Effect of viscoelasticity on the rotation of a sphere in shear flow

When particles are dispersed in viscoelastic rather than Newtonian media,the hydrodynamics will be changed entailing differences in suspension rheology. The disturbance velocity profiles and stress distributions around the particle will depend on the viscoelastic material functions. Even in inertialess flows, changesin particle rotation and migration will occur. The problem of the rotation of a single spherical particle in simple shear flow in viscoelastic fluids was recently studied to understand the effects of changes in the rheological properties with both numerical simulations [D’Avino et al., J. Rheol. 52 (2008) 1331-1346] and experiments [Snijkers et al., J. Rheol. 53 (2009) 459-480]. In the simulations, different constitutive models were used to demonstrate the effects of different rheological behavior. In the experiments, fluids with different constitutive prop-erties were chosen. In both studies a slowing down of the rotation speed of the particles was found, when compared to the Newtonian case, as elasticity increases. Surprisingly, the extent of the slowing down of the rotation rate did not depend strongly on the details of the fluid rheology, but primarily on theWeissenberg number defined as the ratio between the first normal stress difference and the shear stress. In the present work a quantitative comparison between the experimental results and the results from the simulations is made mainly by using a multimodeGiesekus model with Newtonian solvent as constitutive model. The model is fitted to the experimentally obtained linear and nonlinear fluid properties andused to simulate the rotation of a torque-free sphere in a range of Weissenberg numbers similar to those in the experiments. A good agreement between theexperimental and numerical results is obtained. The local torque distribution on the particle surface calculated by simulations is shown.