Cavitation–bubble interaction with an initially perturbed free surface

The interaction of a spark-generated cavitation bubble with an initially perturbed free surface is investigated experimentally, numerically and analytically. By exploiting contact-line pinning, we accurately prescribe an initial meniscus with a thin, hydrophilic-coated rod inserted into the liquid. A pronounced surface cavity, driven by the oscillating bubble, forms and penetrates downward to a scale comparable to the bubble itself. The coupled cavity–bubble system exhibits two distinct regimes – coalescence and non-coalescence – separated by a critical condition governed by the non-dimensional stand-off parameter γ and the initial meniscus height hₘ . In the non-coalescence regime, the cavity evolves through inception, expansion and rebound/jetting. The maximum cavity length hc follows a power-law scaling hc∝ γ α with α =-2.7 (experiments) and α =-2.6 (simulations) for 1.5 γ 3 , where inertia dominates. Deviations emerge for γ 1.5 (strong nonlinearity) and γ 3 (surface tension and viscosity become noticeable). An analytical model based on the Rayleigh–Plesset equation combined with nonlinear Rayleigh–Taylor instability theory captures the trend and confirms that hₘ plays only a secondary role relative to γ . In the coalescence regime, atmospheric air vents into the bubble through the merged cavity, weakening the collapse intensity and reducing the associated pressure peak. We also examine air/liquid compressibility and boundary layer effects, whose significance grows as γ decreases. These findings are relevant to surface-jetting technologies, cavitation-erosion mitigation and underwater-noise suppression.