A delicate balance: How Faraday waves and vortex shedding drive air entrainment

Bubble-turbulence interactions are central to many processes of great fundamental interest, including ocean science, metal casting, and industrial power systems. A versatile yet simple analog for many of these processes is the plunging liquid jet, wherein bubbles are formed by the entrainment of air as the jet impinges upon a quiescent pool. The formation of these bubbles is initiated by the conditions of the jet and free surface, with major roles played by the dynamic effects of gravity, surface tension and the local subsurface flow. When air entrainment begins, it is the result of disturbances upon the interface overcoming surface tension via interactions with the subsurface flow. However, this competition of forces has escaped comprehensive description thus far. To better understand the inception of air entrainment, we employ time-resolved particle image velocimetry to extract pressure and velocity data from a well-characterized plunging jet flow that is well-controlled and harmonically perturbed. We fuse these data with high speed movies and quantitative bubble flow rate measurements to uncover the mechanisms that transform micron-scale free surface disturbances into centimeter-scale bubbles.