Residence time of sphere or air bubble in sheared non-Newtonian fluids

DURING experiments to measure the residence time of air bubbles in a stirred beaker, we observed that for polyacrylamide solutions the bubbles rose more slowly as rotational speed was increased (Fig. 1). This result contradicted our expectation that ‘shear thinning’ of a non-Newtonian fluid would result in a more rapid rise of the bubbles. The multiple-rod impeller used in these initial experiments was similar in design to that used by Steel and Maxon 1 for aeration of a novobiocin-producing actinomycete. Single air bubbles of known volume were generated by a Chromatronix 4-way valve; bubble size could be changed by changing the size or length of the splitting loop on the valve, and the volume of the bubble was checked by measuring its length in a capillary tube of known cross-sectional area. The rise of air bubbles was traced by high speed film.