(108i) Inertially-Driven Migration of Spherical Particles in Microchannel Flow

Suspended particles are known to migrate in pressure-driven flow at finite-inertia conditions. This is the ?tubular pinch? phenomenon, successfully described by theory for the point particle limit in both channel and tube geometries. Rather surprisingly, particle migration is observed to occur regularly in microfluidic applications, particularly when the suspending fluid is aqueous and thus of low viscosity. In this work we focus on the migration and equilibrium positions of suspended spheres in rectangular microchannel geometries. The finite size is found to be a critical determinant in the behavior when the ratio of particle size (a) to the narrow channel dimension (H) satisfies a/H > 1/10. We first review the theory for point particles and existing results for finite-size particles in inertial flows; these show a bifurcation in the equilibrium position for tube and channel flows at large conduit Reynolds number which is apparently due to the finite particle size. We then present a comprehensive examination by both experiment and lattice-Boltzmann based simulation of the migration behavior of particles in rectangular microchannels, showing the dependence of the behavior on the aspect ratio of the channel (W/H), the dimensionless particle size (or a/H), and the Reynolds number. Scaling of the transient data shows that the rate of approach to the ultimate equilibrium position scales linearly with Re, the shear-rate based Reynolds number at the particle scale.