Bubble pinch-off in viscoelastic liquids*

The formation of gas bubbles in a continuous liquid phase is important in many engineering processes, for instance, to generate foam or to provide agitation and mixing in bubbly flows. A challenge in describing the initial formation of a bubble is that the final pinch-off exhibits a singularity in time and space. Past experiments in Newtonian fluids have shown that the evolution of the neck radius can be captured by a power-law of the time before break-up. The corresponding exponent depends on the viscosity of the continuous liquid, from 1/2 for low viscosity to 1 for large viscosity. However, bubble formation in a dilute polymer solution presenting a viscoelastic behavior remains unclear. In this study, we use high-speed imaging to analyze the pinch-off of air bubbles in a dilute solution of polymers. We characterize the time evolution of the neck radius when varying the characteristic relaxation time and describe the influence of the viscoelasticity on the bubble generation process.