Acoustophoretic focusing effects on particle synthesis and clogging in microreactors

Dual-drive acoustic micromixer for rapid nucleation and ultrafast growth of perovskite nanoparticles

All-inorganic cesium lead halide perovskites have garnered significant attention owing to their favorable optical properties. Microfluidics-based acoustic mixers are capable of achieving rapid nucleation and ultrafast growth kinetics. Nevertheless, conventional acoustic mixers rely on the response of microstructures to the acoustic field for mixing fluids, the majority of these disturbances occur in the central region of the channel, with minimal impact on the fluid within the side walls. This paper proposes a novel acoustic mixer that combines the effects of sharp corners and bubbles in response to the acoustic field, thereby producing effective disturbance of the fluid throughout the channel. The combined effect enables the micromixer to achieve complete mixing at different inlet flow ratios with mixing times as low as 5 ms. The superiority of acoustic mixers in controlling the nanocrystal formation stage was further validated through the synthesis of chalcogenide nanocrystals using the LARP method. The millisecond mixing time facilitated the rapid formation of nanocrystals and their subsequent rapid growth. The results demonstrate that the green luminescence intensity at 520 nm of the samples synthesized using the acoustic micromixer is 118% higher than that of the samples synthesized using an intermittent reactor. The novel micromixer broadens the range of applications and offers a promising avenue for the large-scale continuous synthesis of high-quality lead-halide perovskite nanocrystals (NCs).

Ultrasound mechanisms and their effect on solid synthesis and processing: a review

Ultrasound proves to be an effective technique for intensifying a wide range of processes involving solids and, as such, is often used to improve control over both solids formation and post-treatment stages. The intensifying capabilities of ultrasonic processing are best interpreted in the context of the chemical, transport, and mechanical effects that occur during sonication. This review presents an overview of how ultrasound influences the processing and synthesis of solids across various material classes, contextualized within an ultrasound effect framework. By describing the mechanisms underlying the different effects of ultrasound on the solid synthesis and processing, this review aims to facilitate a deeper understanding of the current literature in the field and to promote more effective utilization of ultrasound technology in solid synthesis and processing.

Modular Cascade of Flow Reactors: Continuous Flow Synthesis of Water-Insoluble Diazo Dyes in Aqueous System

Continuous flow synthesis is pivotal in dye production to address batch-to-batch variations. However, synthesizing water-insoluble dyes in an aqueous system poses a challenge that can lead to clogging. This study successfully achieved the safe and efficient synthesis of azo dyes by selecting and optimizing flow reactor modules for different reaction types in the two-step reaction and implementing cascade cooperation. Integrating continuous flow microreactor with continuous stirred tank reactor (CSTR) enabled the continuous flow synthesis of Sudan Yellow 3G without introducing water-soluble functional groups or using organic solvents to enhance solubility. Optimizing conditions (acidity/alkalinity, temperature, residence time) within the initial modular continuous flow reactor resulted in a remarkable 99.5% isolated yield, 98.6 % purity, and a production rate of 2.90 g h-1. Scaling-up based on different reactor module characteristics further increased the production rate to 74.4 g h-1 while maintaining high yield and purity. The construction of this small 3D-printing modular cascaded reactor and process scaling-up provide technical support for continuous flow synthesis of water-insoluble dyes, particularly high-market-share azo dyes. Moreover, this versatile methodology proves applicable to continuous flow processes involving various homogeneous and heterogeneous reaction cascades.

Dielectrophoresis: Measurement technologies and auxiliary sensing applications

Dielectrophoresis (DEP), which arises from the interaction between dielectric particles and an aqueous solution in a nonuniform electric field, contributes to the manipulation of nano and microparticles in many fields, including colloid physics, analytical chemistry, molecular biology, clinical medicine, and pharmaceutics. The measurement of the DEP force could provide a more complete solution for verifying current classical DEP theories. This review reports various imaging, fluidic, optical, and mechanical approaches for measuring the DEP forces at different amplitudes and frequencies. The integration of DEP technology into sensors enables fast response, high sensitivity, precise discrimination, and label-free detection of proteins, bacteria, colloidal particles, and cells. Therefore, this review provides an in-depth overview of DEP-based fabrication and measurements. Depending on the measurement requirements, DEP manipulation can be classified into assistance and integration approaches to improve sensor performance. To this end, an overview is dedicated to developing the concept of trapping-on-sensing, improving its structure and performance, and realizing fully DEP-assisted lab-on-a-chip systems.

