Excitation of airwaves by bubble bursting in suspensions : regime transitions and implications for basaltic volcanic eruptions

Kana Hashimoto,Ikuro Sumita

doi:10.1186/s40623-021-01472-7

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) and a Newtonian liquid. We show that depending on phi and V, airwaves with diverse waveforms are excited, covering a frequency band of f = {mathcal {O}}(10-10^4) Hz. In a suspension of phi le 0.3 or in a suspension of phi = 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 Hz) with an irregular waveform, which likely originates from film vibration. However, in a suspension of phi = 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) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency (f = {mathcal {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 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.

Highlights

In Strombolian and Hawaiian eruptions, bubbles burst upon surfacing and excite airwaves whose main energy is contained in the infrasonic band (Johnson and Ripepe 2011; Fee and Matoza 2013)
For runs using a suspension of φ = 0.4 and bubbles larger than critical (V ≥ 5 cm3 ), the bubble bursts while it is rising when it partially protrudes above the surface and excites Helmholtz resonance and/or air flow (Regimes III and IV)
We showed that when the fluid level is low, an air column resonance including its higher modes is triggered by bubble bursting, which itself excites resonant frequencies that differs from fA (Fig. 15)

Summary

Introduction

In Strombolian and Hawaiian eruptions, bubbles burst upon surfacing and excite airwaves whose main energy is contained in the infrasonic band (Johnson and Ripepe 2011; Fee and Matoza 2013). For runs using a suspension of φ = 0.4 and bubbles larger than critical (V ≥ 5 cm3 ), the bubble bursts while it is rising when it partially protrudes above the surface and excites Helmholtz resonance (mostly with HF wave component) and/or air flow (Regimes III and IV).

Helmholtz resonance 6a Airflow 6b Airflow

Findings

Conclusions