Film drop distributions from bubbles bursting in seawater

Abstract

The liquid droplets produced from the breaking up of the film cap of oceanic bubbles have long been considered as constituting a significant fraction of the marine aerosol. The size and number distributions of these film drops are investigated for single bubbles that burst in seawater under laboratory conditions. Optical particle detection and counting techniques are used to size the droplets generated from bubbles varying in diameter from about 1 to 6 mm. Parallel measurements are also made using a laser holographic setup for bubbles up to about 10 mm in diameter. Taken together, a film drop size spectrum extending from 0.8 to 500 μm is covered. From these results, an experimental correlation between the number of film drops generated and the parent bubble diameter is proposed. Based on this correlation and published data concerning the number of jet drops produced per bubble, a crossover point for bubbles that produce predominantly either film drops or jet drops is located at a bubble diameter of about 1.8 mm. In other words, bubbles larger than this threshold diameter produce more film drops than jet drops, while smaller bubbles generate more jet drops. A couple of film drops as large as 500 μm for a 10 mm bubble are detected. Although these maximum sizes diminish with decreasing bubble diameter, they happen to be much larger than previously thought.