Microchannel cross-section

Unlike capillary electrophoresis, wherein absorbance detection is probably the most commonly utilized technique, absorbance detection on lab-on-a-chip devices has seen only a handful of applications. This can be attributed to the extremely small microchannel depths evident on microchip devices, which are typically on the order of 10 pm. These extremely small channel depths result in absorbance pathlengths that seriously limit the sensitivity of absorbance-based techniques. The Collins group has shown, however, that by capitalizing on low conductivity non-aqueous buffer systems, microchannel depths can be increased to as much as 100 pm without seeing detrimental Joule heating effects that would otherwise compromise separation efficiencies in such a large cross-sectional microchannel [38],... 

Xu, B., Cai, W., Liu, X., Zhang, X. (2013). Mass transfer behavior of liquid-liquid slug flow in circular cross-section microchannel. Chemical Engineering Research and Design, 91, 1203-1211. 

Li and Olsen used microPIV to measure velocity fields in microchannels for Reynolds numbers ranging from 272 to close to 3,000 in two sets of experiments. In the first set of experiments [9], Li and Olsen measured velocity fields in square cross section microchannels with hydraulic diameters ranging from 200 to 640 pm, and in the second set of experiments [10], they repeated these measurements for microchannels with hydraulic diameters equal to 480 pm and aspect ratios ranging from 0.97 to 5.69. In both sets of experiments, they observed transition in the same range of Reynolds numbers as Sharp and Adrian, indicating transitional behavior in agreement with classical macroscale results. 

Estimate the asymptotic Sh for mass transfer with chemical reaction for a circular cross section microchannel with the diameter of 500 pm for Ball equal to 0.1,1, and 100. Also, estimate the mass transfer coefficient for a gas-phase system with molecular diffusivity (B ) of 10" m s . For circular tube Sh = 3.66 and Sh = 4.36. 

The parallel flow can also be formed at low flow velocity, especially in rectangular cross section microchannel, if the interfacial tension between two fluids is very low, for example, wedge-shaped parallel flow contactor [38], or modifying the wettability of the channel walls (see Figure 7.28). The flow could be stabilized by placing membranes or sieve plates inside the channels [9, 39]. 

J. Berthier, K.A. Brakke and E. Berthier, A general condition for spontaneous capillary flow in uniform cross-section microchannels. Microfluid Nanofluid. 16, 779-785 (2014). 

The data on pressure drop in irregular channels are presented by Shah and London (1978) and White (1994). Analytical solutions for the drag in micro-channels with a wide variety of shapes of the duct cross-section were obtained by Ma and Peterson (1997). Numerical values of the Poiseuille number for irregular microchannels are tabulated by Sharp et al. (2001). It is possible to formulate the general features of Poiseuille flow as follows ... 

The test module consisted of inlet and outlet manifolds that were jointed to the test chip (Fig. 6.20). The tested chip with heater is shown in Fig. 6.21. It was made from a square shape 15 x 15mm and 0.5 mm thick silicon wafer, which was later bonded to a 0.53 mm thick Pyrex cover. On one side of the silicon wafer 26 microchannels were etched, with triangular shaped cross-sections, with a base of 0.21 mm... 

The surface tension is of great importance when dealing with bubbles and particulate contaminations in microchannels and determining how strong the capillary forces are in a microchannel. For a cylindrical cross-section, the capillary force, Fcap, can be expressed quantitatively as shown in the following equation,... 

FIGURE 11.32 Flow profiles in microchannels, (a) A pressure gradient, - AP, along a channel generates a parabolic or Poiseuille flow profile in the channel. The velocity of the flow varies across the entire cross-sectional area of the channel. On the right is an experimental measurement of the distortion of a volume of fluid in a Poiseuille flow. The frames show the state of the volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule, (b) In electroosmotic flow in a channel, motion is induced by an applied electric field E. The flow speed only varies within the so-called Debye screening layer, of thickness D. On the right is an experimental measurement of the distortion of a volume of fluid in an electroosmotic flow. The frames show the state of the fluorescent volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule [165], Source http //www.niherst.gov.tt/scipop/sci-bits/microfluidics.htm (see Plate 12 for color version). 

