantages of the system. Reaction conditions identified through droplet screening translate directly to continuous synthesis with minor platform modifications. [ABSTRACT FROM AUTHOR]

Liquid–liquid extraction is an important separation and purification method; however, it faces a challenge in reducing the energy consumption and the environmental impact of solvent (extractant) recovery. The reversible chemical reactions of switchable solvents (nitrogenous bases) with carbon dioxide (CO2) can be implemented in reactive liquid–liquid extraction to significantly reduce the cost and energy requirements of solvent recovery. The development of new effective switchable solvents reacting with CO2 and the optimization of extraction conditions rely on the ability to evaluate and screen the performance of switchable solvents in extraction processes. We report a microfluidic strategy for time- and labour-efficient studies of CO2-mediated solvent extraction. The platform utilizes a liquid segment containing an aqueous extractant droplet and a droplet of a solution of a switchable solvent in a non-polar liquid, with gaseous CO2 supplied to the segment from both sides. Following the reaction of the switchable solvent with CO2, the solvent becomes hydrophilic and transfers from the non-polar solvent to the aqueous droplet. By monitoring the time-dependent variation in droplet volumes, we determined the efficiency and extraction time for the CO2-mediated extraction of different nitrogenous bases in a broad experimental parameter space. The platform enables a significant reduction in the amount of switchable solvents used in these studies, provides accurate temporal characterization of the liquid–liquid extraction process, and offers the capability of high-throughput screening of switchable solvents. [ABSTRACT FROM AUTHOR]

Nanoparticle (NP) dispersions are extensively used for the producing nanocomposite materials. Optimization of formulations of NP dispersions is a time- and labor-consuming process that can benefit from high-throughput preparation of composite materials, followed by studies of their structure-property relationships. This paper describes a microfluidic platform for producing photopolymerized films from NP-monomer dispersions. The platform enables rapid and reproducible preparation of films with varying compositions by tuning the flow rates of the monomer and NP-monomer mixtures, along with suppressed evaporation of volatile monomers, no need of an inert atmosphere, and the reduced exposure to potentially harmful monomers and NPs. This work paves the way for efficient studies of composition-dependent properties of nanocomposite materials. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) sequestration, storage and recycling will greatly benefit from comprehensive studies of physical and chemical gas-liquid processes involving CO2. Over the past five years, microfluidics emerged as a valuable tool in CO2-related research, due to superior mass and heat transfer, reduced axial dispersion, well-defined gas-liquid interfacial areas and the ability to vary reagent concentrations in a high-throughput manner. This Minireview highlights recent progress in microfluidic studies of CO2-related processes, including dissolution of CO2 in physical solvents, CO2 reactions, the utilization of CO2 in materials science, and the use of supercritical CO2 as a "green" solvent. [ABSTRACT FROM AUTHOR]

We introduce oscillatory segmented flow as a compact microfluidic format that accommodates slow chemical reactions for the solution-phase processing of colloidal nanomaterials. The strategy allows the reaction progress to be monitored at a dynamic range of up to 80 decibels (i.e., residence times of up to one day, equivalent to 720–14 400 times the mixing time) from only one sensing location. A train of alternating gas bubbles and liquid reaction compartments (segmented flow) was initially formed, stopped and then subjected to a consistent back-and-forth motion. The oscillatory segmented flow was obtained by periodically manipulating the pressures at the device inlet and outlet via square wave signals generated by non-wetted solenoid valves. The readily implementable format significantly reduced the device footprint as compared with continuous segmented flow. We investigated mixing enhancement for varying liquid segment lengths, oscillation amplitudes and oscillation frequencies. The etching of gold nanorods served as a case study to illustrate the utility of the approach for dynamic characterization and precise control of colloidal nanomaterial size and shape for 5 h. Oscillatory segmented flows will be beneficial for a broad range of lab-on-a-chip applications that require long processing times. [ABSTRACT FROM AUTHOR]

Frustrated Lewis pairs (FLPs) comprising sterically hindered Lewis acids and bases offer the capability to reversibly capture CO2 under mild reaction conditions. The determination of equilibrium constants and thermodynamic properties of these reactions should enable assessment of the efficiency of a particular FLP system for CO2sequestration and provide insights for design of new, efficient formulations of FLP catalysts for CO2capture. We have developed a microfluidic approach to studies of FLP–CO2reactions, which provides their thermodynamic characterization that is not accessible otherwise. The approach enables the determination of the equilibrium reaction constants at different temperatures, the enthalpy, the entropy, and the Gibbs energy of these reactions, as well as the enhancement factor. The microfluidic methodology has been validated by applying it to the well-characterized reaction of CO2with a secondary amine. The microfluidic approach can be applied for fundamental thermodynamic studies of other gas–liquid reactions. [ABSTRACT FROM AUTHOR]

