Microfluidic particle separation via elasto-inertial focusing in straight rectangular microchannels

Label-free separation of target particles or cells in a continuous flow is a crucial process in many commercial and industrial applications. Among the various particle and cell separation techniques, microfluidic separator based on elasto-inertial forces in the flow of non-Newtonian fluids has received increasing attention in the past decade. However, current studies have been mainly focused upon the use of viscoelastic forces to manipulate particles and cells. Little work has been done to obtain a fundamental understanding on how the fluid rheological properties, such as viscoelasticity and shear thinning, affect the motion of particles. This dissertation is aimed to address this question through a systematic experimental study. We first designed a series of experiments to study both the individual and combined effects of fluid rheology and inertia on the migration of rigid spherical particles in a (nearly) square microchannel. The sole effects of fluid inertia, elasticity and shear thinning were investigated by re-suspending the same particles into Newtonian (water), purely elastic (polyvinylpyrrolidone, PVP) and inelastic shear thinning (xanthan gum, XG) fluids, respectively. The combined effects of fluid elasticity and inertia or fluid shear thinning and inertia were investigated in the flow of PVP and XG solutions over a wide range of flow rates. The combined effects of fluid elasticity, shear thinning, and inertia were investigated in two types of elastic fluids with varying shear-thinning properties (polyethylene oxide, PEO and polyacrylamide, PAA). We found that fluid elasticity directs particles toward the channel centerline while fluid shear thinning causes particles to migrate towards both the centerline and corners. In the second part of this dissertation, we performed a comprehensive study of the separation of particles and cells in the flow of PEO solutions through straight rectangular microchannels. We investigated the effects of flow rate, solvent viscosity, PEO concentration, and channel height on the elasto-inertial separation of spherical polystyrene particles. We proposed to explain the observed elasto-inertial particle focusing using a competition of center- (because of fluid elasticity) and wall- (because of fluid shear thinning) directed viscoelastic forces. We also applied this sheath-free separation technique in the flow of biocompatible PEO solutions to sort drug-treated Cryptococcus neoformans by morphology. Three metrics were used to evaluate the parametric effects on the cell separation performance: efficiency, purity, and enrichment ratio. In the last part of this dissertation, we performed another systematic experimental study of the motion of rigid spherical particles in the flow of inelastic shear-thinning XG solutions through straight rectangular microchannels. We found that the number and location of equilibrium particles position are both a strong function of channel dimension, particle size, and XG concentration. Inspired by this study, we demonstrated for the first time a continuous sheath-free separation of polystyrene particles in XG solutions through a straight high width/depth-ratio microchannel. This separation was found to remain effective over a much wider range of flow rates than those reported in the flow of viscoelastic fluids. We attempted to explain the particle migrations in XG solutions using the competition of a strong wall-directed (because of the strong shear thinning effect) and a weak center-directed (because of the weak elasticity effect) lateral force induced by normal stresses in a Poiseuille flow