Figure 8. Cross section of a microchannel with detailed left and right sides and bottom of the microchannel with a thin film layer of Pt/Al203 catalyst. (The researchers wish to express their gratitude to DARPA for funding Grant N66001-02-1-8942.)...

Three types of measurements were performed in this study. First, photodissociation cross sections were measured, in which the total photofragment yield was measured as a function of dissociation photon energy. In these experiments, the electron signal generated by the microchannel plates is collected with a flat metal anode, so that only the total charge per laser pulse is measured. The beam block is 3 mm wide for these measurements. 

The Nernst-Einstein relationship plotted in Fig. 36.1 motivated the development of DiagnoSwiss technology, namely reducing the analysis time and reaction volumes by replacing the commonly used microtitre plate wells with typical dimensions of 100 pL volume and 1 cm2 surface area by a microchannel with a volume of 60 nL and a surface area of 0.03 cm2. The microchannel is an isotropically etched microstructure (see Section 36.2) with a minimal cross-section dimension of 40 pm,... 

Figure 36.2. Left cross section of a microchannel with an inlet port, a channel, an outlet port and integrated gold electrodes. Right a cartridge containing eight parallel microchannels, embedded in a white polymer support, serving as rigidifier, guide and reservoir.

Novel microreactors with immobilized enzymes were fabricated using both silicon and polymer-based microfabrication techniques. The effectiveness of these reactors was examined along with their behavior over time. Urease enzyme was successfully incorporated into microchannels of a polymeric matrix of polydimethylsiloxane and through layer-bylayer self-assembly techniques onto silicon. The fabricated microchannels had cross-sectional dimensions ranging from tens to hundreds of micrometers in width and height. The experimental results for continuous-flow microreactors are reported for the conversion of urea to ammonia by urease enzyme. Urea conversions of >90% were observed. 

Fig. 2. A B SEMs of test patterns of silver fabricated using pCP, followed by selective etching C cross-sectional SEM of microchannels etched in Si(100) and D optical micrograph of a microtransformer fabricated using pCP, selective etching and electroplating...

In contrast, microfabricated microchannels in general exhibit more or less rectangular shaped cross sections depending on the fabrication process. The advantage of rectangular channels with respect to heat dissipation are obvious (even better surface-to-volume ratio) [37,42]. 

FIGURE 2.20 SEM representation of the cross section of a PET microchannel made by photoablation. The trapezoidal form is typical for this type of microfabrication procedure. The lamination on the top is evidenced by the interface between the polymer layers. The channel dimensions are 30 pm wide in the bottom, 40 pm wide in the top, and 40 pm deep [191]. Reprinted with permission from Elsevier Science. 

An infrared C02 laser (1060 nm) was used to cut through a polycarbonate PC (black) (carbon-coated) wafer of 250-pm thickness to create microfluidic channels. The laser-machined black PC wafer was then thermally bonded between two transparent PC wafers at 139°C under two tons of pressure for 45 min [937], A C02 laser was also used to ablate Mylar sheets (with adhesive). Then the machined Mylar sheets were laminated together [1051]. Moreover, a PMMA substrate was machined by a C02 laser [201,202]. However, the microchannel ( 200 pm deep) has a Gaussian-like cross section and a certain degree of surface roughness [201], The C02 laser has also been used to machine PET substrates [203],... 

The temperature gradient can also be achieved at the junction of two microchannels of two different cross-sectional areas in the presence of an electric field. Since there is a higher current density in the narrower channel than in the wider channel, it is hotter in the narrower channel [597]. 

FIGURE 7.7 Cross-sectional view of a microchemical chip integrated with some optical components, (a) The microchemical chip showing the microlens (entrance and exit), the 3000-A Cr aperture layers (entrance and exit), and microchannel (25 pm deep) (b) the microchemical chip together with the pinhole (800 pm) and interference filter for detection by the photodetector [688]. Reprinted with permission from the American Chemical Society. 

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