Migration and chaining of noncolloidal spheres suspended in a sheared viscoelastic medium. Experiments and numerical simulations

Migration and chaining of noncolloidal spheres in a worm-like micellar, viscoelastic solution under shear flow have been studied both experimentally and by numerical simulations. The microstructure dynamics have been experimentally investigated in the flow-gradient and in the flow-vorticity planes. 2D simulations in the flow-gradient plane have been performed for the same geometry, and with a proper selection for the constitutive equation of the suspending liquid. Experimental results show the formation of particle chains in the bulk, along with migration of a considerable fraction of spheres to the walls. At long times, chains in the bulk are stable, and cross-flow migration of individual spheres is suppressed. Numerical simulations with a standard viscoelastic constitutive equation (Giesekus fluid) reproduce the same phenomena observed experimentally, both in terms of fast particle migration to the wall and bulk chain stability. No alignment is, instead, found in simulations with a constant-viscosity, elastic fluid (Oldroyd-B model), in agreement with previous experimental results with Boger fluids