Earth, Planets and Space (Jul 2021)
Excitation of airwaves by bubble bursting in suspensions : regime transitions and implications for basaltic volcanic eruptions
Abstract
Abstract Basaltic magma becomes more viscous, solid-like (elastic), and non-Newtonian (shear-thinning, non-zero yield stress) as its crystal content increases. However, the rheological effects on bubble bursting and airwave excitation are poorly understood. Here we conduct laboratory experiments to investigate these effects by injecting a bubble of volume V into a refractive index-matched suspension consisting of non-Brownian particles (volumetric fraction $$\phi$$ ϕ ) and a Newtonian liquid. We show that depending on $$\phi$$ ϕ and V, airwaves with diverse waveforms are excited, covering a frequency band of $$f = {\mathcal {O}}(10-10^4)$$ f = O ( 10 - 10 4 ) Hz. In a suspension of $$\phi \le 0.3$$ ϕ ≤ 0.3 or in a suspension of $$\phi = 0.4$$ ϕ = 0.4 with a V smaller than critical, the bubble bursts after it forms a hemispherical cap at the surface and excites a high-frequency (HF) wave ( $$f \sim 1-2 \times 10^4$$ f ∼ 1 - 2 × 10 4 Hz) with an irregular waveform, which likely originates from film vibration. However, in a suspension of $$\phi = 0.4$$ ϕ = 0.4 and with a V larger than critical, the bubble bursts as soon as it protrudes above the surface, and its aperture opens slowly, exciting Helmholtz resonance with $$f = {\mathcal {O}}(10^3)$$ f = O ( 10 3 ) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency ( $$f = {\mathcal {O}}(10)$$ f = O ( 10 ) Hz) air flow vented from the deflating bubble, which becomes dominant at a large V. We interpret this transition as a result of the bubble film of a solid-like $$\phi = 0.4$$ ϕ = 0.4 suspension, being stretched faster than the critical strain rate such that it bursts by brittle failure. When the Helmholtz resonance is excited by a bursting bubble in a suspension whose surface level is further below the conduit rim, an air column (length L) resonance is triggered. For L larger than critical, the air column resonance continues longer than the Helmholtz resonance because the decay rate of the former becomes less than that of the latter. The experiments suggest that a bubble bursting at basaltic volcanoes commonly excites HF wave by film vibration. The Helmholtz resonance is likely to be excited under a limited condition, but if detected, it may be used to track the change of magma rheology or bubble V, where the V can be estimated from its frequency and decay rate.
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