High resolution optical investigation of laser intensity and solution temperature effects on thermocavitation

• A non-intrusive optical technique to measure bubble dynamics of thermocavitation. • Effects of laser intensity, temperature, and absorption coefficient on bubble dynamics. • Twin cavitation cases, where a subsequent cavitation bubble appears before the collapse of the previous one. This paper studies cavitation induced by focusing a continuous wave laser in a highly absorbing aqueous copper nitrate solution. The Spatial Transmittance Modulation method was used to detect the cavitation bubbles and verified by high-speed shadowgraphy. The influence of intensity, solution temperature, and absorption coefficient on the cavitation dynamics was investigated by observing the bubble diameters, bubble lifetimes, cavitation times, and cavitation frequencies. The intensity was varied by changing the optical power between 1.2 W and 4.7 W and shifting the laser focal point from 4.5 mm outside to 3.5 mm inside of the cavitating solution by 1 mm increments. Additionally, six solution temperatures between 22 °C to 74 °C were tested, and three concentrations of copper nitrate (10.7, 6.35 and 3.49 g copper nitrate per 10 mL of water) were used to change the solution’s absorption coefficient. The results reveal that on average, larger intensities result in shorter cavitation times, higher frequencies, and higher percentages of small bubbles. Increasing the copper nitrate concentration has a similar effect on the cavitation events. With higher solution temperature, the time to initiate cavitation decreases, and higher percentage of larger bubbles appear due to a decrease of surface tension. However, the solution temperature does not change the cavitation randomness significantly. The findings of this investigation may allow for effective control of thermocavitation dynamics where current laser cavitation applications require managed outcomes.