Single cavitation bubbles collapsing very close to a solid wall form a torus that collapses at two intense spots — the regime responsible for cavitation erosion. A 2026 study (Mur, Petkovšek and Ohl) reported the first observation that these highly asymmetric collapses also emit sonoluminescence, detectable only in a narrow stand-off window (0.1<γ<0.30.1<\gamma<0.3), localized exclusively at the two collapse spots, yet statistically uncorrelated per event with both surface erosion and emitted shock-wave energy. The open problem is to provide a mechanism-level account of this phenomenon: by what physical pathway does a strongly non-spherical, toroidal near-wall collapse compress the bubble content sufficiently to emit visible light, and how does this in-bubble energy-focusing pathway relate to the in-liquid pathways (shock-wave self-focusing and surface Rayleigh waves) that damage the surface — in particular, why the two phenomena are co-localized on average yet statistically decoupled event by event.

Background

Cavitation is the formation and violent collapse of vapor bubbles in a liquid; during collapse, the liquid's kinetic energy is focused by many orders of magnitude, emitting shock waves and, in some conditions, light. Sonoluminescence is this light emission from a collapsing bubble, conventionally understood as arising when the compressed gas and vapor inside the bubble reach temperatures of several thousand kelvin and pressures 10310^3–10410^4 times atmospheric on sub-nanosecond timescales. The emblematic case, single-bubble sonoluminescence, involves a bubble trapped at an acoustic antinode that collapses nearly spherically in each cycle, and even for this best-controlled case the exact emission microphysics (hot dense gas versus weak plasma, bremsstrahlung versus recombination radiation) remains debated (Brenner, Hilgenfeldt and Lohse, Rev. Mod. Phys. 74, 425, 2002). A long-standing experimental picture holds that near-sphericity is essential: luminescence from laser-induced bubbles drops by orders of magnitude or vanishes as the collapse becomes aspherical (Ohl, Lindau and Lauterborn, Phys. Rev. Lett. 80, 393, 1998; Ohl, Phys. Fluids 14, 2700, 2002); near rigid boundaries, measured spectra persist but the fitted interior temperature decreases as the bubble nears the wall (Brujan and Williams, Phys. Rev. E 72, 016304, 2005); and bubble deformation by a uniform pressure gradient quenches single-collapse luminescence beyond a threshold of the anisotropy parameter ζ\zeta (the dimensionless Kelvin impulse, i.e. the normalized liquid momentum imparted to the bubble), which coincides with the point where the micro-jet pierces the bubble (Supponen et al., Phys. Rev. E 96, 033114, 2017). Here a micro-jet is the thin liquid column that traverses the bubble when it collapses asymmetrically, for instance near a wall.

Separately, the same wall-proximate collapses are the classical agents of cavitation erosion. The key geometric parameter is the stand-off distance γ=h/Rmax⁡\gamma=h/R_{\max}, the distance hh of the bubble's nucleation site from the boundary divided by the maximum bubble radius Rmax⁡R_{\max} measured perpendicular to the surface. For very small γ\gamma, the bubble collapses into a torus with two strong collapse spots, and erosion has been attributed to self-focusing of the emitted shock waves (Reuter, Deiter and Ohl, Ultrason. Sonochem. 90, 106131, 2022), to nanosecond shock-wave-induced surface acoustic (Rayleigh) waves at the liquid–solid interface (Zhang et al., Phys. Rev. Res. 1, 033068, 2019; Gutiérrez-Hernández et al., Phys. Fluids 34, 112013, 2022), with single-bubble erosion phenomenology established by Philipp and Lauterborn (J. Fluid Mech. 361, 75, 1998). Rayleigh (surface) waves are elastic waves guided along the solid surface that can concentrate stress at the interface.

Mur, Petkovšek and Ohl (Ultrason. Sonochem. 128, 107815, 2026) brought these two strands together with multi-modal single-bubble measurements (ultra-high-speed bubble dynamics, nanosecond-timed single-photon imaging, shock-wave pressure imaging, and per-event erosion metrology). They found that highly asymmetric toroidal collapses near surfaces do emit light — detectable only for γ<0.3\gamma<0.3, maximal at γ=0.12\gamma=0.12–0.150.15 for aluminum, quartz and glass boundaries, and below detection for γ≥0.5\gamma\geq0.5 — with emission occurring exclusively at the sites and times of the most intense flow focusing (the two collapse spots). Yet, event by event, the occurrence of light emission is not statistically correlated with erosion or with emitted shock-wave energy, although the shape and dynamic conditions for emission coincide with those for strong erosion. The authors conclude that the energy focusing inside the bubble (which produces photons) and in the liquid (which produces shocks and damage) proceed through distinct pathways, and they speculate that Rayleigh waves excited by the shock waves drive the erosion. This observation contradicts the expectation, built on the quenching results above, that strong asphericity precludes sonoluminescence; no mechanism has been proposed or simulated for emission in this regime. Related but distinct precedents exist for light from violently asymmetric cavity collapse driven by an external shock (Bourne and Field, Phil. Trans. R. Soc. A 357, 295, 1999), which do not address the wall-proximate, atmospheric-pressure-driven toroidal collapse considered here.

Problem Statement

By what physical mechanism does a highly asymmetric, toroidal cavitation-bubble collapse near a solid boundary — the regime that erodes the surface — compress the bubble content sufficiently to emit visible light, and how does this in-bubble energy-focusing pathway relate to the in-liquid pathways (shock-wave self-focusing and surface Rayleigh waves) that damage the boundary? A complete answer should provide a quantitative, physically grounded account — a validated model, resolved simulation, or discriminating experimental characterization — that (i) explains why emission arises only in a narrow stand-off window, with a maximum around γ=0.12\gamma=0.12–0.150.15 and detection only for γ<0.3\gamma<0.3, and why it is localized exclusively at the two torus collapse spots where flow focusing is most intense; (ii) accounts for the reported absence of per-event correlation between light emission, emitted shock-wave energy, and erosion, despite both phenomena occurring under very similar bubble shape and dynamics conditions; and (iii) reconciles the observation with the established picture that bubble asphericity quenches sonoluminescence (Ohl 1998, 2002; Brujan and Williams 2005; Supponen et al. 2017). The account should also clarify how far these relations extend to more generalized strong collapses near surfaces, the direction of further work explicitly identified by the source study.

References

  1. Jaka Mur, Rok Petkovsek, Claus-Dieter Ohl, 'Sonoluminescence from single cavitation bubbles near solid surfaces', Ultrasonics Sonochemistry 128 (2026) 107815, DOI 10.1016/j.ultsonch.2026.107815, https://doi.org/10.1016/j.ultsonch.2026.107815 (open access; full text also at PMC13018995)
  2. C. D. Ohl, O. Lindau, W. Lauterborn, 'Luminescence from Spherically and Aspherically Collapsing Laser Induced Bubbles', Physical Review Letters 80 (1998) 393–396, DOI 10.1103/PhysRevLett.80.393, https://doi.org/10.1103/PhysRevLett.80.393
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