Experimental investigation of the behaviors of highly deformed bubbles produced by coaxial coalescence

In this paper, the highly deformed bubbles produced in the regime of bubbling with coalescence are studied systematically, starting from their formation up to their collapse dynamics. We investigate the bubbling dynamics by performing experiments with various liquids (ultrapure water and silicone fluids), gas flow rates (5-300 ml/min) and nozzle radii (0.18-0.55 mm). The evolution of the coalescing process is analyzed in detail, differentiating among four phases: liquid film drainage, neck formation, wave propagation along the bubble surface and bubble detachment. A phase diagram for coaxial coalescence and no coalescence is established in terms of the Weber (to describe the effects of inertia) and the Morton number numbers (to describe the effects of liquid property). Scaling arguments show that the coalescing dynamics is driven by inertia-capillary and viscosity can be neglected. We then present the interesting experimental investigation of large and elongated bubbles just after pinch-off. We deduce the dependence of the geometrical properties of these highly deformed bubbles on the bubbling conditions and a simple model is derived. Finally, we explore the dynamics of the inertial liquid jets arising from the collapse of such highly deformed bubbles. Regardless of the details of the collapse process, we propose a simple scaling law for the jet velocity and unravel the intricate roles of bubble geometry and liquid parameters.