Microfluidic Devices and Microflow Systems

We carry out synthesis in a 19th-century style - we have better glass, better analytical tools. But there hasn t been a real advance. 

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Stemming from the small sizes and high surface-to-volume ratio of the microstructures, microflow systems composed of microfluidic devices have the following characteristic features (Table 7.2) ... 

As demonstrated in this chapter, a number of microfluidic devices of various structures and sizes for extremely fast mixing, heat exchanging and residence time control have been developed based on conventional and modern fabrication technologies. Microflow systems composed of such microfluidic devices are expected to serve as powerful tools for conducting extremely fast, highly exothermic reactions in a highly controlled manner to effect flash chemistry, where desired products are formed within milliseconds to seconds. 

Liquid flow is incompressible, so, in micrometer-scale channels, the flow has a small Reynolds number (Re), usually less than 1, and the flow in simple microchannels is laminar, thus chaotic or turbulent flows are not observed [1]. Many types of microfluidic device have been developed on the basis of this flow behavior. Functional flow control methods based on laminar flow profiles have been proposed and applied in microflow devices and systems. Passive and active flow control methods and their applications are introduced in this section. 

To obtain exact fluid dynamic rules of multiphase microflows, constant interfacial tensions are highly preferred. This is easy to achieve when using purified gas/Hquid or Hquid/Hquid systems without any dissolved substances, such as surfactants or reactants. However, only a few dispersed systems can be operated stably in plastic microfluidic devices without additive, and the reason is the similar wetting properties of both phases on microcharmel wall. For example, water/alkanes are ideal systems (Bremond et al, 2008) for PDMS devices. Water/alcohols are infrequent combinations... 

FIGURE 2.5 Illustration of a SIA-LOV microflow network as assembled for in-valve sorptive preconcentration using renewable sorbent materials prior to further detection via peripheral analytical instruments. SP, syringe pump HC, holding coil. The inset at the right shows how the sorptive beads are retained within the column positions. (Reprinted with permission from Miro, M., and E. H. Hansen. 2007. Miniaturization of environmental chemical assays in flowing systems The lab-on-a-valve approach vis-a-vis lab-on-a-chip microfluidic devices. Anal. Chim. Acta 600 46-57.)... 

The design in this case-study prototype device remains an issue for the reversible motion of the liquid. This case-study prototype device could be effectively used for the one shot of liquid in both single-output and multi-output modes on demand. For continuous random switch function, another problem appears if we turn on microchannel X first and desire to next turn on only microchannel Y where X>T. In this continuous switch function, the microchaimel X -f 1) will be also turned on in this case-study prototype device. Anyway, above issues could be resolved via the two-way bubble actuator design [18, 19], which could make the fluid flow backward in the unwanted microchannel. In addition, this microfluidic switch has the potential to be integrated into a wider fluidic network system with stop valves or microflow discretizers which could be used for separation of liquid segments from a continuous source. 

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