Exploring operational boundaries for acoustic concentration of cell suspensions

The development of a standardized, generic method for concentrating suspensions in continuous flow is challenging. In this study, we developed and tested a device capable of concentrating suspensions with an already high cell concentration to meet diverse industrial requirements. To address typical multitasking needs, we concentrated suspensions with high solid content under a variety of conditions. Cells from Saccharomyces cerevisiae, Escherichia coli, and Chinese hamster ovary cells were effectively focused in the center of the main channel of a microfluidic device using acoustophoresis. The main channel bifurcates into three outlets, allowing cells to exit through the central outlet, while the liquid evenly exits through all outlets. Consequently, the treatment separates cells from two-thirds of the surrounding liquid. We investigated the complex interactions between parameters. Increasing the channel depth results in a decrease in process efficiency, attributed to a decline in acoustic energy density. The study also revealed that different cell strains exhibit distinct acoustic contrast factors, originating from differences in dimensions, compressibility, and density values. Finally, a combination of high solid content and flow rate leads to an increase in diffusion through a phenomenon known as shear-induced diffusion. KEY POINTS: • Acoustic focusing in a microchannel was used to concentrate cell suspensions • The parameters influencing focusing at high concentrations were studied • Three different cell strains were successfully concentrated.

Heterogeneous photochemical reaction enabled by an ultrasonic microreactor

The presence of solids as starting reagents/reactants or products in flow photochemical reactions can lead to reactor clogging and yield reduction from side reactions. We address this limitation with a new ultrasonic microreactor for continuous solid-laden photochemical reactions. The ultrasonic photochemical microreactor is characterized by the liquid and solid residence time distribution (RTD) and the absorbed photon flux in the reactor via chemical actinometry. The solid-handling capability of the ultrasonic photochemical microreactor is demonstrated with a silyl radical-mediated metallaphotoredox cross-electrophile coupling with a solid base as a reagent.

Microreactor-based micro/nanomaterials: fabrication, advances, and outlook

Micro/nanomaterials are widely used in optoelectronics, environmental materials, bioimaging, agricultural industries, and drug delivery owing to their marvelous features, such as quantum tunneling, size, surface and boundary, and Coulomb blockade effects. Recently, microreactor technology has opened up broad prospects for green and sustainable chemical synthesis as a powerful tool for process intensification and microscale manipulation. This review focuses on recent progress in the microreactor synthesis of micro/nanomaterials. First, the fabrication and design principles of existing microreactors for producing micro/nanomaterials are summarized and classified. Afterwards, typical examples are shown to demonstrate the fabrication of micro/nanomaterials, including metal nanoparticles, inorganic nonmetallic nanoparticles, organic nanoparticles, Janus particles, and MOFs. Finally, the future research prospects and key issues of microreactor-based micro/nanomaterials are discussed. In short, microreactors provide new ideas and methods for the synthesis of micro/nanomaterials, which have huge potential and inestimable possibilities in large-scale production and scientific research.

Emulsification mechanism in an ultrasonic microreactor: Influence of surface roughness and ultrasound frequency

An ultrasonic microreactor with rough microchannels is presented in this study for oil-in-water (O/W) emulsion generation. Previous accounts have shown that surface pits or imperfections localize and enhance cavitation activity. In this study cavitation bubbles are localized on the rough microchannels of a borosilicate glass microreactor. The cavitation bubbles in the microchannel are primarily responsible for emulsification in the ultrasonic microreactor. We investigate the emulsification mechanism in the rough microchannels employing high-speed imaging to reveal the different emulsification modes influenced by the size and oscillation intensity of the cavitation bubbles. The effect of emulsification modes on the O/W emulsion droplet size distribution for different surface roughness and frequency is demonstrated. The positive effect of the frequency on minimizing the droplet size utilizing a reactor with large pits is presented. We also demonstrate microreactor systems for a successful generation of miniemulsions with high dispersed phase volume fractions up to 20%. The observed emulsification mechanism in the rough microchannel offers new insights into the utility and scale-up of ultrasonic microreactors for emulsification.

