11:00 〜 13:00
[SVC28-P05] Flow-to-fracture transition of linear Maxwell-type/yield strength fluid by air injection; implications in magma fracture
キーワード：Viscoelasticity , Yield strength , Rheology, Ductile, Brittle, Magma
The 2011–12 eruption of Cordón Caulle in Chile exhibited hybrid explosive–effusive eruption dynamics that led to the emplacement of a large obsidian lava flow during sustained pyroclastic venting (Schipper et al., 2013). It provided a piece of evidence for the interplay of both ductile and brittle behaviors of the same magma.
The transition has been explained based on the linear Maxwell viscoelastic model; magma flows when it is deformed slowly, while it breaks in a brittle way under rapid deformation. However, the brittleness of fracture has not been quantitatively discussed. We report the experimental study of the degassing bubble dynamics through two analogous fluids of magma; an aqueous solution of cetyltrimethylammonium bromide and sodium salicylate (CTAB/NaSal) and gel-solution. We use CTAB/NaSal, which is known to have simple Maxwell-type viscoelasticity and exhibit brittle fracture. Besides, the gel-fluid is characterized by the presence of yield strength, represented by the Bingham model. Namely, it flows at large stresses instead of fracturing. We observed a wide range of dynamic behaviors of air injection depending on the injection rate, Q, and found distinct differences between the two fluids. The viscoelastic fluid (CTAB) behaved like a ductile material at low Q (< 0.5mL/s), while it became brittle at high Q (> 0.5mL/s), forming cracks growing at 40% of the shear wave speed. At the transitional Q, the bubble showed both ductile expansion and brittle fracture. We observed the increase of the number of fractures with the increase of Q. At a high flux of Q >1.25mL/s, we registered as many as three fractures in the same time scale experiment. In contrast, we observed no fracture in the yield-stress fluid (gel) at even higher Q. Brittle fragmentation was not observed, and the growth speed of bubble nose in the Bingham fluid was much lower than its shear-wave speed, estimated using the rigidity measured in the small-oscillation tests. These results will fill the gap between brittle and non-brittle fragmentations of fluids possessing different elasticities.
The transition has been explained based on the linear Maxwell viscoelastic model; magma flows when it is deformed slowly, while it breaks in a brittle way under rapid deformation. However, the brittleness of fracture has not been quantitatively discussed. We report the experimental study of the degassing bubble dynamics through two analogous fluids of magma; an aqueous solution of cetyltrimethylammonium bromide and sodium salicylate (CTAB/NaSal) and gel-solution. We use CTAB/NaSal, which is known to have simple Maxwell-type viscoelasticity and exhibit brittle fracture. Besides, the gel-fluid is characterized by the presence of yield strength, represented by the Bingham model. Namely, it flows at large stresses instead of fracturing. We observed a wide range of dynamic behaviors of air injection depending on the injection rate, Q, and found distinct differences between the two fluids. The viscoelastic fluid (CTAB) behaved like a ductile material at low Q (< 0.5mL/s), while it became brittle at high Q (> 0.5mL/s), forming cracks growing at 40% of the shear wave speed. At the transitional Q, the bubble showed both ductile expansion and brittle fracture. We observed the increase of the number of fractures with the increase of Q. At a high flux of Q >1.25mL/s, we registered as many as three fractures in the same time scale experiment. In contrast, we observed no fracture in the yield-stress fluid (gel) at even higher Q. Brittle fragmentation was not observed, and the growth speed of bubble nose in the Bingham fluid was much lower than its shear-wave speed, estimated using the rigidity measured in the small-oscillation tests. These results will fill the gap between brittle and non-brittle fragmentations of fluids possessing different elasticities.