Influence of liquid-phase rheological properties on bubble rising motion

In this paper, the rising behaviors of single bubbles in Newtonian, shear-thinning, constant-viscosity viscoelastic, and shear-thinning viscoelastic liquids are numerically investigated. The gas-liquid interface is tracked by the volume of fluid (VOF) method, and the viscoelastic and shear-thinning effects of the liquid phase are described by the viscoelastic FENE-CR model and the shear-thinning Carreau model, respectively. The effects of Eötvös number (Eo), Weissenberg number (Wi) and rheological index (n) on bubble dynamics are investigated in detail. The numerical results reproduce the phenomena of bubble pointed tail and negative wake in viscoelastic liquids. The present study shows that the liquid-phase rheological properties significantly affect bubble dynamics. In a definite Newtonian liquid, the deformation of bubbles with a definite size is directly controlled by surface tension, and the flow field around the bubble is affected by bubble rising motion. In the liquids with strong shear-thinning effects, bubbles are easier to deform due to a decrease in viscous drag. When the bubble shows spherical cap or flat shape, the "viscosity blind zone" appears in the tail region. In viscoelastic liquids, a bubble takes on the unique pointed tail under the influence of liquid-phase elasticity. The phenomenon of negative wake even occurs in the region of bubble tail, which can be attributed to the relaxation behavior of polymer molecules. The comparison of bubble dynamics between in constant-viscosity and shear-thinning viscoelastic liquids shows the liquid-phase viscoelasticity is a necessary condition for the appearance of negative wake, and the shear-thinning effect can promote the appearance of negative wake. It is discovered that the liquid-phase apparent viscosity is relatively high in the region where the negative wake occurs in shear-thinning viscoelastic liquids.