Flow Measurements in Microchannels Using a MicroPIV system

High quality PIV experiments in micro-channel flow geometries to get detailed and accurate flow measurements can be difficult and involves paying attention to a variety of aspects. In the experiments, the reasons could be ranging from uncontrolled transients in the fluidic flows systems to fouling of the microchannels by impurities, bubbles, the seed particles themselves. Another factor which could have a significant effect is the distortion of the boundary of the channel caused by the difference in the refractive indices. Experimental measurements were taken on two channels in a PDMS (polydimethylsiloxane) micro-channel test facility shown in Figure 1. The two microchannels under study were (a) the expansion channel with fully developed flow in a 150 μm wide rectangular section to a 750 μm wide channel, and (b) a ‘T’ junction with two 100 μm wide outflow channels branch at two 90 degree elbow turns from a 100 μm inflow channel. Both micro-channels are 100 μm deep across the entire channel length. The measurements were performed for Reynolds number of 1, based on the centerline velocity of the channels. The experiments were performed using a PIV system equipped with an inverted microscope. Fluorescent particles of about 1 μm in size were employed as seed particles. Typical particle number concentration was around 2 to 3 particles per interrogation volume. Hence, the ensemble average of the correlation function was used to obtain the velocity vectors. Channel geometry imperfections, fouling and bubble formation affect the flow development and the nature of the flow, especially near the wall. Introduction Flows in micro-channels are of interest in a wide range of areas such as aerodynamics, fluid mixing, propulsion, microsprays, chemical and biological analysis, bio-MEMS applications and bio-fluid mechanics [1]. The high surface-to-volume ratio and the small volume of flow associated with microchannels provide unique advantages in the area of transport, especially heat transfer. Generally, micro-channels are defined to be smaller than 1mm, and for measurement purposes, more than tens of microns. Fabrication of the channels is a complex process that can have strong impact on the surface texture, cross sectional geometry and hence the affect the velocity field, especially, close to the wall boundaries. In microflows, unlike in ordinary flows, the entire flow field is influenced by the presence of wall boundaries. Hence, the entire flow field could be affected by the imperfections in the channels caused during the manufacturing process. Various types of fabrication techniques are used to obtain different desired microchannel (cross section) properties [2]. Lithography is one of the most common techniques for fabricating microchannels. Although the channels could be made out of different polymeric materials, PDMS (polydimethylsilooxane) is often chosen for the many properties it exhibits that are suitable for flow measurement applications. Transparency to laser light illumination, mechanical durability, good surface chemistry and stability against humidity and temperature are among them. A rapid prototyping method based on lithography is used to fabricate the microchannels [3 5]. The microchannel flow model used here is a test bed that contains multiple flow elements or channels of different geometries. Velocity measurements are carried out in two different channels in the test bed. 15 Australasian Fluid Mechanics Conference The University of Sydney, Sydney, Australia 13-17 December 2004 Figure 1: PDMS Microchannel test bed Experimental details The microchannel flow model used here for the experiments is a Model 101-100P test bed. The details of the construction of the Model 101-100P PDMS test facility are as follows. The mask for photolithography containing a negative image of the micro-channel device is created in a CAD program then printed on a transparency by a high-resolution printer. The mold is made from a silicon wafer coated by a 100 micron thick layer of photo resist (Nano XP SU-8 100, Microchem Corp., Newton, MA) by spin-coating at 2900 rpm for 30 s, followed by a softbake (95C for 30 min.), UV lithography (650 mJ/cm2), a postbake (95C for 30 min), and development for 10 min. A prepolymer of PDMS is then cast onto the mold and cured. The PDMS replica is then peeled from the silicon wafer leaving the channels cast into the surface of the cured PDMS. To create closed channels a cover glass slip is bonded to the PDMS surface by oxidizing both the surface of the PDMS replica and glass by oxygen plasma treatment (70W, 85mTorr for 20s). When the two oxidized surfaces are brought into contact they bond covalently creating a seal, which can withstand up to 5 bars. The tightly sealed channels are necessary for pressure driven flow experiments to be conducted over a wide range of speed. Since the surfaces of the glass and the PDMS are each hydrophilic, filling the channels with liquids is relatively easy. A gravity feed approach (pressure driven) was used to generate the flow in the microchannel. The two reservoirs (Fig.1) in the test bed provide a stable pressure differential to drive the flow. The two micro-channels in the test bed used are (a) expansion channel with fully developed flow in a 150 μm wide rectangular section to a 750 μm wide channel and (b)‘T’ junction with two 100 μm wide outflow channels branch at two 90 degree elbow turns from a 100 μm inflow channel. Both micro-channels are 100 μm deep across the entire channel length. Distilled water was used as the fluid medium. The flow was seeded using fluorescent particles of about 1 μm in size. Since volume illumination (as opposed using a light sheet illumination in macro flow PIV applications) is used in measuring microflows, the number concentration of seed particles need to be controlled [6]. The out-of-focus images becomes the background noise that, if not properly controlled, could overwhelm the particle images and hence can result in poor signal quality. The number of out-of-focus images can be reduced by controlling the seed number concentration, C or selecting the proper depth, L, of the channel. The visibility V of the in-focus images [6] is