Investigation of Secondary (Dean) Flows in Curved Microchannels and Application to Microparticle Manipulation in Various Fluids

Separation, solution exchange, and detection of microparticles and microorganisms, such as DNA, bacteria, and cancer cells are essential steps in a wide range of biomedical applications. Conventional methods such as centrifugation and mechanical filtration rely on laborious processes and deal with the possibility of damaging particles and cells. Microfluidic methods such as inertial manipulation of particles in microchannels, on the other hand, offer low cost and fast sample processing down to the single cell manipulation and detection level. The Dean flow-coupled inertial and elasto-inertial systems, taking advantage of secondary vortices lateral to the direction of the flow in curved microchannels, have provided an improved level of precision over particle separation throughput compared to straight channels. However, the dynamics of fluid flow and particle focusing in fluids with various rheological characteristics like blood and milk still requires a thorough fundamental investigation.

In this thesis, we attempt to fully investigate the control parameters of both fluids and particles in a curvilinear microchannel, with an aim to provide fundamental understanding of the fluid dynamics and particle focusing in various aqueous microenvironments, with a focus on non-Newtonian fluids. In objective 1 of the thesis, we focused on understanding the physics of the secondary Dean flow of viscoelastic fluids and shear-thickening nanofluids in curved microchannels. Various parameters such as channel dimensions and fluid properties were investigated to obtain a comprehensive knowledge of the secondary vortices. Two empirical correlations were developed for the average Dean velocity (VDe) of viscoelastic PEO solutions and SiO2 nanofluids, which significantly reduced the prediction error compared to the existing water-based VDe correlations in the literature. In objective 2, the particle dynamics in Dean-coupled elasto-inertial systems were experimentally investigated to understand the effects of different channel geometries and fluid viscosity on particle focusing behavior in curved microchannels. In objective 3, we demonstrated a proof-of-concept duplex particle washing process in viscoelastic PEO solutions. The developed knowledge of particle and fluid interactions in Dean-coupled elasto-inertial systems could be vital in various biomedical applications that require a target particle washing process. In objective 4, for the first time, we presented the particle behavior analysis in SiO2 nanofluids and investigated the effects of channel curvature, fluid axial velocity, and viscosity on particles focusing at the channel outlet. Our investigations could be utilized to enhance the throughput and efficiency of microdevices to address real life challenges in microparticle purification and detection in fluids with various rheological properties.