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We present an integrated flow chemistry strategy using two membrane-based flow reactors to enhance the extraction and recovery rates of switchable hydrophilicity solvents (SHSs) by five times compared to batch reactors. The developed green flow chemistry strategy achieves an overall single-pass recovery efficiency of 60.1% for 2-(dibutyl amino)ethanol. [ABSTRACT FROM AUTHOR]

Green solvent utilization and recovery enabled by switchable hydrophilicity solvents (SHSs), using carbon dioxide as the switching trigger, offer intriguing advantages in sustainable chemistry. To further elevate SHSs, an intensified continuous flow strategy is presented, providing an accurate in situ reaction monitoring and a scalable green solvent extraction route. [ABSTRACT FROM AUTHOR]

Over the past decade, continuous flow reactors have emerged as a powerful tool for accelerated fundamental and applied studies of gas–liquid reactions, offering facile gas delivery and process intensification. In particular, unique features of highly gas-permeable tubular membranes in flow reactors (i.e., tube-in-tube flow reactor configuration) have been exploited as (i) an efficient analytic tool for gas–liquid solubility and diffusivity measurements and (ii) reliable gas delivery/generation strategy, providing versatile adaptability for a wide range of gas–liquid processes. The tube-in-tube flow reactors have been successfully adopted for rapid exploration of a wide range of gas–liquid reactions (e.g., amination, carboxylation, carbonylation, hydrogenation, ethylenation, oxygenation) using gaseous species both as the reactant and the product, safely handling toxic and flammable gases or unstable intermediate compounds. In this highlight, we present an overview of recent developments in the utilization of such intensified flow reactors within modular flow chemistry platforms for different gas–liquid processes involving carbon dioxide, oxygen, and other gases. We provide a detailed step-by-step guideline for robust assembly and safe operation of tube-in-tube flow reactors. We also discuss the current challenges and potential future directions for further development and utilization of tubular membrane-based flow reactors for gas–liquid processes. [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]

We present an automated microscale flow chemistry platform for rapid performance evaluation of continuous and discrete reaction parameters in homogeneous hydroformylation reactions. We demonstrate the versatility of the developed microfluidic platform through a systematic study of the effects of a library of phosphine-based ligands on catalytic activity and regioselectivity. [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 report an automated flow chemistry platform that can efficiently perform a wide range of chemistries, including single/multi-phase and single/multi-step, with a reaction volume of just 14 μL. The breadth of compatible chemistries is successfully demonstrated and the desired products are characterized, isolated, and collected online by preparative HPLC/MS/ELSD. [ABSTRACT FROM AUTHOR]

Continuous multiphase flow strategies are commonly employed for high-throughput parameter screening of physical, chemical, and biological processes as well as continuous preparation of a wide range of fine chemicals and micro/nano particles with processing times up to 10 min. The inter-dependency of mixing and residence times, and their direct correlation with reactor length have limited the adaptation of multiphase flow strategies for studies of processes with relatively long processing times (0.5–24 h). In this frontier article, we describe an oscillatory multiphase flow strategy to decouple mixing and residence times and enable investigation of longer timescale experiments than typically feasible with conventional continuous multiphase flow approaches. We review current oscillatory multiphase flow technologies, provide an overview of the advancements of this relatively new strategy in chemistry and biology, and close with a perspective on future opportunities. [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]

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. [ABSTRACT FROM AUTHOR]

A multi-phase flow strategy, based on oscillatory motion of a bi-phasic slug within a fluorinated ethylene propylene (FEP) tubular reactor, under inert atmosphere, is designed and developed to address mixing and mass transfer limitations associated with continuous slug flow chemistry platforms for studies of bi-phasic catalytic reactions. The technique is exemplified with C–C and C–N Pd catalyzed coupling reactions. [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]

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]

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. [ABSTRACT FROM AUTHOR]

