A Review of Pressure Drop and Mixing Characteristics in Passive Mixers Involving Miscible Liquids

The present review focuses on the recent studies carried out in passive micromixers for understanding the hydrodynamics and transport phenomena of miscible liquid-liquid (LL) systems in terms of pressure drop and mixing indices. First, the passive micromixers have been categorized based on the type of complexity in shape, size, and configuration. It is observed that the use of different aspect ratios of the microchannel width, presence of obstructions, flow and operating conditions, and fluid properties majorly affect the mixing characteristics and pressure drop in passive micromixers. A regime map for the micromixer selection based on optimization of mixing index (MI) and pressure drop has been identified based on the literature data for the Reynolds number (Re) range (1 ≤ Re ≤ 100). The map comprehensively summarizes the favorable, moderately favorable, or non-operable regimes of a micromixer. Further, regions for special applications of complex micromixer shapes and micromixers operating at low Re have been identified. Similarly, the operable limits for a micromixer based on pressure drop for Re range 0.1 < Re < 100,000 have been identified. A comparison of measured pressure drop with fundamentally derived analytical expressions show that Category 3 and 4 micromixers mostly have higher pressure drops, except for a few efficient ones. An MI regime map comprising diffusion, chaotic advection, and mixed advection-dominated zones has also been devised. An empirical correlation for pressure drop as a function of Reynolds number has been developed and a corresponding friction factor has been obtained. Predictions on heat and mass transfer based on analogies in micromixers have also been proposed.

Droplet Microfluidic Devices: Working Principles, Fabrication Methods, and Scale-Up Applications

Compared with the conventional emulsification method, droplets generated within microfluidic devices exhibit distinct advantages such as precise control of fluids, exceptional monodispersity, uniform morphology, flexible manipulation, and narrow size distribution. These inherent benefits, including intrinsic safety, excellent heat and mass transfer capabilities, and large surface-to-volume ratio, have led to the widespread applications of droplet-based microfluidics across diverse fields, encompassing chemical engineering, particle synthesis, biological detection, diagnostics, emulsion preparation, and pharmaceuticals. However, despite its promising potential for versatile applications, the practical utilization of this technology in commercial and industrial is extremely limited to the inherently low production rates achievable within a single microchannel. Over the past two decades, droplet-based microfluidics has evolved significantly, considerably transitioning from a proof-of-concept stage to industrialization. And now there is a growing trend towards translating academic research into commercial and industrial applications, primarily driven by the burgeoning demands of various fields. This paper comprehensively reviews recent advancements in droplet-based microfluidics, covering the fundamental working principles and the critical aspect of scale-up integration from working principles to scale-up integration. Based on the existing scale-up strategies, the paper also outlines the future research directions, identifies the potential opportunities, and addresses the typical unsolved challenges.

Minimize Precolumn Band Broadening with Immiscible Solvent Sandwich Injection

We investigated the possibility of reducing the effect of precolumn band broadening (PreCBB) by sandwiching the sample between two small plugs of an immiscible liquid. It has been found that in cases of severe PreCBB, improvements in peak efficiency can amount up to 20 times for the early-eluting compounds. For smaller degrees of PreCBB, the gain on the efficiency of early-eluting compounds is smaller (order of 50%), yet it is still significant. It has been verified that the presence of the immiscible fluid sandwich does not affect the repeatability of the analysis nor the linearity of the calibration curves used for analyte quantitation. It is also shown that the main effect of the two sandwich plugs is the minimization of the dispersion in the precolumn transfer tubing itself, which makes the method fundamentally different from pure on-column focusing methods such as the performance optimizing injection sequence (POISe) method. It is further demonstrated that both halves of the sandwich are needed, since the beneficial effect is clearly much smaller when only one plug is present. A drawback of the method is that some of the late-eluting peaks are slightly adversely affected by the presence of the sandwich liquid in the case where 127 μm i.d. tubing was used. The mechanism for this peak deterioration effect is at present still unclear but only occurs under gradient conditions and is clearly linked to the size of the sandwich plugs (the smaller the plugs, the smaller the adverse effect) and the internal diameter of the tubing used between the injection valve and the column.

