Sphere migration in oscillatory Couette flow of a viscoelastic fluid

Abstract

Observations are presented of the migration of spherical particles in large-amplitude, oscillatory flows between concentric cylinders. The ratio of the gap between the cylinders to the cylinder radii was 1:20, yielding an approximately linear shear flow in the absence of any particles. The particles had diameters d that were comparable to the gap width, in the range 0.3 mm<d<0.6 mm, and were suspended in a 13% solution of polyisobutylene in tetradecane. Particle motion was observed by using horizontal and vertical charge coupled device cameras that were interfaced with a computer. The results show that, in a linear velocity field, in low frequency and large-amplitude oscillations, spherical particles near solid boundaries generally migrate toward the nearest boundary, in keeping with previous simulation results for steady flows. However, the migration velocity depends strongly on the sphere diameter, frequency, and amplitude of the oscillation. For example, at small amplitudes large “dead zones” are apparent, in which the sphere does not migrate at all, and these zones are not centered in the gap. In addition, at frequencies greater than 1 Hz, the particles do not migrate all the way to the outer wall, but stop short by a frequency-dependent distance. When the particles do migrate, the migration velocity is determined by a single dimensionless parameter that is derived here by a scaling analysis.