Continuous rise velocity of air bubbles in non-Newtonian biopolymer solutions

We have studied the free rise of small air bubbles in aqueous solutions of xanthan gum and carboxymethylcellulose. The rheological behavior of the biopolymer solutions used in our experiments was thoroughly characterized. The terminal rise velocity of the bubbles was measured by imaging their motion with a high-speed video camera. The rise velocity increased as a power law in bubble volume for the smallest bubbles studied, as expected in the Stokes flow regime, but became approximately constant for larger bubbles. We observed no discontinuity in the rise velocity over the range of bubble volumes studied, from 1 mu l to 4000 mu l. The shape of the bubbles changed from spherical to cusped to spherical cap as the volume increased. While the smaller bubbles rose vertically through the polymer solutions, the larger bubbles rose in a spiral or zig-zag path. (c) 2013 Elsevier Ltd. All rights reserved.