Alternation of Stress Profile around Bubbles Associated with Velocity Discontinuities*

This study focuses on the mechanism of bubble velocity discontinuity (BVD) for a bubble rising in a viscoelastic fluid. Using high-speed polarization measurement, the behavior of the bubble and the surrounding flow structure were observed. The velocity of the rising bubble before and after BVD matched the non-slip and slip conditions, respectively, indicating that BVD is caused by a boundary condition change. This is further supported by the observation that the bubble velocity immediately after its onset matched the slip condition for all bubble volumes. Additionally, particle orientation profiles differed before and after BVD, suggesting variations in polymer relaxation positions. We assumed that the transition from non-slip to slip conditions was due to polymer adsorption and the Marangoni effect. Despite differences in stress components, the consistent rising velocities before and after BVD suggest complex dynamics in viscoelastic fluids, warranting further research.