Analysis of dynamic acoustic resonance effects in a sonicated gas-liquid flow microreactor

In this work, we characterize acoustic resonance phenomena occurring between gas bubbles in a segmented gas-liquid flow in a microchannel irradiated with a frequency around 500 kHz. A large acoustic amplitude can be reached, leading to gas-liquid interface deformation, atomization of micrometer sized droplets, and cavitation. A numerical approach combining an acoustic frequency-domain solver and a Lagrangian Surface-Evolver solver is introduced to predict the acoustic deformation of gas-liquid interfaces and the dynamic acoustic magnitude. The numerical approach and its assumptions were validated with experiments, for which a good agreement was observed. Therefore, this numerical approach allows to provide a description and an understanding of the acoustic nature of these phenomena. The acoustic pressure magnitude can reach hundreds of kPa to tens of MPa, and these values are consistent with the observation of atomization and cavitation in the experiments. Furthermore, volume of fluid simulations were performed to predict the atomization threshold, which was then related to acoustic resonance. It is found that dynamic acoustic resonance gives rise to atomization bursts at the gas bubble surface. The presented approach can be applied to more complex acoustic fields involving more complex channel geometries, vibration patterns, or two-phase flow patterns.

Non-fouling flow reactors for nanomaterial synthesis

This review provides a holistic description of flow reactor fouling for wet-chemical nanomaterial syntheses. Fouling origins and consequences are discussed together with the variety of flow reactors for its prevention.

A versatile non-fouling multi-step flow reactor platform: demonstration for partial oxidation synthesis of iron oxide nanoparticles

In the last decade flow reactors for material synthesis were firmly established, demonstrating advantageous operating conditions, reproducible and scalable production via continuous operation, as well as high-throughput screening of synthetic conditions. Reactor fouling, however, often restricts flow chemistry and the common fouling prevention via segmented flow comes at the cost of inflexibility. Often, the difficulty of feeding reagents into liquid segments (droplets or slugs) constrains flow syntheses using segmented flow to simple synthetic protocols with a single reagent addition step prior or during segmentation. Hence, the translation of fouling prone syntheses requiring multiple reagent addition steps into flow remains challenging. This work presents a modular flow reactor platform overcoming this bottleneck by fully exploiting the potential of three-phase (gas-liquid-liquid) segmented flow to supply reagents after segmentation, hence facilitating fouling free multi-step flow syntheses. The reactor design and materials selection address the operation challenges inherent to gas-liquid-liquid flow and reagent addition into segments allowing for a wide range of flow rates, flow ratios, temperatures, and use of continuous phases (no perfluorinated solvents needed). This "Lego®-like" reactor platform comprises elements for three-phase segmentation and sequential reagent addition into fluid segments, as well as temperature-controlled residence time modules that offer the flexibility required to translate even complex nanomaterial synthesis protocols to flow. To demonstrate the platform's versatility, we chose a fouling prone multi-step synthesis, i.e., a water-based partial oxidation synthesis of iron oxide nanoparticles. This synthesis required I) the precipitation of ferrous hydroxides, II) the addition of an oxidation agent, III) a temperature treatment to initiate magnetite/maghemite formation, and IV) the addition of citric acid to increase the colloidal stability. The platform facilitated the synthesis of colloidally stable magnetic nanoparticles reproducibly at well-controlled synthetic conditions and prevented fouling using heptane as continuous phase. The biocompatible particles showed excellent heating abilities in alternating magnetic fields (ILP values >3 nH m² kg_Fe^-1), hence, their potential for magnetic hyperthermia cancer treatment. The platform allowed for long term operation, as well as screening of synthetic conditions to tune particle properties. This was demonstrated via the addition of tetraethylenepentamine, confirming its potential to control particle morphology. Such a versatile reactor platform makes it possible to translate even complex syntheses into flow, opening up new opportunities for material synthesis.

