On the births of film drops from bubbles bursting on seawater surfaces

Abstract

The parameters describing the birth of film droplets originating from bubbles bursting on seawater surfaces are presented. Results are given for bubble sizes Db from 2 to 14.6 mm equivalent volume diameter. It is shown, contrary to earlier reports, that the films of all bubbles with Db up to at least 14.6 mm burst in an orderly manner in which a hole appears at a well-defined location, usually the film's edge, and propagates from there gathering up the film's mass into a toroidal ring as it progresses. This process is enabled because surface tension provides the force required to sustain the centripetal accelerations. Film drops are created when beads, of sufficient size, form along the length of the toroidal ring and surface tension is insufficient to maintain the centripetal accelerations at these accumulation points. Pieces of the ring break loose and leave the toroidal ring along paths tangential to the bubble's cap. It is shown that only bubbles larger than 2.4 mm diameter can launch film droplets by this means and that this begins when the film has rolled up through an angle of about 31° independent of both bubble size and (theoretically) surface tension. Film drop spray patterns recorded on MgO-coated cylindrical shells surrounding the burst bubbles yield film drop numbers and trajectories. In addition, film drop size distributions, their speed of launch, and the speed at which the film opens have been determined as a function of bubble size. The droplet sizes cited here are substantially larger than most previous estimates, and with a high probability, these droplets follow downward trajectories which lead them to impact the surface. A strong inference may be drawn that these impacts give birth to secondary droplets that are smaller than their parents and which have upward velocity components.