Hadi Mohammadigoushki, Pengtao Yue & James J. Feng
J. Rheol. 58, 1809-1827 (2014)
Abstract - We report experiments on the migration of a large bubble in an otherwise monodisperse two-dimensional foam sheared in a wide-gap Couette device. The bubble migrates away from the walls toward an equilibrium position between the center of the gap and the inner cylinder. This differs from the situation in a narrow-gap Couette device, where the equilibrium position is at the center of the gap (Mohammadigoushki and Feng, Phys. Rev. Lett. 109, 084502, 2012). The shift in equilibrium position is attributed to the non-Newtonian rheology of the foam, which is brought out by the non-homogeneous shearing in a wide-gap geometry. Two aspects of the rheology, shear-thinning and the first normal stress difference, are examined separately by comparing with bubble migration in a xanthan gum solution and a Boger fluid. Shear-thinning shifts the equilibrium position inward while the normal stress does the opposite. Bubble migration in the two-dimensional foam is the outcome of the competition between the two effects.