Ultrasound, which is defined as sound waves with frequencies above 20 kHz, is widely used in science and technology, ranging from medical applications to industrial cleaning. When intense ultrasound is applied to a liquid, it generates a fascinating phenomenon known as cavitation. Cavitation occurs when rapid pressure changes result in the formation, growth, and collapse of miniscule bubbles within the liquid. These bubbles can generate extremely high temperatures and pressures upon collapse, thus facilitating chemical reactions and mechanical effects that are otherwise challenging to achieve.

Despite its importance in fields such as medicine, chemistry, and engineering, the fundamental mechanism of cavitation remains poorly understood. A primary challenge is the wide range of scales involved: the process originates from microscopic molecular motions (phase transitions between liquid and gas) but generates macroscopic effects observable at much larger scales. In particular, the initial stage of bubble formation, known as nucleation, is extremely challenging to examine both experimentally and theoretically.

This problem is addressed in this study via an unprecedented large-scale molecular dynamics simulation involving approximately 100 billion atoms. By directly solving Newton’s equations of motion for all atoms in the system, this approach avoids the introduction of artificial assumptions regarding bubble formation and naturally captures the process from a purely atomistic perspective. The simulation is performed under conditions corresponding to ultrasonic irradiation, thus enabling observations of bubbles emerging and evolving in real time.

The results reveal that numerous bubbles were simultaneously generated near the ultrasound source. These bubbles grew and merged into large clusters, which repeatedly split and recombined synchronously with the ultrasound oscillation. Such collective dynamics of bubble clusters, which highlight the importance of many-body interactions in cavitation, have not been reproduced in previous simulations.

This study provides a unified illustration of cavitation dynamics from the initial phase transition to large-scale collective behavior. The findings not only improve the understanding of nonequilibrium phenomena in liquids but also contribute to practical applications, including medical therapies, sonochemical reactions, and material processing. Furthermore, the simplicity and scalability of the simulation method render it applicable to more complex systems, such as liquids containing impurities or dissolved gases, thus facilitating future research directions.

(Written by Yuta Asano on behalf of all authors.)

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