Microreactor Technology: Identifying Focus Fields and Emerging Trends by Using CiteSpace II

Microreactors have gained widespread attention from academia and industrial researchers due to their exceptionally fast mass and heat transfer and flexible control. In this work, CiteSpace software was used to systematically analyze the relevant literature to gain a comprehensively understand on the research status of microreactors in various fields. The results show that the research depth and application scope of microreactors are continuing to expand. The top 10 most popular research fields are photochemistry, pharmaceutical intermediates, multistep flow synthesis, mass transfer, computational fluid dynamics, μ-TAS (micro total analysis system), nanoparticles, biocatalysis, hydrogen production, and solid-supported reagents. The evolution trends of current focus areas are examined, including photochemistry, mass transfer, biocatalysis and hydrogen production and their milestone literature is analyzed in detail. This article demonstrates the development of different fields of microreactors technology and highlights the unending opportunities and challenges offered by this fascinating technology.

Micromixing Intensification within a Combination of T-Type Micromixer and Micropacked Bed

The combination of microstructural units is an effective strategy to improve the micromixing of liquid phase systems, especially viscous systems. However, how the microstructural combination influences micromixing is still not systematically investigated. In this work, the Villermaux/Dushman reaction is used to study the micromixing performance of the viscous system of the glycerol-water in the combination of a T-type micromixer and a micropacked bed. Micromixing performances under various structural parameters and fluid characteristics are determined and summarized, and the micromixing laws are revealed by dimensionless analysis considering the specific spatial characteristics and temporal sequence in the combined microstructures. It achieves good agreement with experimental results and enables guidance for the design and scaling-up of the combined T-type micromixer and micropacked bed towards micromixing intensification in viscous reaction systems.

Microflow chemistry and its electrification for sustainable chemical manufacturing

Sustainability is vital in solving global societal problems. Still, it requires a holistic view by considering renewable energy and carbon sources, recycling waste streams, environmentally friendly resource extraction and handling, and green manufacturing. Flow chemistry at the microscale can enable continuous sustainable manufacturing by opening up new operating windows, precise residence time control, enhanced mixing and transport, improved yield and productivity, and inherent safety. Furthermore, integrating microfluidic systems with alternative energy sources, such as microwaves and plasmas, offers tremendous promise for electrifying and intensifying modular and distributed chemical processing. This review provides an overview of microflow chemistry, electrification, their integration toward sustainable manufacturing, and their application to biomass upgrade (a select number of other processes are also touched upon). Finally, we identify critical areas for future research, such as matching technology to the scale of the application, techno-economic analysis, and life cycle assessment.

A novel observation platform for determining the micro-dispersion performance in practical reaction systems

A novel observation platform by placing the downstream observation window vertically has been constructed, and the actual micro-dispersion state can be exactly reflected in the observation window.

Revisiting aromatic diazotization and aryl diazonium salts in continuous flow: highlighted research during 2001–2021

Aryl diazonium salts play an important role in chemical transformations; however their explosive nature limits their applications in batch.

Kinetic study of o-nitrotoluene nitration in a homogeneously continuous microflow

Determination of nitration kinetics of o-nitrotoluene with a homogeneously continuous microflow system.

Geometrical pH mapping of Microfluids by principal-component-analysis-based xyz-spectrum conversion method

The absorption spectra of bromothymol blue (BTB) solution introduced in microfluidic devices were reproduced by principal component analysis (PCA)-based xyz-spectrum conversion methods for geometric mapping of the pH values of fluids. We fabricated PDMS-made microfluidic devices with a channel depth of 1 mm to overcome the lower detection limits of transmittance image acquisition. Aqueous solutions of pH indicators under various pH conditions were hydrodynamically introduced into the channel, and RGB values of the region of interest (ROI) were obtained via image analysis. The xyz values were then converted into absorption spectral data of the pH indicator using the PCA-based spectral reproduction previously proposed by the authors. The high reproducibility of the spectra was confirmed to be comparable to that of the conventional method using a spectrophotometer. We applied the present method to elucidate the pH gradient at an aqueous biphasic interface in the microfluidic channels generated by contacting multiple laminar flows of two or three buffered solutions. We confirmed that the pH gradient ranged from approximately 70 to 140 μm, which is consistent with the results reported using other approaches. The results demonstrate the applicability of the present method to the fluctuation field in micro/nanospaces to acquire spectrophotometric information in the order of milliseconds without monochromating equipment.

