Microfluidic flow chemistry

The reasons to perform electrochemistry, in particular, electrosynthesis, in a microfluidic system are the following (Rode et al., 2009) (1) reduction of ohmic resistance in the electrochemical cell, by decreasing the distance between anode and cathode, (2) enhancement of mass transport by increase of electrode surface to cell volume ratio, also realized by small interelectrode gaps, (3) performing flow chemistry to establish single-pass conversion, and (4) coupling of cathode and anode processes, permitting simultaneous formation of products at both electrodes. The latter... 

A fifth reason for using microfluidics in electrochemistry would be the possibility to combine flow chemistry with an ultrafast mixer, which allows the generation and subsequent use of short-lived reactive ions or radicals, for example, in a "cation flow" process (Suga et al., 2001 Yoshida, 2008). Finally, a sixth reason for performing electrochemistry in a microfluidic system may be the desire to efficiently remove reaction heat (or joule heat due to high currents in combination with a high ohmic resistance) in fast electrochemical reactions (Yoshida, 2008). 

Figure 9. Formation of droplets in microfluidic flow-focusing devices. The micrographs on the left illustrate the process of formation of aqueous drops in an organic continuous fluid [S. Makulska, P. Garstecki, Institute of Physical Chemistry PAS]. The chart on the right shows the dependence of the volume of liquid droplets formed in a planar flow focusing device on the value of the capillary number (Adapted from Ref [24]).

Fig. 25.11 Schematics depicting two mechanisms of droplet foimation in a microfluidic flow focusing device (FFD) (Reprinted from [43]. With permission. Copyright 2007 the Royal Society of Chemistry)...

Utilization of flow reactors can overcome many of the problems associated with traditional batch reactors. Such reactors often have veiy short path lengths and may have better control of temperature. As demonstrated in many flow chemistry applications, scale-up is only limited by time. Over the past decade there have been many developments in the technology for photochemical reactions in flow, ranging from complex microfluidic devices to simple tubing based reactors, all of which have advantages and disadvantages." ... 

Since microreactor technology was first seen as an effective method for the synthesis of chemical compounds, enormous advances have been made in this area. The examples discussed in this chapter and many other illustrations in the Hterature, prove the potential of flow chemistry in chemical and phamiaceutical production and confirm the expected benefits and the intensification of chemical processes. The above-mentioned flow processes furthermore illustrate the flexibiHty of microfluidic devices, as flow chemistry allows the Hnking of individual reactions into multi-step reactions as well as preparing a series of analogues by simple modifications. A variety of technical approaches can additionally be considered for the implementation of flow processes, such as the automated and real-time in-line analysis and... 

The Heck-Matsuda reaction has also been subjected to continuous flow chemistry conditions, as reported by Wirth et al. [100]. The reaction that was studied involved the reaction between an in situ generated diazonium ion and/r-fluorostyrene to give ( )-/ -fluorostilbene (Scheme 1.35) and the device used by this group relied on a segmented flow (liquid-liquid slug flow) to increase reaction rates in microfluidic flow. They used perfluorodecalin as an inert and immiscible liquid spacer. 

Boleininger, J., Kurz, A., Reuss, V. and Sonnichsen, C. (2006) Microfluidic continuous flow synthesis of rod-shaped gold and silver nanocrystals. Physical Chemistry Chemical Physics, 8, 3824-3827. 

Thus the study of surfaces has emerged as an important focus in the chemical sciences, and the relationship between surfaces of small systems and their performance has emerged as a major technological issue. Flow in microfluidic systems—for example, in micromechanical systems with potential problems of stiction (sticking and adhesion) and for chemistry on gene chips—depends on the properties of system surfaces. Complex heterogeneous phases with high surface areas—suspensions of colloids and liquid crystals—have developed substantial... 

One way to ease any difficulties that may arise in fabricating a membrane, especially in design configurations that are not planar, is to go membraneless. Recent reports take advantage of the laminar flow innate to microfluidic reactors ° to develop membraneless fuel cells. The potential of the fuel cell is established at the boundary between parallel (channel) flows of the two fluids customarily compartmentalized in the fuel cell as fuel (anolyte) and oxidant (catholyte). Adapting prior redox fuel cell chemistry using a catholyte of V /V and an anolyte of Ferrigno et al. obtained 35 mA cmr at... 

FIGURE 9.16 Principle and procedure of the sequence-specific DNA detection in the microfluidic system under laminar flow conditions. The labeled DNA was indicated by a circular dot. Fluorescence of the duplex was measured at B, and that of labeled target was measured at A [1177]. Reprinted with permission from the Royal Society of Chemistry. 

The field of electrokinetic phenomena has had a long history. For over a century, it has been recognized that ions in solution move in the presence of electric fields. It is also well known that bulk flow can occur when electric fields are applied and when the surfaces containing the fluid are charged (electroosmosis). These two phenomena, ion and bulk flow movement in the presence of electric fields, and their applications in both analytical chemistry and microfluidics are covered in depth in this book. 

Notably, two classes of microreactors exist, referring to applications in analysis, especially in the field of biochemistry and biology (e.g., continuous flow microfluidic devices, micro total analysis systems - (xTAS, etc.) or chemical engineering and chemistry. Although these fields are distinct, there are clear overlaps and common areas of development. 

The development of truly versatile chips for analytical, synthetic and biological chemistry will require not only the understanding of the flow in networks, but also the development of modules for active control over the flow, merging, splitting and above all, formation of droplets. Below we review the field of active control over formation of droplets and provide an example of a microfluidic droplet-on-demand system. 

Figure 3. A microscope image of microfluidic cell culture chips having (A) channels (type 1) (Reprinted with permission from Ref [5], copyright 2005 The Royal Society of Chemistry) and (B) chambers (type 2) (Reprinted with permission from Ref [6], copyright 2006 American Chemical Society). A schematic view and finite element simulation of the vertical flow profile (velocity field) of type 1 (C and E) and type 2 (D and F).

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