Study on the mechanism of microbubble collision and fusion in water

Microbubbles, as key triggering factors of cavitation phenomena, are of significant importance for research on cavitation effects. This paper systematically investigates the collision and coalescence dynamics of microbubbles through a combined approach of theoretical analysis, experimental observation, and numerical simulation. First, a compact microbubble generation device was designed. By integrating a high-speed impeller-accelerated flow field with gas–liquid mixing pressurization technology, efficient and accurate controlled generation of microbubbles was achieved. Second, an observation system based on a high-magnification microscopic lens and optimized reflective optical path was developed, overcoming the technical challenge of capturing transient behaviours at the micrometer scale. Furthermore, a mesh-free numerical model based on Smoothed Particle Hydrodynamics (SPH) was established. By incorporating surface tension corrections and interfacial force coupling algorithms, the simulation accuracy of microscale microbubble motion was significantly enhanced. Experimental and numerical results demonstrate that: Microbubble coalescence requires multiple collisions, with the number of collisions positively correlated with the amplitude of surface oscillations. Microjets and local pressure differentials are the key physical mechanisms triggering coalescence. The coalescence process follows a transient evolutionary pattern described as a ‘contact-compression-rupture-reconstruction’ cycle. This study provides novel observation techniques and theoretical models for microscale microbubble dynamics, offering valuable reference for cavitation effect prediction, drag and noise reduction in underwater equipment, and the optimal design of hydraulic machinery.