Microflow techniques

Finally, and what may prove in the immediate near future to be the most important aspect of investigating the bloodstream with microflow techniques is the finding that many processes in the bloodstream are actually activated due to the flow itself. It has long been accepted that endothelium-derived NO is actually stimulated due to a shear stress placed upon the endothelium. However, it is also becoming increasingly clear that other events in the microcirculation are also stimulated due to the forces placed upon certain cells when flowing through open tubes (vessels). These same forces ean be applied to cells in an in vitro format in a microflow system and the resultant cellular response ean be quantitatively measured or monitored. In order to better understand the importance of flow for the induction of many cellular events in vivo, it is necessary to understand the properties of blood flow in vivo. 

Using advances in computer reconstruction methods (see e.g. Kikkinides and Burganos, 2000 Torquato, 2001) and past experience with discrete particle deposit simulations (Konstandopoulos, 2000), we have developed algorithmic as well as process-based reconstruction techniques to generate three-dimensional (3D) digital materials that are faithful representations of DPF microstructures. We refer to this approach as DPF microflow simulation (MicroFlowS). MicroFlowS is thus a short name for a computational approach, which combines... 

In most electroosmotic flows in microchannels, the flow rates are very small (e.g., 0.1 pL/min.) and the size of the microchannels is very small (e.g., 10 100 jm), it is extremely difficult to measure directly the flow rate or velocity of the electroosmotic flow in microchannels. To study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems [18-22]. In the microPIV technique, the fluid motion is inferred from the motion of sub-micron tracer particles. To eliminate the effect of Brownian motion, temporal or spatial averaging must be employed. Particle affinities for other particles, channel walls, and free surfaces must also be considered. In electrokinetic flows, the electrophoretic motion of the tracer particles (relative to the bulk flow) is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. 

An electrochemical microflow reactor, which is composed of diflone and stainless steel bodies produced by a mechanical manufacturing technique, is shown in Figure 7.24. The reactor consists of a two-compartment electrochemical cell, which is divided by a PTFE membrane. Carbon felt (7 mm x 7 mm x 5 mm) made of carbon fibers = 10 im) is used as the electrode. 

Many organic reactions suffer from the formation of significant amounts of polymeric by-products. Faster reactions than mixing might be responsible in many cases. To avoid such undesirable side reactions, slow addition and high dilution techniques are often used. The examples shown here, however, indicate that such reactions can be conducted much more easily and selectively using a microflow system without deceleration by slow addition or high dilution conditions. 

The advent and development of micromachining techniques, which enabled microflow based systems, has opened a potentially very important new area for development. The small size and potential for low cost means that new chemical production concepts such as point of use manufacturing could be realized. Microfabrication techniques also offer the potential to imbed analytical measurement within the flow path for close coupled control of the production process. Finally, new chemistries, which are difficult or just dangerous to practice in conventional equipment, are much more easily controlled in microscale equipment. However, given the conservative nature of the chemical processing industry this technology, like all new innovations, will have a slow introduction into mainstream manufacturing. 

In the present study, we investigated a series of systematically modified Y zeolites as catalysts for the vapour-phase nitration of benzene. The activity, selectivity and stability of the samples was measured in a microflow fixed bed integral reactor, at 443 K and atmospheric pressure. The influence of sodium and aluminum content, and of the textural properties on the catalytic performance was investigated. The effect of the different treatments on some physico-chemical properties of the catalysts was investigated by means of standard characterization techniques (SEM, AAS, XRD, low pressure Ar adsorption, 29Si MAS solid state NMR). 

Alum NR, White J (1998) A fast integral equation technique for analysis of microflow sensors based on drag force calculations. In Proceedings of the inter-natitmal conference on modeling and simulation of microsystems, semiconductors, sensors and actuators, Santa Clara, pp 283-286... 

With typical microchannel flow rates usually smaller than 10 pL/min, a microflow has to be capable of continuous and uniform perfusion of media as well as steady culturing conditions. The flow must have a distribution such that adherent cells are not exposed to signiflcant shear stress. Cells in suspension are assayed either while carried by bulk microflow or, more often, after immobilization in the chip. Common immobilization techniques in microchannels are hydrodynamic trapping [6, 7] and adsorbing cells to a chemically treated surface [8]. 

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