The importance of (un)dissolved gases on early-stage cavitation dynamics within an acoustic field.

Gas content strongly affects cavitation dynamics; however, most studies rely solely on dissolved gas measurements, overlooking the influence of undissolved bubbles. This study investigates their role by introducing air bubbles (<200 µm) into water samples with identical dissolved gas levels and analyzing cavitation at both inception and developed phases using high-speed imaging and hydrophone measurements. The results show that the presence of pre-existing bubbles alters cavitation dynamics at the inception and developed phases. Under low dissolved gas and in the absence of air bubbles, cavitation can initiate from a single nucleus, and the developed phase exhibits transient vaporous cavitation with the highest acoustic intensity. In contrast, pre-existing bubbles promote the formation of conical-like bubble structures early in the inception phase and affect their dynamics in the developed phase, reducing the acoustic pressure and attenuating the noise spectrum. These effects are reversible upon bubble removal and independent of dissolved gas concentration, demonstrating that dissolved gas alone cannot represent gas-related influences on cavitation. Characterizing not only dissolved but also undissolved gas content is therefore essential for cavitation studies and applications.