Air Entrainment Processes and Bubble Size Distributions in the Surf Zone

A new optical instrument was deployed in the surf zone in a trial experiment to measure bubble size distributions and visualize air entrainment and bubble formation mechanisms within breaking surf. Images of bubbles and the evolving air–water mixture inside and beneath breaking wave crests are presented. The images resolve features of the air–water mixture to length scales of hundreds of microns across a 3.7-cm field of view. Two qualitatively different large-scale air entrainment processes are observed. First, intrusions of air and water, thought to be created by jets penetrating the water's surface, fragment into plumes of bubbles. Second, an air cavity trapped by the overturning wave crest is observed to disintegrate into bubbles. The timescale for the evolution from a compacted air–water mass to individual bubbles was on the order of 90 ms or less for both of these processes. In addition, small-scale air filaments hundreds of microns wide and millimeters long have been discovered beneath wave-induced water jets. The filaments were observed in the process of fragmenting into bubbles hundreds of microns in diameter.