Behavior of laser-induced cavitation bubbles in liquid nitrogen

Abstract

Initial behavior and the subsequent motion of a bubble in liquid nitrogen are investigated experimentally using high-speed photography. A bubble is generated by focusing a pulsed ruby laser into liquid nitrogen at 78.0 K, changing the ambient pressures up to 253.2 kPa which corresponds to the applied pressure (or overpressure), Δp, being 147.1 kPa. When the energy level of the laser beam at the focus exceeds an irradiance threshold, for instance 5.4×1011 W/cm2 for Δp=4.9 kPa, the optical breakdown occurs in the liquid nitrogen, followed by a series of high-speed phenomena such as plasma formation, shock wave emission, and vapor bubble generation. It is found that during a very short period after the plasma formation a bubble grows nonspherically reflecting from the plasma shape, but the bubble volume itself varies with time in the same way for all cases performed in the present experiment. The liquid inertia is a dominant factor affecting the bubble growth, while the thermal effect becomes remarkable during the bubble collapse, resulting in the retardation of the bubble motion. The characteristic behavior of a laser-induced cavitation bubble in liquid nitrogen is significantly influenced by the phase change of vapor at the bubble surface as well as by the vapor pressure inside the bubble. Immediately after the bubble rebound, instabilities are amplified over the bubble surface similar to those caused in the water case.