Hybrid laser-ultrasound cavitation for cloud evolution studies
Creators: name: Gerold, Bjoern
name: Kotopoulis, Spiros
name: Cochran, Sandy
name: Postema, Michiel
name: Prentice, Paul
Publisher: name: Zenodo
Year: 2012

description: We report on the development of an instrument for hybrid ‘sonoptic’ cavitation studies. A focused ultrasound transducer is housed in a custom-built chamber, which permits optical access to the focal volume, without perturbing the propagating acoustic waves. This configuration allows pulsed-laser irradiation of the liquid at the focus, and simultaneous high-speed observation of cavitation activity in this region. In this paper we provide a brief description of the apparatus and present preliminary data on the distinct cavitation regimes we have observed; specifically, laser-induced cavitation in an established field, and a new phenomenon that we refer to as laser-nucleated acoustic cavitation. The former involves a laser-pulse of energy above the threshold value for optical breakdown for the medium, in a pre-established ultrasound field. Here, a cavity rapidly expands to a maximum diameter of a few 100 μms, from the plasma generated on absorption of the optical energy, and collapses to form debris that is subsequently driven by the ultrasound radiation. By contrast, laser-nucleated acoustic cavitation is initiated by a pulse of energy below the breakdown threshold, in a pre-established field. For this regime, either form of radiation does not result in cavitation activity without the other. In combination, the role of the laser-pulse is to initiate activity which is dominated by the ultrasound exposure from the outset. Crucially, the spatial and temporal precision afforded to the occurrence of cavitation by laser-nucleation, allows us to consolidate our assertion of acoustic cavitation. With observations at unprecedented resolutions, we compare the size of constituent cavities within a single acoustical cycle to theoretical predictions, based on the frequency of the ultrasound driving the activity. It is expected that such observations will contribute to a greater understanding of cavitation in focused ultrasound, including for potential future therapeutic applications.
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