| Summary: | This thesis is based on the studies of microfluidic flow characteristics and separation of microparticles in lab-on-a-chip systems. We investigated on the effects of different surface modifications and channel dimensions on microfluidic flow. These investigations were applied to design and fabricate the lab-on-a-chip systems for the maximum efficiency of microparticles separation. The SU8 (photo-curable polymer) microfluidic devices were fabricated by maskless lithography. The polymethylmethacrylate (PMMA) devices were fabricated by hot embossing. The polymer microchannel surfaces were modified by air dielectric barrier discharge (DBD) plasma processing, nitrogen plasma treatment in Plasma Enhanced Chemical Vapour Deposition (PECVD), hydrogenated amorphous carbon (a-C:H) coating, and Si-doped a-C:H coating. The channel lid must be sealed properly on the microchannel substrate, so that there will be no leakage of liquid between the lid and the substrate. The channel lid was sealed on the microchannel substrate by indirect bonding technique (for SU8 devices) and direct bonding technique (for PMMA devices). The microstructures (microchannels and micropillars) were characterised by Surface stylus profilometer and Optical microscope. The pristine and modified microchannel surfaces were characterised by sessile drop methods, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The microfluidic flow was characterised by a CMOS camera capturing 25 frames per second with a time resolution of 0.04 second. The effect of surface wettability was studied on microfluidic flow by different surface modifications on polymer microchannel surfaces. In this work, polymer micropillars were used as designed surface roughness elements on the channel bottom wall. The effect of polymer micropillars was studied on the microfluidic flow. The channel aspect ratio (channel height/channel width) is an important parameter to control the microfluidic flow in any microfluidic device. The effect of channel aspect ratio was investigated on the microfluidic flow.The laminar motion of liquid is highly important criteria for integrated microfluidic device as lab-on-a-chip system. The laminar flow characteristics of dyed water were experimentally investigated in the SU8 microchannels with or without polyimide square micropillars on the bottom wall. The Reynolds number was evaluated to be less than unity in each microfluidic flow. The micropillar side length has significant effect on the speed of the liquid front in microfluidic devices. The microfluidic flow was slower for larger side length of polyimide square micropillars. The PMMA microfluidic devices were fabricated by hot embossing technique. The PMMA micropillars were used as the surface roughness elements on the bottom wall of the device. The bottom wall and side wall surfaces of the PMMA microchannels were modified by plasma treatments and coatings of diamond-like carbon (DLC) films. The range of static water contact angle was from 81.2o to 44.3o after surface modifications including the pristine PMMA surface. The dyed water flow was faster on the microchannel surface having higher polar component of surface free energy. The dyed water flow was significantly faster on the microchannel surface of lower static water contact angle. Also, the dyed water flow was faster in the device containing micropillars of lower side length on the bottom wall. The effect of channel aspect ratio on microfluidic flow was studied by water, ethanol and ethylene glycol as working liquids. The flow of any of these liquids was higher for larger aspect ratio. Also, we investigated the effect of dynamic viscosity on the speed of microfluidic flow by using few working liquids (water, ethanol, Isopropyl alcohol, liquid mixture and ethylene glycol) of different coefficients of dynamic viscosity. The microfluidic flow of ethylene glycol was slower due to higher coefficient of dynamic viscosity than other liquids. The PMMA microchannel bends of rectangular cross-section were fabricated by hot embossing in the range of channel widths from 55 μm (micron) to 400 μm. Also, the SU8 microchannel bends were fabricated by maskless lithography in the range of channel widths from 100 μm (micron) to 400 μm. The air-water interface motion through these microchannels were recorded and then analysed. The effect of channel aspect ratio on microfluidic flow was studied in these devices. The Reynolds number was evaluated as always less than 1 in each channel of every microchannel bend. The air-water interface velocity has shown a prominent increase at 90o separation angle in microchannel bend. The air-water interface velocity has shown an increasing trend at higher channel aspect ratios. We investigated the separation of 10 μm polystyrene microparticles by prototype microchannel bend structure and we got 86% separation efficiency. But, we obtained 100% separation efficiency of 10 μm polystyrene microparticles in a modified microchannel bend structure for each aqueous solution of different particle densities.
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