Ultrasonic enhanced emulsification process in 3D printed microfluidic device to encapsulate active pharmaceutical ingredients

A new method to prepare polymer encapsulated repaglinide nanoparticles through ultrasonic enhanced microchannel emulsification technique was explored. Using the concept of 3D printing, three different shaped micromixers (T-type, Y-type, and F-type) followed by a serpentine microchannel was fabricated using SS-316. Parametric study was performed on all three fabricated micromixers. The best results were obtained for the Y-microchannel in a microfluidic system alone, which showed a minimum particle size of 513.6 nm with 75.4% encapsulation efficiency (EE). In the selected microchannel, to further reduce the drug particle size and to increase% EE, convective mixing between immiscible fluids was enhanced by implementing ultrasound. Compared to the microfluidic system, particle size and EE were significantly improved in the ultrasonic microfluidic system. The experimental results revealed that the minimum particle size of 75.4 ± 1.3 nm with 82.9 ± 0.2% EE was achieved using an ultrasonic enhanced microfluidic system. The zeta potential of + 29.5 mV was obtained for emulsion prepared using the ultrasonic microfluidic system, whereas + 22 mV was prepared using a microfluidic system. Moreover, a backscattering measurement was performed to predict the stability of prepared emulsions. Integrating the ultrasound with a microfluidic system has proven beneficial for drug encapsulation.

Organic Sonochemistry: A Chemist's Timely Perspective on Mechanisms and Reactivity

Sonochemistry, the use of sound waves, usually within the ultrasonic range (>20 kHz), to boost or alter chemical properties and reactivity constitutes a long-standing and sustainable technique that has, however, received less attention than other activation protocols despite affordable setups. Even if unnecessary to underline the impact of ultrasound-based strategies in a broad range of chemical and biological applications, there is considerable misunderstanding and pitfalls regarding the interpretation of cavitational effects and the actual role played by the acoustic field. In this Perspective, with an eye on mechanisms in particular, we discuss the potentiality of sonochemistry in synthetic organic chemistry through selected examples of past and recent developments. Such examples illustrate specific controlling effects and working rules. Looking back at the past while looking forward to advancing the field, some essentials of sonochemical activation will be distilled.

Acoustic resonance and atomization for gas-liquid systems in microreactors

It is shown that a liquid slug in gas-liquid segmented flow in microchannels can act as an acoustic resonator to disperse large amounts of small liquid droplets, commonly referred to as atomization, into the gas phase. We investigate the principles of acoustic resonance within a liquid slug through experimental analysis and numerical simulation. A mechanism of atomization in the confined channels and a hypothesis based on high-speed image analysis that links acoustic resonance within a liquid slug with the observed atomization is proposed. The observed phenomenon provides a novel source of confined micro sprays and could be an avenue, amongst others, to overcome mass transfer limitations for gas-liquid processes in flow.

Continuous Ultrasonic Reactors: Design, Mechanism and Application

Ultrasonic small scale flow reactors have found increasing popularity among researchers as they serve as a very useful platform for studying and controlling ultrasound mechanisms and effects. This has led to the use of these reactors for not only research purposes, but also various applications in biological, pharmaceutical and chemical processes mostly on laboratory and, in some cases, pilot scale. This review summarizes the state of the art of ultrasonic flow reactors and provides a guideline towards their design, characterization and application. Particular examples for ultrasound enhanced multiphase processes, spanning from immiscible fluid-fluid to fluid-solid systems, are provided. To conclude, challenges such as reactor efficiency and scalability are addressed.

