Flow-To-Fracture Transition of Linear Maxwell-Type Versus Yield Strength Fluids by Air Injection-Implications for Magma Fracturing

To illuminate brittle and ductile fracturing of magma, we investigated bubble expansion and fracturing in two contrasting fluids: a Maxwell-type viscoelastic fluid and a Bingham-type yield-strength fluid. Measurements of the complex shear modulus, G ' + iG '' (i is the imaginary unit), under small-strain oscillation showed that both fluids are elastic (G ' > G '') with similar rigidity. Viscous behavior (G ' G '') appeared at lower frequency in the Maxwell fluid but at larger strain in the Bingham fluid. When we injected air into the Maxwell fluid, bubbles expanded viscously at low flux, fractured in a brittle manner at high flux, and behaved transitionally at intermediate flux. In contrast, we observed no fracturing in the Bingham fluid. This demonstrates that the G ' > G '' condition is insufficient to infer that brittle fracturing can occur. Brittle fracturing of the Maxwell fluid occurred not at a critical strain rate but under decreasing strain rate and increasing stress.

Automated summary

To study brittle and ductile fracturing of magma, bubble expansion and fracturing were investigated in a Maxwell-type viscoelastic fluid and a Bingham-type yield-strength fluid. Both fluids were found to be elastic, but showed different viscous behavior. In the Maxwell fluid, bubbles expanded viscously at low flux, fractured in a brittle manner at high flux, and behaved transitionally at intermediate flux. No fracturing was observed in the Bingham fluid. This suggests that the G ' > G '' condition alone is not sufficient to infer brittle fracturing.