Capillary driven flows in micro-machined open polymer microchannels
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Wettability and surface roughness have been studied for quite some years now especially in the area of microfabrication, in which more interests have been raised for the use of polymeric materials (acrylic family) for various biomedical applications. This study addresses one of those interests, the effect of fabrication parameters on the surface property of micromachined polymethyl methacrylate (PMMA) by varying the micro-milling parameters including the depth of cut, feed-rate, spindle speed, coolant (air), and the size of drilling bits. From a factor analysis based on experimental results, depth of cut has the highest influence on the surface quality. The wettability study and contact angle measurements showed that different combinations of fabrication parameters resulted in varied surface roughness. It was determined that the spindle speed of 20,000 RPM, feed-rate of 800 mm/min, depth of cut of 150 mm, and milling bit of 203μm diameter were optimum fabrication parameters to be used for machining microchannels for the experimental investigation of capillary driven flows. Contact angle measurements show which of the samples has the highest and lowest value for contact angle thereby determining whether the surface is hydrophobic or hydrophilic. Wenzel model was used to understand the wettability of each of the micromachined surfaces, the lowest contact angle measured was 30.6° at the average surface roughness of 0.84μm, and highest contact angle was 48.2° at the average surface roughness of 1.57μm. Although these results contain some outliers, the optimum fabrication parameters provided the lowest surface roughness with an equivalent contact angle of 39.5°. The other goal of this study is to understand the relationship between capillary-driven flow and wettability parameters (contact angle, surface roughness, and design parameters). An experimental investigation on capillary driven flow was carried out based on a pool of design parameters, the length, width, depth, and aspect ratio of machined microchannels. The results were modeled based on the Washburn equation, which governs the advancing of the meniscus with an increase in time. The results showed that the preferred flow rate was achieved at the design parameters such as the aspect ratio of 1.23, 700μm width, and 600μm depth of machined microchannel. Capillary driven flow was relatively fast with test solution mixed with AOT of 0.5g, 1.0g, 2.0g and 3g, except for 3g where the speed is relatively low due to reverse critical micelle concentration.