A flow-focusing geometry is integrated into a microfluidic device and used to study drop formation in liquid–liquid systems. A phase diagram illustrating the drop size as a function of flow rates and flow rate ratios of the two liquids includes one regime where drop size is comparable to orifice width and a second regime where drop size is dictated by the diameter of a thin “focused” thread, so drops much smaller than the orifice are formed. Both monodisperse and polydisperse emulsions can be produced.

/pdf/formation-of-dispersions-using-flow-focusing-in-2zw2hx9w2y.pdf

Formation of dispersions using “flow focusing” in microchannels

This review reports the progress on the recent development of micromixers. The review first presents the different micromixer types and designs. Micromixers in this review are categorized as passive micromixers and active micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, passive micromixers will be the focus of this review. Next, the review discusses the operation points of the micromixers based on characteristic dimensionless numbers such as Reynolds number Re, Peclet number Pe, and in dynamic cases the Strouhal number St. The fabrication technologies for different mixer types are also analysed. Quantification techniques for evaluation of the performance of micromixers are discussed. Finally, the review addresses typical applications of micromixers.

https://iopscience.iop.org/article/10.1088/0960-1317/15/2/R01/pdf

Micromixers?a review

In the past few years, continuous-flow reactors with channel dimensions in the micro- or millimeter region have found widespread application in organic synthesis. The characteristic properties of these reactors are their exceptionally fast heat and mass transfer. In microstructured devices of this type, virtually instantaneous mixing can be achieved for all but the fastest reactions. Similarly, the accumulation of heat, formation of hot spots, and dangers of thermal runaways can be prevented. As a result of the small reactor volumes, the overall safety of the process is significantly improved, even when harsh reaction conditions are used. Thus, microreactor technology offers a unique way to perform ultrafast, exothermic reactions, and allows the execution of reactions which proceed via highly unstable or even explosive intermediates. This Review discusses recent literature examples of continuous-flow organic synthesis where hazardous reactions or extreme process windows have been employed, with a focus on applications of relevance to the preparation of pharmaceuticals.

Continuous‐Flow Technology—A Tool for the Safe Manufacturing of Active Pharmaceutical Ingredients

The application of microstructured reactors in the chemical process industry has gained significant importance in recent years. Companies that offer not only microstructured reactors, but also entire chemical process plants and services relating to them, are already in existence. In addition, many institutes and universities are active within this field, and process-engineering-oriented reviews and a specialized book are available. Microstructured systems can be applied with particular success in the investigation of highly exothermic and fast reactions. Often the presence of temperature-induced side reactions can be significantly reduced through isothermal operations. Although microstructured reaction techniques have been shown to optimize many synthetic procedures, they have not yet received the attention they deserve in organic chemistry. For this reason, this Review aims to address this by providing an overview of the chemistry in microstructured reactors, grouped into liquid-phase, gas-phase, and gas-liquid reactions.

Chemistry in microstructured reactors.

We review transport characteristics of pressure-driven, multiphase flows through microchannel networks tens of nanometres to several hundred of micrometres wide with emphasis on conditions resulting in enhanced mixing and reduced axial dispersion. Dimensionless scaling parameters useful in characterizing multiphase flows are summarized along with experimental flow visualization techniques. Static and dynamic stability considerations are also included along with methods for stabilizing multiphase flows through surface modifications. Observed gas–liquid and immiscible liquid–liquid flows are summarized in terms of flow regime diagrams and the different flows are related to applications in chemistry and materials synthesis. Means to completely separate multiphase flows on the microscale and guidelines for design of scalable multiphase systems are also discussed.

Multiphase microfluidics: from flow characteristics to chemical and materials synthesis

Direct fluorination of toluene using elemental fluorine in gas/liquid microreactors

Two different concepts of contacting gas and liquid media were realized in miniaturized reaction devices. The first one, a micro bubble column, utilizes dispersed systems consisting of small gas bubbles in a liquid phase. A large specific interfacial area of about 9,000 m2/m3 was determined by applying the sulfite oxidation method. Flow equipartition of a high number of parallel operating channels was achieved by using a pressure barrier. First visual observations of the liquid and gaseous flows indicate that this principle leads to a uniform flow equipartition. The second concept was based on the generation of thin stable films in a falling film microreactor. Very narrow residence time distributions of the flows in the channels were achieved also by using a pressure barrier.

Gas / Liquid Microreactors for Direct Fluorination of Aromatic Compounds Using Elemental Fluorine

The micro bubble column [1] was one of the first microstructured devices being specially designed for gas/liquid processing. Within a short time, the experimental performance of this microreactor was demonstrated for the fluorination of toluene using elemental fluorine, as an example of use with industrial relevance. However, the understanding of fundamental reactor characteristics had to be postponed to succeeding, more detailed investigations. In this context, the objective of the work described in this article is to determine specific interfacial areas as a main parameter of reactor performance and to develop on this basis a first reactor model for gas/liquid reactions in the micro bubble column. This reactor model enables then proper prediction of suitable candidate reactions for processing in this microreactor.

Characterization of a Gas/Liquid Microreactor, the Micro Bubble Column: Determination of Specific Interfacial Area

Mass and heat transfer are significantly enhanced in microreactors because of the large surface-to-volume ratio [1–4]. These effects are expected to strongly influence different types of reactions, in particular high temperature gas phase reactions in the field of heterogeneous catalysis [5,6] and highly exothermic, fast two- or three-phase reactions. The extreme temperature conditions and the high reactivity of the process gases in case of heterogeneous catalysis or of the process liquids in case of gas/liquid reactions seriously limits the number of micromaterials to be applied to ceramics, stainless steel or special alloys. Ceramic components may be mainly applied as supports for catalysts as in large-scale reactors. Furthermore, their low thermal conductivity may be favourably used to prevent undesired heat transfer by creating insulating barriers between hot and cold miniaturized components in compact microreaction assemblies.

Fabrication and Interconnection of Ceramic Microreaction Systems for High Temperature Applications

18. Mehrphasenprozesse in Mikroreaktoren‐Konzept, Systeme und Charakterisierung

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