Spatial separation of cavitating bubble populations: the nanodroplet injection model.

Intense Na* emission was observed from 0.1 M Na(2)SO(4) in 95% sulfuric acid during multibubble sonoluminescence (MBSL). As a function of acoustic intensity, three different morphologies are observed for the cloud of light emitting bubbles. Even more strikingly, there is a spatial separation between blue-white emitting bubbles and orange emitting bubbles, easily observed by the naked eye. Surprisingly, no Na* emission was observed on the top of a bubble cloud (blue-white emission) while strong Na* emission predominates at the bottom. This observation provides the first experimental evidence to distinguish between the two types of two-site models of acoustic cavitation and MBSL: spatial separation of emission from nonvolatiles (e.g., Na*) ought not to be observed for the heated-shell model but is predicted from the injected droplet model.