From Optical Breakdown to Bubble Inception: A Coupled Plasma-Thermal Framework for Nanosecond Laser-Induced Cavitation in Water
Laser-induced cavitation under nanosecond optical breakdown is central to applications such as laser-induced forward transfer (LIFT), microsurgery, and microfluidic actuation, yet the physical origin of the earliest cavity and its connection to subsequent bubble growth remain unresolved. Existing models typically describe bubble formation either as a plasma-driven mechanical response or as a thermally driven nucleation process, without resolving how these mechanisms interact during inception. In this study, we developed a coupled plasma-thermal framework that unifies free-electron dynamics, plasma absorption, thermoelastic acoustic response, residual thermal energy retention, and post-inception bubble evolution within a single description. The model shows that bubble inception is governed primarily by plasma-induced thermoelastic acoustic relaxation, which generates transient tensile rarefaction pressures sufficient for cavitation on nanosecond timescales, while residual thermal energy sustains subsequent bubble growth. Because plasma-mediated energy deposition is spatially anisotropic under moving breakdown conditions, the initial cavity is predicted to form as an elongated, asymmetric bubble rather than as a spherical nucleus. This initial bubble shape reflects the spatial structure of the plasma absorption region and provides the starting geometry for subsequent bubble dynamics. Comparison with time-resolved experiments demonstrates that the coupled framework captures both early-time cavity formation and long-time bubble expansion more accurately than plasma-only or thermal-only models. These results show how breakdown-induced asymmetry controls the birth shape of the cavitation bubble and its early fluid-mechanical evolution, providing a physics-based basis for modeling laser-driven liquid motion and material transport.