Fluorescence temperature measurement of optical thermocavitation

Abstract

While optical cavitation creates and directs potent localized flows non-intrusively, the resulting thermal dynamics are uniquely difficult to characterize. The acoustic shockwaves produced by cavitation damage submersed sensors near enough to effectively measure cavitation dynamics. The bubble lifetime is on the order of tens of microseconds, demanding a high sampling rate which is difficult and costly to achieve via infrared imaging, and requires fragile micro-scale temperature sensors. Planar laser-induced fluorescence provides an alternative technique to explore the thermal effects of optical cavitation. The temperature field around the cavitation bubble can be measured non-intrusively with this method using a high speed video camera. A 440 nm continuous laser sheet excites rhodamine B dye to fluoresce. The intensity of the fluorescence is correlated to the liquid temperature. A Miro 310 camera is used to record the fluorescent intensity while cavitation is induced by a focused continuous wave 810 nm laser – a wavelength that does not produce fluorescence in rhodamine. The temperature field is characterized qualitatively at very high frame rates using a shadowgraph method, and quantified by measuring the fluorescent intensity during cavitation.