We introduce a miniature gate valve as a readily implementable strategy for actively controlling the flow of liquids on-chip, within a footprint of less than one square millimetre. Bubble gates provide for simple, consistent and scalable control of liquid flow in microchannel networks, are compatible with different bulk microfabrication processes and substrate materials, and require neither electrodes nor moving parts. A bubble gate consists of two microchannel sections: a liquid-filled channel and a gas channel that intercepts the liquid channel to form a T-junction. The open or closed state of a bubble gate is determined by selecting between two distinct gas pressure levels: the lower level corresponds to the “open” state while the higher level corresponds to the “closed” state. During closure, a gas bubble penetrates from the gas channel into the liquid, flanked by a column of equidistantly spaced micropillars on each side, until the flow of liquid is completely obstructed. We fabricated bubble gates using single-layer soft lithographic and bulk silicon micromachining procedures and evaluated their performance with a combination of theory and experimentation. We assessed the dynamic behaviour during more than 300 open-and-close cycles and report the operating pressure envelope for different bubble gate configurations and for the working fluids: de-ionized water, ethanol and a biological buffer. We obtained excellent agreement between the experimentally determined bubble gate operational envelope and a theoretical prediction based on static wetting behaviour. We report case studies that serve to illustrate the utility of bubble gates for liquid sampling in single and multi-layer microfluidic devices. Scalability of our strategy was demonstrated by simultaneously addressing 128 bubble gates. [ABSTRACT FROM AUTHOR]

In an effort to produce the materials of next‐generation photoelectronic devices, postsynthesis halide exchange reactions of perovskite quantum dots are explored to achieve enhanced bandgap tunability. However, comprehensive understanding of the multifaceted halide exchange reactions is inhibited by their vast relevant parameter space and complex reaction network. In this work, a facile room‐temperature strategy is presented for rapid halide exchange of inorganic perovskite quantum dots. A comprehensive understanding of the halide exchange reactions is provided by isolating reaction kinetics from precursor mixing rates utilizing a modular microfluidic platform, Quantum Dot Exchanger (QDExer). The effects of ligand composition and halide salt source on the rate and extent of the halide exchange reactions are illustrated. This fluidic platform offers a unique time‐ and material‐efficient approach for studies of solution phase‐processed colloidal nanocrystals beyond those studied here and may accelerate the discovery and optimization of next‐generation materials for energy technologies. [ABSTRACT FROM AUTHOR]

Controlled synthesis of semiconductor nano/microparticles has attracted substantial attention for use in numerous applications from photovoltaics to photocatalysis and bioimaging due to the breadth of available physicochemical and optoelectronic properties. Microfluidic material synthesis strategies have recently been demonstrated as an effective technique for rapid development, controlled synthesis, and continuous manufacturing of solution‐processed semiconductor nano/microparticles, due to enhanced parametric control enabling precise tuning of material properties, size, and morphologies. In this review, the basics of microfluidic material synthesis approaches complemented with recent advances in the flow fabrication of metal oxide, chalcogenide, and perovskite semiconductor particles are discussed. Furthermore, advancements in artificial intelligence (AI)‐driven materials–space exploration and accelerated formulation optimization using modular microfluidic reactors are outlined. Finally, future directions for the fabrication of semiconducting materials in flow and the implementation of AI with automated microfluidic reactors for accelerated material discovery and development are presented. [ABSTRACT FROM AUTHOR]

Flow-induced alignment of shape-anisotropic colloidal particles is of great importance in fundamental research and in the fabrication of structurally anisotropic materials; however, rheo-optical studies of shear-induced particle orientation are time- and labor-intensive and require complicated experimental setups. We report a single-droplet oscillatory microfluidic strategy integrated with in-line polarized light imaging as a strategy for studies of shear-induced alignment of rod-shape nanoparticles. Using an oscillating droplet of an aqueous isotropic suspension of cellulose nanocrystals (CNCs), we explore the effect of the shear rate and suspension viscosity on the flow-induced CNC alignment and subsequent relaxation to the isotropic state. The proposed microfluidic strategy enables high-throughput studies of shear-induced orientations in structured liquid under precisely controlled experimental conditions. The results of such studies can be used in the development of structure-anisotropic materials. [ABSTRACT FROM AUTHOR]