Interaction of two spark generated bubbles beneath free surface

The interactions of two oscillating bubbles beneath a free surface are studied with the help of high-speed photography. The collapse can produce liquid jets both in the bulk water and upon the free surface. Different interaction modes and surface jets are observed, which are significantly affected by the relative bubble-bubble distance and the relative depths below the free surface. The phenomena are summarized in a dimensionless parameter plot, where several regions are divided based on the collapse behavior. Particularly, a transition region is observed where different behaviors occur randomly. Based on the experimental results, the boundary lines between the regions are proposed in a wide range of parameters.