Influence of liquid viscosity on the interactions between cavitation bubbles and flat liquid-liquid interfaces

This study investigates the interactions between cavitation bubbles and the interfaces of two immiscible liquids, with practical implications and potential applications in the fields such as ultrasonic emulsification and wastewater treatment. To explore the influence of liquid viscosity on the interaction between the cavitation bubble and flat liquid-liquid interface, visualization experiments were performed on the laser-induced cavitation bubbles near two liquid-liquid interfaces composed of deionized water and two silicone oils with different viscosities (50 mPa·s, 500 mPa·s) by using high-speed photography. Three different positions were employed for the generation of cavitation bubbles, i.e., at the interface, in the water, and in the silicone oil. The evolutions of cavitation bubbles and the corresponding interface deformations at different dimensionless standoff distances γ between the cavitation bubble and the interface were observed. The results show that the difference in the viscosity of silicone oil significantly affects the physical phenomena occurred during the interaction between the millimeter-scale cavitation bubble and the interface. On this basis, the qualitative and quantitative analyses for the cavitation bubble jet dynamics indicate that the critical value of γ for jet penetration through the interface between the water and the higher-viscosity silicone oil (interface 2, γ = 0.33) is lower than that for the interface between the water and the lower-viscosity silicone oil (interface 1, γ = 0.69). Besides, the jet generated by the cavitation bubble near interface 1 possesses a higher maximum velocity. These indicate that increased viscosity inhibits the development of the jet. The cavitation bubbles that initiate in the water near Interface 1 consistently migrate away from the interface and do not split, while those near interface 2 would migrate towards the interface at intermediate γ and would split at γ <0.91. In addition, the jet behaviours of cavitation bubbles near interface 2 at different γ are examined and classified into four types.