Dimensional Control in Microlamination

Microlamination has the capacity to fabricate metal, polymer and ceramic devices with high aspect ratios in large production volumes. This has been demonstrated in industry where microlamination has been used to mass produce ink-jet print heads. Development of microlamination methods for production of MECS devices has required research into new lamina patterning processes, bonding techniques, and registration methods.

14.4.1 Effects of Patterning on Microchannel Array Performance

As suggested, microchannel devices are well known for their superior performance in heat and mass transport applications. Much of this is due to the large surface area to volume ratios made possible by the high-aspect-ratio microchannels (HARM) within these devices. In order for these HARM devices to be efficient, pressure drop through these devices must be minimised. Past research has shown that as the channel size decreases within microchannels, the fluid flow behavior within the laminar flow regime of the channel begins to deviate from the traditional Navier-Stokes theory (Pfahler et al. 1990; Rahman and Gui 1993; Zemel et al. 1984). Wu and Little (1983) and Peng et al. (1994) both found that the wall effect in microchannels had a significant impact on the laminar flow behavior within the microchannels. Parameters given for influencing the wall effect included the method used to form microchannel walls. Typically, in metal microlamination, the device is formed from cold-rolled shim stock, which possesses a very smooth surface finish. Therefore, sidewall geometries in metal microlaminated structures are not expected to play a large role in the microfluidic wall effect. A microchannel, however, (Fig. 14.3) formed by microlamination may have significant endwall surface roughness depending upon the method of machining used during the lamina-patterning step. Many techniques have been used for lamina patterning including laser micromachining, photochemical etching, and electrochemical machining. Laser micromachining has been used to form microchannels for microlamination because of its ability to quickly adapt to complex geometry. The endwalls formed by the laser micromachining process are known to be rough. Photochemical etching has been used in microlamination due to its relatively low cost when compared with laser machining.

Matson et al. (1997) suggested the use of photochemical etching to pattern highly complex stainless steel shims to fabricate laminated microchannel chemical reactors with a relatively low cost per shim (less than US$1). However, in photochemical etching processes, the costs of waste treatment and disposal can be greater than the processing costs (Datta and Harris 1997). An alternative process with fewer waste disposal problems is electrochemical micromachining (EMM). EMM is a well-established technique that has been used in the electronics industry to machine thick and thin films. This method is known to provide a very smooth endwall surface. It is expected that for microlaminated channels there exists an aspect ratio limit at which the method of lamina patterning no longer significantly influences the microchannel pressure drop. Knowledge of this limit would be beneficial for device designers of cross-flow and counter-flow microchannel devices where maximum aspect ratio limits are known to exist. Efforts to quantify the end wall effect in microlamination have been reported in Paul et al. (1999). In this study, stainless steel microchannels with various aspect ratios were fabricated with machining techniques resulting in both good (EMM) and poor (deep UV laser micromachining) surface finish. Pressure drop was then tested across the microchannels. The results were compared to see whether there is a difference in the pressure drop performance of the microchannels due to these two machining methods and at what aspect ratio the difference becomes insignificant.

Fig. 14.3. Schematic defining terms associated with a microlaminated geometry

14.4.2 Theory

For fluid flow in a channel, it is well known that the friction factor is not only a function of Reynolds number, but it also depends on the shape of the channel cross section. With the same flow area, the friction factors are different if the channel cross sections are different. Surface roughness of the channel is another important factor for flow in the microchannels because of large relative roughness although the effect is negligible in macro scale fluid (Wu and Little 1983; White 1994). In this experiment, two different shapes of the channel cross-section were considered—rectangular and trapezoid. The difference in the shapes of cross section is caused by different machining methods used in the experiment. The EMM process provides a trapezoidal cross section with smooth end walls whereas laser machining gives a rectangular cross section with rougher end walls. In this study, the range of the volumetric flow rate Q for the microchannel test was set and the pressure drop Ap across the test module was measured. The test module was assumed to have a specific cross-sectional area A and a channel length L. The results are presented in form as a friction factor f and hydraulic diameter Dh. The friction factor is defined as:

Where, p is the density of liquid and the hydraulic diameter Dh can be calculated from:

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