Liquid-Liquid Flows with Non-Newtonian Dispersed Phase in a T-Junction Microchannel

Immiscible liquid–liquid flows in microchannels are used extensively in various chemical and biological lab-on-a-chip systems when it is very important to predict the expected flow pattern for a variety of fluids and channel geometries. Commonly, biological and other complex liquids express non-Newtonian properties in a dispersed phase. Features and behavior of such systems are not clear to date. In this paper, immiscible liquid–liquid flow in a T-shaped microchannel was studied by means of high-speed visualization, with an aim to reveal the shear-thinning effect on the flow patterns and slug-flow features. Three shear-thinning and three Newtonian fluids were used as dispersed phases, while Newtonian castor oil was a continuous phase. For the first time, the influence of the non-Newtonian dispersed phase on the transition from segmented to continuous flow is shown and quantitatively described. Flow-pattern maps were constructed using nondimensional complex We^0.4·Oh^0.6 depicting similarity in the continuous-to-segmented flow transition line. Using available experimental data, the proposed nondimensional complex is shown to be effectively applied for flow-pattern map construction when the continuous phase exhibits non-Newtonian properties as well. The models to evaluate an effective dynamic viscosity of a shear-thinning fluid are discussed. The most appropriate model of average-shear-rate estimation based on bulk velocity was chosen and applied to evaluate an effective dynamic viscosity of a shear-thinning fluid. For a slug flow, it was found that in the case of shear-thinning dispersed phase at low flow rates of both phases, a jetting regime of slug formation was established, leading to a dramatic increase in slug length.

An intensified chlorination process of 4-nitroaniline in a liquid–liquid microflow system

A strategy of dissolving chlorine gas in 1,2-dichloroethane for a liquid–liquid two-phase chlorination reaction was proposed to resolve the problems of low efficiency, strong corrosivity, and poor controllability of gas–liquid chlorination.

Contactless mass transfer for intra-droplet extraction

This study demonstrates the possibility of “contactless” mass transfer between two aqueous slugs (droplets) separated by an oil slug in Taylor flow inside milli-channels. Separation of the alternating aqueous slugs at the outlet was performed by switching a couple of solenoid valves at branched outlets according to signals obtained by an optical sensor at the branch. Transfer of bromothymol blue (BTB) from acidic to basic aqueous slugs was performed for demonstration. In some cases, aqueous slugs separated by oil, merged catching on each other due to the velocity difference. Interfacial tension which was affected by the solute concentration was responsible for the velocity difference. Position-specific mass transfer activity at the rear end of the aqueous slugs was found on the course of the experiment. A meandering channel decreased the velocity difference and enhanced mass transfer. Almost complete (93%) transfer of BTB was achieved within a short residence time of several minutes under optimized conditions. The presented system opens a way for advanced separation using minimum amounts of the oil phase and allows concentrating the solute by altering relative lengths of the sender and receiver slugs.

Continuous Synthesis of Nanocrystals via Flow Chemistry Technology

Modern nanotechnologies bring humanity to a new age, and advanced methods for preparing functional nanocrystals are cornerstones. A considerable variety of nanomaterials has been created over the past decades, but few were prepared on the macro scale, even fewer making it to the stage of industrial production. The gap between academic research and engineering production is expected to be filled by flow chemistry technology, which relies on microreactors. Microreaction devices and technologies for synthesizing different kinds of nanocrystals are discussed from an engineering point of view. The advantages of microreactors, the important features of flow chemistry systems, and methods to apply them in the syntheses of salt, oxide, metal, alloy, and quantum dot nanomaterials are summarized. To further exhibit the scaling-up of nanocrystal synthesis, recent reports on using microreactors with gram per hour and larger production rates are highlighted. Finally, an industrial example for preparing 10 tons of CaCO₃ nanoparticles per day is introduced, which shows the great potential for flow chemistry processes to transfer lab research to industry.