Synergistic effects of the alternating application of low and high frequency ultrasound for particle synthesis in microreactors

Ultrasound (US) is a promising method to address clogging and mixing issues in microreactors (MR). So far, low frequency US (LFUS), pulsed LFUS and high frequency US (HFUS) have been used independently in MR for particle synthesis to achieve narrow particle size distributions (PSD). In this work, we critically assess the advantages and disadvantages of each US application method for the case study of calcium carbonate synthesis in an ultrasonic microreactor (USMR) setup operating at both LFUS (61.7 kHz, 8 W) and HFUS (1.24 MHz, 1.6 W). Furthermore, we have developed a novel approach to switch between LFUS and HFUS in an alternating manner, allowing us to quantify the synergistic effect of performing particle synthesis under two different US conditions. The reactor was fabricated by gluing a piezoelectric plate transducer to a silicon microfluidic chip. The results show that independently applying HFUS and LFUS produces a narrower PSD compared to silent conditions. However, at lower flow rates HFUS leads to agglomerate formation, while the reaction conversion is not enhanced due to weak mixing effects. LFUS on the other hand eliminates particle agglomerates and increases the conversion due to the strong cavitation effect. However, the required larger power input leads to a steep temperature rise in the reactor and the risk of reactor damage for long-term operation. While pulsed LFUS reduces the temperature rise, this application mode leads again to the formation of particle agglomerates, especially at low LFUS percentage. The proposed application mode of switching between LFUS and HFUS is proven to combine the advantages of both LFUS and HFUS, and results in particles with a unimodal narrow PSD (one order of magnitude reduction in the average size and span compared to silent conditions) and negligible rise of the reactor temperature.

Enabling low power acoustics for capillary sonoreactors

Capillary reactors demonstrate outstanding potential for on-demand flow chemistry applications. However, non-uniform distribution of multiphase flows, poor solid handling, and the risk of clogging limit their usability for continuous manufacturing. While ultrasonic irradiation has been traditionally applied to address some of these limitations, their acoustic efficiency, uniformity and scalability to larger reactor systems are often disregarded. In this work, high-speed microscopic imaging reveals how cavitation-free ultrasound can unclog and prevent the blockage of capillary reactors. Modeling techniques are then adapted from traditional acoustic designs and applied to simulate and prototype sonoreactors with wider and more uniform sonication areas. Blade-, block- and cylindrical shape sonotrodes are optimized to accommodate longer capillary lengths in sonoreactors resonating at 28 kHz. Finally, a novel helicoidal capillary sonoreactor is proposed to potentially deal with a high concentration of solid particles in miniaturized flow chemistry. The acoustic designs and first principle rationalization presented here offer a transformative step forward in the scale-up of efficient capillary sonoreactors.

Pulsed ultrasound for temperature control and clogging prevention in micro-reactors

Ultrasonic micro-reactors are frequently applied to prevent micro-channel clogging in the presence of solid materials. Continuous sonication will lead to a sizeable energy input resulting in a temperature increase in the fluidic channels and concerns regarding microchannel degradation. In this paper, we investigate the application of pulsed ultrasound as a less invasive approach to prevent micro-channel clogging, while also controlling the temperature increase. The inorganic precipitation of barium sulfate particles was studied, and the impact of the effective ultrasonic treatment ratio, frequency and load power on the particle size distribution, pressure and temperature was quantified in comparison to non-sonicated experiments. The precipitation reactions were performed in a continuous reactor consisting of a micro-reactor chip attached to a Langevin-type transducer. It was found that adjusting the pulsed ultrasound conditions prevented microchannel clogging by reducing the particle size to the same magnitude as observed for continuous sonication. Furthermore, reducing the effective treatment ratio from 100 to 12.5% decreases the temperature rise from 7 to 1 °C.

Focusing of Microcrystals and Liquid Condensates in Acoustofluidics

Manipulation of high-density materials, such as crystals and liquid condensates, is of great importance for many applications, including serial crystallography, structural and molecular biology, chemistry, and medicine. In this work, we describe an acoustic technique to focus and harvest flowing crystals and liquid condensates. Moreover, we show, based on numerical simulations, that the acoustic waves can be used for size-based particle (crystals, droplets, etc.) separation. This is an essential technological step in biological research, medical applications, and industrial processes. The presented technology offers high precision, biocompatibility, ease of use and additionally, is non-invasive and inexpensive. With the recent advent of X-ray Free Electron Laser (XFEL) technology and the associated enormous importance of a thin jet of crystals, this technology might pave the way to a novel type of XFEL injector.