On dynamics of inflating a soap bubble

The physical behaviour of bubbles, though often encountered, involves complex dynamics that are not fully understood or observed, and we still continue to discover unexplored nuances. This study investigates the inflation dynamics of a soap bubble using a controlled airflow through a nozzle coated with a thin film. By varying the source pressure, we analyze and predict how it affects the rate of bubble growth during inflation. We visualise the previously unobserved internal airflow, discovering that air enters the bubble as a round jet, which exits the nozzle and impacts the growing concave surface of the inflating bubble, forming a toroidal vortex. Several scaling laws are identified that govern the vortical flow throughout the bubble interior and the vortex core. This bubble-confined vortex ring interacts with the incoming jet, exhibiting destabilisation that leads to the inception of a turbulence-like event beyond a very low jet Reynolds number (Re~250). These findings enhance our understanding of bubble physics and cast a basis for studying confined toroidal vortices continually being fed with mass flux, unlike self-propelled free vortex rings, where the feeding is discontinued after it pinches off from the orifice.