Capitalizing on the benefits of microscale segmented flows, e.g., enhanced mixing and reduced sample dispersion, so far requires specialist training and accommodating a few experimental inconveniences. For instance, microscale gas-liquid flows in many current setups take at least 10 min to stabilize and iterative manual adjustments are needed to achieve or maintain desired mixing or residence times. Here, we report a cruise control strategy that overcomes these limitations and allows microscale gas-liquid (bubble) and liquid-liquid (droplet) flow conditions to be rapidly "adjusted" and maintained. Using this strategy we consistently establish bubble and droplet flows with dispersed phase (plug) velocities of 5-300 mm s(-1), plug lengths of 0.6-5 mm and continuous phase (slug) lengths of 0.5-3 mm. The mixing times (1-5 s), mass transfer times (33-250 ms) and residence times (3-300 s) can therefore be directly imposed by dynamically controlling the supply of the dispersed and the continuous liquids either from external pumps or from local pressurized reservoirs. In the latter case, no chip-external pumps, liquid-perfused tubes or valves are necessary while unwanted dead volumes are significantly reduced.

We present an automated microfluidic (MF) approach for the systematic and rapid investigation of carbon dioxide (CO(2)) mass transfer and solubility in physical solvents. Uniformly sized bubbles of CO(2) with lengths exceeding the width of the microchannel (plugs) were isothermally generated in a co-flowing physical solvent within a gas-impermeable, silicon-based MF platform that is compatible with a wide range of solvents, temperatures and pressures. We dynamically determined the volume reduction of the plugs from images that were accommodated within a single field of view, six different downstream locations of the microchannel at any given flow condition. Evaluating plug sizes in real time allowed our automated strategy to suitably select inlet pressures and solvent flow rates such that otherwise dynamically self-selecting parameters (e.g., the plug size, the solvent segment size, and the plug velocity) could be either kept constant or systematically altered. Specifically, if a constant slug length was imposed, the volumetric dissolution rate of CO(2) could be deduced from the measured rate of plug shrinkage. The solubility of CO(2) in the physical solvent was obtained from a comparison between the terminal and the initial plug sizes. Solubility data were acquired every 5 min and were within 2-5% accuracy as compared to literature data. A parameter space consisting of the plug length, solvent slug length and plug velocity at the microchannel inlet was established for different CO(2)-solvent pairs with high and low gas solubilities. In a case study, we selected the gas-liquid pair CO(2)-dimethyl carbonate (DMC) and volumetric mass transfer coefficients 4-30 s(-1) (translating into mass transfer times between 0.25 s and 0.03 s), and Henry's constants, within the range of 6-12 MPa.

We report the development of a versatile microfluidic (MF) reactor with multiple analytical probes, which can be used for (i) quantitative characterisation of molecular vibrational signatures of reactants or products, (ii) the localised real-time monitoring of temperature and (iii) site-specific measurements of pH of the reaction system. The analytical probes utilised for in situ reaction analysis include an ATR-FTIR probe, a temperature probe, and a pH probe. We demonstrate the applications of the MF reactor with integrated probes for the parallel monitoring of multiple variables in acid/base neutralisation reaction, of changes in buffer pH, temperature, and vibrational absorption bands, and for monitoring the kinetics of the reaction between CO(2) and a buffer system with therapeutic applications.

We report a microfluidic (MF) approach to studies of temperature mediated carbon dioxide (CO(2)) transfer between the gas and the liquid phases. Micrometre-diameter CO(2) bubbles with a narrow size distribution were generated in an aqueous or organic liquid and subsequently were subjected to temperature changes in the downstream channel. In response to the cooling-heating-cooling cycle the bubbles underwent corresponding contraction-expansion-contraction transitions, which we term 'bubble breathing'. We examined temperature-controlled dissolution of CO(2) in four exemplary liquid systems: deionized water, a 0.7 M aqueous solution of NaCl, ocean water extracted from Bermuda coastal waters, and dimethyl ether of poly(ethylene glycol), a solvent used in industry for absorption of CO(2). The MF approach can be extended to studies of other gases with a distinct, temperature-dependent solubility in liquids.