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 $\gamma$ and the initial meniscus height $h_m$ . In the non-coalescence regime, the cavity evolves through inception, expansion and rebound/jetting. The maximum cavity length $h_{\textit{c}}$ follows a power-law scaling $h_{\textit{c}}\propto \gamma ^{\alpha }$ with $\alpha =-2.7$ (experiments) and $\alpha =-2.6$ (simulations) for $1.5\lesssim \gamma \lesssim 3$ , where inertia dominates. Deviations emerge for $\gamma \lesssim 1.5$ (strong nonlinearity) and $\gamma \gtrsim 3$ (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 $h_m$ plays only a secondary role relative to $\gamma$ . 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 $\gamma$ decreases. These findings are relevant to surface-jetting technologies, cavitation-erosion mitigation and underwater-noise suppression.