Microchannels downstream

The characteristics of electrokinetically controlled fluid flow in microchannel manifolds has been studied in a systematic way by Harrison and coworkers [28, 30]. An illustrative demonstration of the potential of this approach is shown in Fig. 2 for the controlled dilution of a fluorescein solution under voltage control. In parallel with a stepwise decrease of the potential applied to the fluorescein reservoir, a decrease of fluorescence signal downstream after the junction is visible in Fig. 2. As long as the ionic strength and pH in each supply channel is the same (same jieo), mass balance is automatically fulfilled, and the incoming flows at the intersection will be exactly balanced by the outgoing flow of the mixed components (otherwise, mass balance would be enforced by additional hydrodynamic or secondary internal flows). This way of mixing fluids was also... 

Multiphase microflows are dominated by pressures (Aota et al., 2007a, 2009a). One important parameter needed to describe the multiphase microflows is the pressure that drives the fluids. The pressure decreases in the downstream part of the flow because of the fluids viscosity. When two fluids in contact with one another have different viscosities, the pressure difference (APfiow) between the two phases is a function of the contact length and the flow velocity. Another important parameter is the Laplace pressure (APLapiace) caused by the interfacial tension between two phases. The position of the interface is fixed within a point in the microchannel by the balance established between the APLaplace and APFlow. 

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 the latter (washing) area, the m-xylene phase containing the Co chelates and the coexisting metal chelates from the former (reaction and extraction) area is interposed between the HC1 and NaOH solutions, which were introduced through the other two inlets at a constant flow rate. Then a three-phase flow, HCl/m-xylene/NaOH, forms in the microchannel. The decomposition and removal of the coexisting metal chelates proceed along the microchannel in a similar manner as described above. Finally, the target chelates in m-xylene are detected downstream by TLM. 

For the reported experiments, the heat transfer coefficient h is representative of the entire length of the microchannels, calculated either at the downstream end of the microcharmels, or based on the bulk mean wall-fluid temperature difference over the entire length of the microchannels. 

Instead of the optical labels, enzymes are also used as a labeling material [7]. ELISA is one of the most popular techniques in the conventional immunoassay and suitable in the microchip format. By using a fluorogenic reaction, the fluorescent signal can be taken at the downstream microchannel without interruption by microbeads. In the case of chromogenic reaction, a TLM is useful [8]. 

FIGURE 38.17 Plot of the width of the sample plug (10 nM Alexa Fluor 488) focused at the interface of LE (750 mM Tris-Hcl) and TE (25 mM Tris-Hepes) at various locations downstream in a 50 jjim wide (20 ixm deep) microchannel. Each data set represents five realizations of a constant current FTP experiment. 

FIGURE 38.19 Two 100 aM sample electropherograms of ITP/CE separation of Alexa Fluor 488 (the peaks near 73.5 s) and Bodipy (peaks near 76.5 s). A glass microchip (microchannel cross-sectional dimensions are 50 xm wide and 20 p.m deep) and 60x water immersion objective (N.A. = 0.9) were used. The detector was located 30 mm downstream of the injection region. 

Characterization of Microfiuidic Devices Using Microparticies, Fig. 3 (a) Fluorescent images at four downstream positions illustrating ordering of 7.32 pm-diameter microparticles at = 120 in a high-aspect-ratio microchannel (50 pm x 100 pm), (b) Fluorescent intensity line scans at four downstream positions (7.32 pm-diameter particles), (c) Measurement of FWHM as a function of downstream position for four... 

The gradient evolves as the fluid flows downstream, such that the cells in the microchannel do not all experience the same concentration gradient. 

Heat-Transfer-Detection-Based Flow Sensors These thermal-anemometer-based flow sensors can sense very low flows in microchannels. The measurement principle is based on the thermal time of flight. The length of the heating pulse and the time of flight used in the measurement are measured in milliseconds. An example of the structure of a flow sensor is shown in Fig. 5 [1]. The structure consists of a heater in the middle, with an upstream and a downstream temperature sensor integrated into the wall of the channel. When there is no flow in the channel, heat diffuses into the two temperature sensor regions and no differential temperature is detected. An increase in the flow rate in the channel favors convection of heated fluid in the direction of the flow, and the differential temperature detected by the sensors increases. 

The micro-PIV measurement reported in this experiment was carried out with a 4x objective lens. With a CCD sensor size of 6.3 x 4.8 mm, the size of an image pixel is 2.475 pm and the size of the measured area is 1584 x 1188 pm. Fluorescent particles with a diameter of 3 pm were used to trace the flow. A microchannel with the cross section of 910 x 50 pm and the length of 5 mm was used. The liquids used in the experiment were the aqueous NaCl solution (concentration 10 " M) and aqueous glycerol (volume concentration 24 %). The integration area is 32 x 32 pixels. Previous studies showed that the entry length of liquid flow in microchannel was very short [11]. The measurement was taken at 1 mm downstream of the entrance thus stable velocity field was obtained. 

In general, for diagnostic applications, the requirements for blood cell removal are extremely onerous, i.e., a 100 % cell removal efficacy. Even a few cells (out of the millions present in a typical sample) have the ability to clog up microchannels and adversely affect sensitive biosensors downstream. The sensitivity of the biosensor and the concentration ranges of the target biomarkers also determine how much the original solution may be diluted. For... 

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