Microscale flow system

Kawaguchi T, Miyata H, Ataka K, Mae K, Yoshida J (2005) Room-Temperature Swern Oxidations by Using a Microscale Flow System. Angew Chem Int Ed 44 2413-2416... 

Figure 5.22 Schematic diagram of the microscale flow system for theSwernoxidation (bycourtesyofWiley-VCH VerlagCmbH) [58].

Kawaguchi, T., et al., Room-temperature swern oxidations by using a microscale flow system. Angewandte Chemie-Intemational Edition, 2005,44 2413-2416. 

Kawaguchi T., Minuata H., Ataka K., Mae K., Yoshida J. Room-temperature Swem oxidations by using a microscale flow system. Angew. Chem. Int. Ed. 2005 44(16) 2413-2416. 

The remaining chapters in this book are organized as follows. Chapter 2 provides a brief introduction to the mesoscale description of polydisperse systems. There, the mathematical definition of a number-density function (NDF) formulated in terms of different choices for the internal coordinates is described, followed by an introduction to population-balance equations (PBE) in their various forms. Chapter 2 concludes with a short discussion on the differences between the moment-transport equations associated with the PBE and those arising due to ensemble averaging in turbulence theory. This difference is very important, and the reader should keep in mind that at the mesoscale level the microscale turbulence appears in the form of correlations for fluid drag, mass transfer, etc., and thus the mesoscale models can have non-turbulent solutions even when the microscale flow is turbulent (i.e. turbulent wakes behind individual particles). Thus, when dealing with turbulence models for mesoscale flows, a separate ensemble-averaging procedure must be applied to the moment-transport equations of the PBE (or to the PBE itself). In this book, we are primarily... 

Microscale fluidic systems use small volumes so sensitivity of detection can be a challenge. Any detector for chip-based LC needs to be small and ideally have low power consumption. It is generally a problem of interfacing. Flow cell geometry is also a big factor, e.g. a U cell instead of linear flow cell can give a ten-fold increase in sensitivity for absorbance measurements. Electrochemical detection is very common, mainly ampero-metric and potentiometric, and very amenable to detection on chip. Fluorescence is more sensitive than UV-Vis absorbance and chemiluminescence is sensitive down to a single molecule, similar to LIF. 

Young EWK, Simmons CA (2010) Macro- and microscale fluid flow systems for endothelial cell biology. Lab Chip 10(2) 143-160... 

Typically in fluid mechanics, a wealth of information about any given flow system can be fotmd from the velocity field, which is often visualized and quantified through the use of tracer particles. As mentioned above, micro-PIV and micro-PTV are both well-established tools for extracting quantitative information from microscale fluid systems [3], However, special care must be taken in applying these techniques to very nearwall flows with evanescent wave illumination. Both techniques require imaging the instantaneous positimis of tracer particles seeded in the flow at two different instances in time to infer fluid velocities. 

Visualization has played a central role in the development of the field of microfluidics and associated applications. It is both a natural desire and highly informative to see what is going on inside these small-scale systems. The inherent scale of these systems results in significant deviations from macroscale fluid behavior, most notably due to an increase in the role of surface effects and diffusive transport of mass, momentum, and energy. Unique micro- and nanoscale transport phenomena present many opportunities for advanced functionality as exploited by the many applications described in this encyclopedia and elsewhere. The scale of these systems, however, also presents some unique challenges with respect to visualization. Most notably, on the microscale the nonintrusive requirement precludes the use of many techniques commonly applied to macroscale flows, such as hot-wire anemometry and the injection of dye by mechanical means. In addition, it is not possible to observe the phenomena without the use of a microscope. Several visualization techniques have, however, been successfully applied to microscale flows. Like much of the research in microfluidics and nanofluidics, many proof-of-concept contributions have appeared in the 1990s, and subsequent advancement has been rapid. Most visualization techniques applied to microstructures may be conveniently divided into two categories particle-based methods and scalar-based methods [1]. 

Microscale flow visualization has become an important tool for characterizing the performance of microchannels, micromixers and microreactors. Due to the rapid improvement in computer power and the great efforts in optical instrumentation, new measuring systems with high spatial and temporal resolution are available even for smaller companies and research institutes. In connection with the recent success in numerical simulation of microscale flow, a signiflcant acceleration in clariflcation of microscale phenomena and technical development can be expected. By ensuring the development of reliable systems, microscale flow visualization will provide an important contribution to the further spread of micro process engineering applications in the chemical and biochemical industries. 

Within the specific context of micro- and nanofluidics, the fabrication of self-assembled nanostrucures can represent an enabling technology for novel engineering applications. One such application - and the specific motivation for this article - lies in the creation of catalytic nanostructures for micro-Znanoscale reacting flow systems. A specific example is the catalyzed chemical decomposition of monopropellant fuels for the purposes of small satellite micropropulsion [1] (Fig. 2). Here, the size and density of nanorod formations naturally provide the high surface area-to-volume ratios desirable for efficient catalysis in microscale geometries. Certainly other important appli-... 

In traditional DSMC simulations of supersonic flows, the Dirichlet type of velocity boundary conditions has generally been used. This approach is often applied in external-flow simulations, which require the downstream boundary to be far away from the base region. However, the flows in microscale systems are often suhsonic flows, and the boundary conditions which can he obtained from the experiment always refer to pressure and temperature, instead of velocity and number density. Wang and Li [5] have proposed a new implicit treatment for a pressure boundary condition, inspired by the characteristic theory of low-speed microscale flows. This new implementation of boundary conditions not only overcomes the instability of particle-based approaches, hut also has a higher efficiency than any other existing methods. The new method is easy to extend to gas flows where the downstream and upstream directions are not opposite, such as in L-shaped and T-shaped channels. 

A variety of microscale separation methods, performed in capillary format, employ a pool of techniqnes based on the differential migration velocities of analytes under the action of an electric field, which is referred to as capillary electromigration techniques. These separation techniques may depend on electrophoresis, the transport of charged species through a medium by an applied electric field, or may rely on electrically driven mobile phases to provide a true chromatographic separation system. Therefore, the electric field may either cause the separation mechanism or just promote the flow of a solution throughout the capillary tube, in which the separation takes place, or both. 

A direct injection nebuliser (DIN) was used to interface LC with ICP-MS (Shum et al., 1992a). The DIN transferred all of the sample into the inductively coupled plasma. Microscale LC separations in small packed columns were studied because the column flow rates of about 30 ml min 1 were compatible with the DIN. The low dead volume (less than 1 ml) of the interface prevented excessive band broadening. Eluents containing up to 85% methanol were accommodated. The analyte signal varied by about 20% as the eluent changed from 20% to 80% methanol in water. Detection limits for arsenic and tin species using the HPLC-DIN-ICP-MS system were 0.2-0.6 and 8-10pg, respectively. 

---