Particle Migration in Confined Shear Flows

In this thesis, we examine the cross streamline migration of dilute suspensions of non colloidal particles undergoing simple shear flow via theory and experiments. Such migration behavior has a profound impact not only on the bulk rheological properties of the suspension (for example, total stress, viscosity etc.) but also on mixing. The major focus of our work is to experimentally understand the motion of rigid particles and deformable blood cells in the shear flow of a parallel plate device. The anisotropy of the interaction between the particle/cell with the plane results in a migration velocity towards the centerline. Historically, the migration behavior of particles and cells in plane poiseuille flow is fairly well understood and there is very good agreement between the theory and experiments. Migration in simple shear flow, in contrast, is a bit of an enigma with multiple migration theories and almost no experimental data available in literature. In Chapter 2, we investigate the migration of rigid polystyrene particles via a balance between buoyancy and inertia in an attempt to resolve the discrepancy between the various available models. In Chapter 3, the migration of red blood cells is examined using the same approach and compared to various drop, vesicle and actual blood cell models. It is demonstrated that even though the blood cell is not really a drop, its migration behavior can be reasonably approximated by the rather simple drop model of Chan & Leal (1979) with a single adjustable parameter. In Chapter 4, we examine migration in a counter rotating parallel plate geometry. The presence of inertial secondary currents due to the higher gap widths and rotation rates employed for these experiments gives rise to a range of intriguing dynamics depending on the degree of counter-rotation. In particular, particles can be driven to discrete equilibrium positions in the gap in both the radial and vertical directions. We further demonstrate that this geometry can be used to effect a size based separation, segregating differently sized particles to different radial positions in the device. In the final chapter, we describe a rather curious mechanism for fluid dispersion resulting from a tight confinement of rigid particles in a simple shear flow. It is demonstrated that the mixing produced by such tightly confined spheres, rather than being chaotic over the system, is limited to about central two-thirds of the gap.