Air entrainment processes and bubble size distributions in the surf zone

A new optical instrument was deployed in the surf zone in a trial experimen t to measure bubble size distributions and visualize air entrainment and bu bble formation mechanisms within breaking surf. Images of bubbles and the e volving air-water mixture inside and beneath breaking wave crests are prese nted. 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 di fferent large-scale air entrainment processes are observed. First: intrusio ns 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 ti mescale for the evolution from a compacted air-water mass to individual bub bles was on the order of 90 ms or less for both of these processes. In addi tion, small-scale air filaments hundreds of microns wide and millimeters lo ng have been discovered beneath wave-induced water jets. The filaments were observed in the process of fragmenting into bubbles hundreds of microns in diameter.