Microchannels liquid interface

To further stabilize multiphase flow, the liquid-liquid interface can be created at the boundary formed at a constricted opening [427]. Moreover, guide structures (5 pm high) were fabricated in a microchannel (20 pm deep) to stabilize a three-phase flow, as shown in Figure 3.16. These structures were etched on quartz using... 

Hibara, A., Nonaka, M., Hisamoto, H., Uchiyama, K., Kikutani, Y., Tokeshi, M., Kitamori, T., Stabilization of liquid interface and control of two-phase confluence and separation in glass microchips by utilizing octadecylsilane modification of microchannels. Anal. Chem. 2002, 74(7), 1724-1728. 

When APFiow exceeds the maximum APLapiace, the organic phase flows toward the aqueous phase (Figure 13b). When APFiow is lower than the minimum APLaPiace/ the aqueous phase flows toward the organic phase (Figure 13c). When the flow rate ratio is changed, the pressure balance is maintained by changing the position of the liquid-liquid interface. This model indicates that the important parameters for microfluid control are the interfacial tension, the dynamic contact angle, and the depth of the microchannel. This model can also be applied to gas-liquid microflows. 

A similar distinction between a system with pre-electrolysis with only one electrode (in this case anodic) process, and a system with simultaneous anodic and cathodic processes (in which anode and cathode are on opposite walls of a microchannel so that each liquid is only in contact with the desired electrode potential, analogous to the fuel cell configurations discussed above) was made by Horii et al. (2008) in their work on the in situ generation of carbocations for nucleophilic reactions. The carbocation is formed at the anode, and the reaction with the nucleophile is either downstream (in the pre-electrolysis case) or after diffusion across the liquid-liquid interface (in the case with both electrodes present at opposite walls). The concept was used for the anodic substitution of cyclic carbamates with allyltrimethylsilane, with moderate to good conversion yields without the need for low-temperature conditions. The advantages of the approach as claimed by the authors are efficient nucleophilic reactions in a single-pass operation, selective oxidation of substrates without oxidation of nucleophile, stabilization of cationic intermediates at ambient temperatures, by the use of ionic liquids as reaction media, and effective trapping of unstable cationic intermediates with a nucleophile. 

In addition to the inherently safe features of microchannel reactors, they allow rapid catalyst testing and have the option of the same catalyst type use, for example, washcoated, throughout the whole development cycle, up to pilot and production. Furthermore, mass transfer is improved through the increase in gas-liquid interfaces, and thermal control is also better due to larger exchange surfaces. 

The development and application of chip-based solvent extraction is gradually expanding as shown in this chapter. With methods for surface modification, the liquid-liquid interface in a microchannel can be stabilized, and so special techniques for solvent extraction are not required. Therefore, since solvent extraction is one of the basic chemical processes, it is expected that it will be used more often in the future. 

Hisamoto, H., et al., Fast and high conversion phase-transfer synthesis exploiting the liquid-liquid interface formed in a microchannel chip. Chemical Communications, 2001, 2662-2663. 

Mixing in microreactors is almost exclusively assumed to be laminar dne to the small flow channel width. Laminar flow through microchannels reqnires the phase flow to be alternated in some fashion in order to create the mixing environment. This operation is important because mass transfer, in this case, is driven only by molecular diffusion. Hence, the creation of larger gas-liquid interfaces is the only practical course of action for gas-limited operations. Miniaturized bubble columns are able to accomplish this task well. For example, a standard reactor can create... 

EOF is generated when an external electrical field is applied to the liquid in a microchannel tangentially, due to the presence of net charges within a thin layer near the solid-liquid interface. This thin layer is called the electrical double layer (EDL), and the net charges in the EDL are the result of charge separation in the bulk liquid induced by the surface charge, which originates... 

Liquid-Liquid Stratified Flow in Mkrochannels, Fig. 1 Schematic of liquid-liquid interfaces in an H-shaped microchannel, (a) the laminar fluid diffusion... 

A microreaction system was developed for the carbonylation of nitrobenzene as well [28]. Under lower CO gas pressure [9.5 bar much lower than those in conventional ones (>100 bar)], phenylisocyanate was produced. A gas-liquid slug flow of the reactant mixture was formed in the microchannel for efficient mass transfer across the gas-liquid interfaces. The isocyanate yield of the microflow reaction was shown to be three to six times higher than that of the batch reaction, depending on the inner diameter (i.d.) of the microtube. A higher isocyanate yield was obtained in a narrow-bore tube (0.5 mm i.d.) than in a wide-bore tube (1.0 mm i.d.). The catalyst they applied was Pd(py)2Cl2 and pyridine system. 

Shi et al. demonstrated another optofluidic microlens utilizing active pressure control of an air-liquid interface [19]. The working mechanism of this microlens is shown in Figure 7.18. DI water was introduced into a straight microchannel. The microchannel made a T-junction with an air reservoir. As the water flowed past the T-junction, air was trapped in the reservoir and a movable air-water interface formed at the T-junction, with the air bending into the water due to the air-liquid contact angle and the hydrophobic-hydrophilic interaction between the surface and the liquid. The air-water interface acted as a divergent lens. 

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