When, how, and why the path of an air bubble rising in pure water becomes unstable

Recently, [Herrada, M. A. and Eggers, J. G., Proc. Natl. Acad. Sci. U.S.A. 120, e2216830120 (2023)] reported predictions for the onset of the path instability of an air bubble rising in water and put forward a physical scenario to explain this intriguing phenomenon. In this Brief Report, we review a series of previously established results, some of which were overlooked or misinterpreted by the authors. We show that this set of findings provides an accurate prediction and a consistent explanation of the phenomenon that invalidates the suggested scenario. The instability mechanism actually at play results from the hydrodynamic fluid-body coupling made possible by the unconstrained motion of the bubble which behaves essentially, in the relevant size range, as a rigid, nearly spheroidal body on the surface of which water slips freely.

Automated summary

Recently, a study published in Proceedings of the National Academy of Sciences reported predictions and a physical scenario explaining the path instability of an air bubble rising in water. However, in this Brief Report, the authors review previously established results and show that the suggested scenario is invalidated. The actual instability mechanism is due to the hydrodynamic fluid-body coupling facilitated by the unconstrained motion of the bubble, which behaves as a rigid, nearly spheroidal body on the water's surface.