Single Cavitation Bubble Collapse: Phenomena, Theoretical Models, and Thermodynamic Effects

5. Conclusions

A single cavitation bubble provides a controlled system for investigating one of the strongest forms of hydrodynamic energy focusing. The bubble grows when local pressure decreases or energy is deposited, reaches a maximum volume, and collapses when the surrounding pressure recovers. During collapse, liquid inertia produces rapid radial acceleration, while finite sound speed, bubble contents, geometry, and thermodynamic exchange determine how the available energy is partitioned.

The principal collapse phenomena include shock wave emission, rebound, microjet formation, toroidal transition, fragmentation, localized wall loading, heating, light emission, and chemical activity. A spherical collapse concentrates energy into internal compression and a compact pressure wave. A nonspherical collapse diverts part of that energy into a jet, translation, vortex motion, and multiple shock sources. Near a rigid wall, the bubble generally migrates toward the boundary and the jet is directed toward it, but erosion is governed by the complete jet and toroidal collapse sequence rather than by one pressure peak.

The Rayleigh model remains the appropriate starting point because it provides the collapse time $t_R=0.914681R_{\max }\sqrt{\rho /\mathrm{\Delta }p}$ and the fundamental inertial scale. The Rayleigh Plesset equation adds gas pressure, vapor pressure, viscosity, and surface tension. Keller Miksis and Prosperetti Lezzi formulations introduce first order compressibility, while the Gilmore equation accounts for pressure dependent liquid density and sound speed. Boundary integral methods are required for strong deformation and jetting. Compressible multiphase computational fluid dynamics is required when shocks, viscosity, topology change, and complex wall geometry must be resolved simultaneously.

The thermodynamic effect is a dynamic feedback mechanism rather than a simple correction to vapor pressure. Evaporation cools the interface and suppresses further growth. Condensation releases latent heat, while finite condensation rates can retain vapor within the bubble and cushion collapse. As water temperature rises, saturation pressure increases sharply and collapse driving pressure decreases. Experiments in hot water, liquid nitrogen, and aqueous ammonia confirm larger minimum radii, reduced collapse velocity, weaker shocks or jets, and changes in rebound and stability. Direct measurements also demonstrate transient cooling and heating of several kelvin in the liquid immediately surrounding the bubble.

For engineering prediction, the model must be matched to the required output. Collapse time can often be estimated from Rayleigh scaling. Shock amplitude requires a compressible formulation. Jet loading requires nonspherical dynamics. Peak temperature, vapor content, and cryogenic behavior require coupled heat and mass transfer. Reliable simulations should be validated against multiple observables because agreement with radius history alone does not establish the accuracy of energy partition, internal pressure, thermal behavior, or material loading.