Reproducing absorption spectra of pH indicators from RGB values of microscopic images

Absorption spectra of pH indicators in aqueous solutions were reproduced from RGB values of microscopic images utilizing principal component analysis (PCA) and linear algebraic treatments. The reproduction of absorption spectra comprises the following three steps: (1) determining the loading spectra by PCA, (2) determining the conversion matrix from the RGB values to the score vectors, and (3) reproducing the absorption spectra by linear combination of the loading spectra and the score vectors. The reproducibility of the absorption spectra was demonstrated by employing bromothymol blue and methyl red solutions as pH indicators. The reproduced spectra of both indicators were in good agreement with the spectra measured with a conventional spectrophotometer. The pK_a values of both indicators calculated from the reproduced spectra are in good agreement with those obtained from the spectrophotometric spectra and the literature values, confirming validity of the reproduction. This approach was applied to measure pH of freeze concentrated solutions in micro drains formed in ice. A change in pH was successfully observed on freezing and was compared with that reported in previous literature. Since this method does not necessitate the use of grating systems, spectral changes can be traced in milliseconds; this elucidates the phenomena occurring in fluctuating fields.

Manipulation of gas-liquid-liquid systems in continuous flow microreactors for efficient reaction processes

Gas-liquid-liquid flow in microreactors holds great potential towards process intensification of operation in multiphase systems, particularly by a precise control over the three-phase contact patterns and the associated mass transfer enhancement. This work reviews the manipulation of gas-liquid-liquid three-phase flow in microreactors for carrying out efficient reaction processes, including gas-liquid-liquid reactions with catalysts residing in either liquid phase, coupling of a gas-liquid reaction with the liquid-liquid extraction, inert gas assisted liquid-liquid reactions and particle synthesis under three-phase flow. Microreactors are shown to be able to provide well-defined flow patterns and enhanced gas-liquid/liquid-liquid mass transfer rates towards the optimized system performance. The interplay between hydrodynamics and mass transfer, as well as its influence on the overall microreactor system performance is discussed. Meanwhile, future perspectives regarding the scale-up of gas-liquid-liquid microreactors in order to meet the industrial needs and their potential applications especially in biobased chemicals and fuels synthesis are further addressed.

Homogeneous synthesis of hydroxylamine hydrochloride via acid-catalyzed hydrolysis of nitromethane

A continuous homogeneous synthesis of NH₂OH·HCl was achieved and well described with a segmented semi-empirical kinetics model.

Facile synthesis of anhydrous microparticles using plug-and-play microfluidic reactors

Microfluidic materials synthesis techniques are an ideal approach for controlled synthesis of anhydrous microparticles. In this article, we highlight the recent developments using plug-and-play microreactors for anhydrous microparticle synthesis.

Experiments and computations of microfluidic liquid–liquid flow patterns

A high accuracy model is built using machine learning to predict flow patterns, providing a powerful tool for continuous flow microreactor design.

Compact Steam-Methane Reforming for the Production of Hydrogen in Continuous Flow Microreactor Systems

The implementation of fuel cell deployment requires efficient conversion of fuels into hydrogen in a distributed energy system. Fortunately, continuous flow and microreactor technology provide unique opportunities for the portable production of hydrogen. This study focuses on determining the operation space for a thermally integrated methane reforming system, thereby providing a theoretical basis for the design and optimization of such systems. The steam-methane reforming over rhodium coupled with methane combustion over platinum in a thermally integrated microchannel reactor arranged with rectangular-shaped protuberances was studied numerically in order to improve its operability and stability. Computational fluid dynamic simulations were carried out with detailed reaction mechanisms to identify conditions for the maximum hydrogen yield and the highest output power. Various operating lines were presented, and various performance metrics were evaluated accordingly. The results indicated that the efficient production of hydrogen is made possible through improving transport performance for highly active catalysts. The flow disturbance elements designed for the reactor are of great benefit to intensification of the reforming process. There exists a trade-off between fuel utilization and output power. Autothermal operation advantages from improved transport performance in small physical dimensions were demonstrated for the system, but careful thermal management is always necessary to ensure its efficient and stable operation. The thermal conductivity of the wall separating the exothermic and endothermic reactions plays a significant role in determining the performance of the system. Highly active catalysts are required to intensify the overall reforming process and to achieve efficient thermal management. Adjustment of fluid velocities can serve as a convenient means to achieve efficient operation of the system.