Scuba Bubble Selfie
July 20, 2014
Photographer: Dave Lynch
Summary Author: Dave Lynch
Scuba divers are familiar with the sight of themselves reflected in large air bubbles from divers below them. Silhouetted against Snell’s Window, the diver is seen against a bright blue sky or white if it is cloudy. Here the author's looking straight down on a large ascending bubble that was exhaled by another diver. The bubble was about 20 in (50 cm) away. Viewed from the side, the same bubble has a flat bottom and shallow domed upper surface, like an old-fashioned paper weight.
The optics of large air bubbles in water is extremely complicated, so much so that nobody has computed their shape in any detail. As the bubble rises, it expands. As its size changes, so does its shape. As its shape alters, the flow of water around it changes. Turbulent flow sets up waves at the air-water interface, which induces more flow instability. Pretty soon the bubble starts to oscillate, then fragments into two or three bubbles with many smaller bubbles tagging along. At no time in the life of one of these bubbles is it in equilibrium with its surrounding. No wonder its shape is so hard to calculate. But the quivering silvery bubbles are pretty!