Picosecond-scale dynamic response of a fundamental branched cavitation unit in high frequency ultrasonic standing-wave-like fields

The formation and evolution of branched cavitation structures involve complex multibubble interactions and energy transfer, yet their picosecond-scale dynamics under high-frequency standing-wave-like confined acoustic excitation remain poorly understood. Based on the observed inverted-triangular three-bubble topology, a minimal branched cavitation unit was established using molecular dynamics, with excitation parameters determined by a modified Keller–Miksis equation. Simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator were performed at 400, 800, and 1200 GHz to investigate centroid migration, dynamic responses, and energy evolution. The results reveal strong frequency dependence of multibubble dynamics. With increasing frequency, the onset of near-wall migration of the bottom-corner bubble advances from ∼3.5 to 2.5 ps, while its minimum Z-direction velocity increases in magnitude from −1.9 to −6.0 Å/ps and its peak total acceleration from 2.2 to 8.7 Å/ps2. For upper-corner bubbles, rapid migration begins at approximately 1.6 ps, with minimum Z-direction velocities reaching −7.3 and −7.9 Å/ps, respectively, indicating enhanced interbubble momentum transfer and directional migration. The three-bubble system undergoes cooperative motion and spatial reorganization, evolving from the initial inverted-triangular configuration toward a near-linear arrangement with a larger included angle. Thermal motion and kinetic energy peaks shift toward earlier times, while potential energy decreases, indicating enhanced localized energy accumulation, interfacial compression, and atomic rearrangement. Overall, these findings elucidate the frequency-dependent dynamic response at the atomic scale, providing a basis for multibubble cavitation modeling and high-frequency ultrasonic surface strengthening.