Microchannel filling/coating

Example 7.10. Prins et al. [306] used electrowetting to control fluid motion in microchannels. To do so, they coated aluminum electrodes first with a 12 pm thick layer of parylene and then with a 10 nm thick fluoropolymer film. The channels were 0.35 mm wide. Due to the hydrophobic polymer water does not flow into the capillaries. Only after applying voltages of typically 200 V did the capillaries fill with water. When switching the voltage off, the water flowed out of the capillaries again. 

In order to detect the analyte specifically, a complex has to be formed first. To this end, the revelation moiety (e.g. an enzyme-labelled antigen or antibody) is for instance incubated in the chip so as to bind to the analyte that has previously been captured within the microchannel. In another scheme, the analyte solution is first mixed with the revelation moiety, and the formed complex is then incubated in the chip in order to be captured on the bed of antibodies coating the walls of the micro-channel. After a washing step (to remove the excess affinity partner), the microchannel is filled with the substrate which shall thus react... 

An alternative to filling or coating with a catalyst layer the microcharmels, with the related problems of avoiding maldistribution, which leads to a broad residence time distribution (RTD), is to create the microchannels between the void space left from a close packing of parallel filaments or wires. This novel MSR concept has been applied for the oxidative steam reforming of methanol [173]. Thin linear metallic wires, with diameters in the millimeter range, were close packed and introduced into a macro tubular reactor. The catalyst layer was grown on the external surface of these wires by thermal treatment. 

Since then, various methods have been adopted for fabrication of photoresist-based microfluidic devices. The first method shown in Figure 20.9a begins with a spin coating of photoresist onto a substrate and patterning with a photomask." Once the open microchannels are created, a sacrificial material is filled into the space of the microchannel. Subsequently, a second layer of photoresist is spin coated and patterned on top to define the access holes for inlet and outlet. Finally, the sacrificial layer is dissolved to create the closed microchannels. The major disadvantage in this process is the slow dissolution, therefore only short microchannels are applicable. 

The fraction of intercalated species emerging as electroactive is less stably trapped inside the coating, being mainly localized in microchannels formed inside the film, filled with the electrolytic solution. The presence of these channels is also invoked to explain why neutral and anionic species, e.g., [Mo(CN)8]" and [Fe (CN)6] , can also diffuse into the cationic clay film [6, 21]. 

Additives to suppress particle agglomeration may be added to the suspension. This is crucial for low particle sizes [129]. Pfeifer et al. described a technique for wash-coating copper/zinc oxide catalysts onto aluminium microchannels [135], Copper oxide nanoparticles of 41 nm average particle size were mixed with zinc oxide nanoparticles of 77 nm average particle size either by wet mixing with aqueous hydroxy ethyl cellulose or hydroxy propyl cellulose in isopropyl alcohol. Alternatively, the particles were typically milled and then dispersed in aqueous hydroxy ethyl cellulose. The dispersion then filled in the microchannels, resulting in a catalyst layer of 20 pm thickness, which was then calcined in air at 450 °C. The surface area of the samples was around 20 m g . ... 

Figure 1.1 Different examples of spontaneous capillary flows (SCF) in open-surface microchannels (channels etched in sihcon and coated hy an SiO layer) (a) serial SCF (b) parallel SCF (c) parallel channels (d) winding channels crossing wells (e) filling of a cylindrical cavity (f) capillary filaments in a cylindrical well (g) capillary filaments in corners. Photographs by N. ViUard, D. Gosselin and J. Berthier (CEA-Leti).

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