Sonoluminescence in Liquid Metals

Multibubble sonoluminescence experiments have been performed in two liquid metals: mercury and the eutectic alloy of Ga, In, and Sn, commonly known as galinstan. The experiments were executed at ambient temperature, using a quartz horn to serve the two-fold purpose of inducing ultrasonic cavitation and providing an optically transparent window to detect the sonoluminescence (SL) radiated from bubbles generated in proximity of the quartz/metal interface. The typical emission lines of the metal under test could be detected, and their intensity was found to be correlated with the level of applied acoustic pressure. Their origin could be associated with two concurrent physical mechanisms: electrical microdischarges and true SL emissions. The contribution of these different processes was singled out by the deposition of an electrically conductive optically transparent film on the end face of the quartz horn. Typical SL broad-band emission could be detected in both types of liquid metals, while the emission lines corresponding to Sn and In excited states appeared only when testing galinstan in special experimental conditions. This point requires further investigations, and observation suggests the possible contribution of mechanoluminescence phenomena. Finally, in a preliminary experiment performed while maintaining mercury under a steady argon flow, we observed emission lines associated with the noble gas excited states, even in the presence of the conductive transparent film: this result excludes a significant contribution of the microdischarge mechanism and represents a strong evidence in favor of the formation of a warm and dense plasma.