Viscous-Flow-to-Fracture Transition in Linear Maxwell Fluids vs. Yield-Strength Materials: Implications for Magma Fracturing and Acoustic Emission

During magma ascent, the growth, ascent, and bursting of bubbles play a pivotal role, intricately linked with the rheological properties of magma. In the realm of this complexity, viscoelasticity stands out as a crucial factor influencing infrasound generation resulting from bubble bursting in volcanic systems. The linear Maxwell model, a fundamental representation of magma viscoelasticity, serves as a starting point for our investigation.

In this study, we explore acoustic wave generation induced by bubble bursting in two distinct fluids: a Maxwell-type viscoelastic fluid (CTAB) and a Bingham-type yield-strength fluid (GEL). Despite both fluids exhibiting similar rigidity in the linear elastic regime, their fracture and flow behaviors diverge significantly. The acoustic signals generated by these fluids display pronounced variations, contingent upon factors such as flux (Q) and the depth of air injection (h).

In the case of CTAB, bursting sounds are observed exclusively in the brittle regime at elevated Q, characterized by successive fractures within the fluid. At shallow injection depths, these fractures extend from the nozzle to the fluid surface, creating distinct acoustic waves. Deeper air injection results in crack growth, detachment from the nozzle, ascent to the surface, and subsequent acoustic wave generation. Interestingly, a round cavity, exclusive to small Q, does not produce acoustic waves.

In contrast, GEL consistently forms a round air cavity, with the initial bursting being silent within the same range of Q and h as observed in CTAB. Brittle fractures are notably absent, and acoustic wave generation occurs only with continued air injection, indicating that rigidity alone does not dictate bursting sound; rather, it hinges on fluid brittleness and gas-flow conditions, including rate, depth, and injection history. This experimental exploration sheds light on the nuanced interplay between fluid properties and gas-flow dynamics in the context of acoustic wave generation during bubble bursting.