Microfluidics of Complex Fluids

Microfluidics is very important in the world of BioMEMS, particularly for lab-ona-chip devices. There are many unusual differences from conventional fluidics, such as the significance of surface forces, the high shear/extensional rate, the high heat transfer rate, the low Reynolds number, and the high Weissenberg number. In this study, aqueous solutions of high molecular-weight polymers, polyethylene oxide (PEO) and hydroxyethyl cellulose (HEC), as well as biomacromolecules, protein BSA and DNA fragments, have been chosen as the model materials for the complex fluids involved in BioMEMS applications. The presence of large molecules brings in different interfacial properties, hydrodynamic properties, and electrokinetic properties. This complexity, together with the traits of microdevices, may lead to phenomena like vortex formation, flow instability, wall slip, and solute degradation. Four research directions have been explored in this study, focusing on different microfluidic characteristics. First, the significance of surface forces in microfluidics was used to develop two microfluidic functions, capillary valving and capillary pumping. Different surface properties are preferred in different functions. Secondly, an on-chip capillary electrophoresis system was developed, benefiting from the high heat transfer rate in microchannels. DNA separation was realized in this novel system, rather than the common laser induced fluorescence (LIF) method. A resolution as high as 10bp is ii achievable with a polymer solution as the sieving material. The dynamic coupling mechanism works in polymer solutions, different from the separation mechanism in polymer gels. Thirdly, the characteristic of high shear rate was used to expand current rheological measurements to higher shear rates. With the careful evaluation of the end correction (Bagley correction) and wall slip correction, rheological data have been successfully derived for shear rates as high as 10 s. The polymer degradation was observed on PEO solutions as a result of the in-channel shear stress and the end flow. The degradation mechanism was explored. Lastly, the characteristics of high Weissenberg number and strong extensional flow at the entries triggered the study of end (entrance/exit) flow in microfluidics. Advanced flow visualization revealed the vortex enhancement and flow instability phenomena in some complex fluids. The correlation between these phenomena and the hydrodynamic properties or the chemical structures of polymers has been explored.