Three-dimensional measurements of microstructure and particle migration in suspension flows

Suspensions of discrete particles dispersed within a Newtonian viscous medium, commonly found in nature and manipulated in industrial processes, have macroscopic fluid properties deviating from those of Newtonian fluids. For example, at moderate particle volume fractions, these systems show particle migration apparently resulting from normal stresses in nonlinear shear. These normal stress is predicted through numerical studies and measured in recent experiments (Deboeuf et al., Phys. Rev. Lett., 2009); it is believed to be responsible particle microstructural anisotropy in turn stemming from multi-particle hydrodynamic interactions. Few previous studies have experimentally explored suspension microstructure and shear migration in complicated process-relevant flows.;Our experiments present the evidence showing that microstructural anisotropy creates normal stresses in sheared suspensions. Particle velocimetry of weakly Brownian suspensions in pressure-driven microchannel flows combines with three-dimensional particle locations obtained via confocal microscopy after arresting the flow to show this anisotropy. Volume fractions 0.2 < &phis; < 0.56 and shear rates g&d2; yield Peclet numbers 0 < Pe < 1700 in our experiments. Experimentally, at high g&d2; the pairwise particle distribution correlates strongly along the axis of compression, similar to Stokesian Dynamics simulations under identical conditions by our collaborator, Jeffrey F. Morris at the City College of New York. At the channel center, where g&d2; → 0, concentrated &phis; = 0.56 suspensions behave as a confined isotropic fluid.;We investigate the concentration gradients resulting from competition between advection and shear-induced migration of suspensions in steady open flows using microscale channels designed to enhance the mixing of Newtonian fluids by inducing transverse advection (Stroock et al., Science, 2001). Under pressure-driven flow in straight channels, particles migrate from high shear regions at channel walls to low shear regions at the channel center. A one-dimensional suspension balance model (Miller and Morris, J. Non-Newton. Fluid Mech., 2006) is modified to describe the behavior at the channel center. The degree of mixing is then calculated as a function of bulk volume fraction, and used to predict the volume fraction resulting in the maximum degree of segregation. Herringbone channels form a concentration profile without the particle focusing found in straight channels. Transients can result from Kelvin-Helmholtz-like buckling instability during the onset of migration near the channel entrance when particle-depleted fluid is injected into particle-rich fluid. In staggered herringbone channel chaotic flows, better mixing is seen at lower than higher bulk volume fractions. Thus, the ability of static mixers to reduce segregation due to shear-induced migration is limited.;The behavior of bi-dispersed suspension in size and blood flow through these channels is also investigated. With the addition on 0.5 microm microspheres, at the same volume fraction, to the 1.0 microm suspension we see enhanced mixing in straight and staggered herringbone channels and enhanced segregation of the larger species at moderate volume fractions in herringbone channels. This suggests that mixing and separation can be tailored through adjustment of both suspension properties and channel geometry. For blood flow through these channels, the density mismatch between red blood cells and plasma presents a challenge in the evaluation of mixing and shear migration. However, generally cell migration is similar to that in monosized suspensions. Large particles migrate faster and in herringbone channels the Kelvin-Helmholtz-like instability seen in monosized suspensions is stronger; this creates periodic concentration fluctuations along the flow axis.