Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-022-05252-5

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s40623-020-01320-0

Language：English   Publishing type：Research paper (scientific journal)  

In this study, we examined the rheology of polyurethane foam (PUF) as an analogue system of magma undergoing vesiculation, deformation, and solidification. Solid PUF was formed by mixing two polymeric liquids, followed by chemical reactions of foaming and curing. The transient rheology during the processes was investigated using an instrument based on a stress-controlled rheometer. First, the oscillatory rheology was measured applying small-amplitude oscillations with cyclic frequency sweeps. In the foaming stage, the frequency-dependence of the rheology depends on the dynamic capillary number in a similar way to the existing bubbly flow model. In the curing stage, the viscoelasticity of the liquid becomes dominant, and the rheology of PUF shows Maxwell-type viscoelasticity with relaxation time exponentially increasing with time. When the reaction is rapid because of high temperature, the viscoelasticity during curing is not scaled by the single relaxation time. Instead, PUF passes through a condition called gelation, which is characterized by a power-law relaxation spectrum. Next, we examined the relationship between the dynamic viscosity measured by the small-strain oscillation and the shear viscosity measured with a fixed shear rate at a large strain. We found that PUF obeys the Cox-Merz rule, which states that the two viscosities are equivalent when the shear rate and the angular frequency are equal. Specifically, the shear viscosity tends to be smaller than the dynamic viscosity by a few tens of percent. The difference was shown to be in the range expected for bubbly fluid in the foaming stage, while it went beyond what can be explained by the bubbly fluid model when the viscoelasticity of the liquid becomes significant. X-ray computed tomography analyses revealed that samples in these cases contained large bubbles, indicating bubble coalescence. We infer that the difference is caused by bubble coalescence as well as non-Newtonian rheology during solidification, including gelation. All these processes might occur in actual magma during eruptions. We show that the ranges of the capillary number comparing the deformation time and the bubble shape relaxation time and Deborah number comparing the deformation time and the viscoelastic relaxation time in our experiments are comparable with their ranges expected for magma ascending in an eruptive conduit. The characteristic time of viscosity increase of PUF was also shown to be realizable for magma in natural volcanoes. We conclude that PUF is useful for simulating magma processes in eruptive conduits. (C) 2020 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jvolgeores.2020.106771

Language：English   Publishing type：Research paper (scientific journal)  

Tube pumice, the bubbles of which are highly elongated in one direction, is a common product of explosive silicic eruptions, especially caldera-forming eruptions. To understand the conditions necessary to generate tube-like bubbles, we performed extensional experiments on polyurethane foam, which experiences vesiculation, deformation, and solidification at room temperature. The strain and strain rate of pure shear deformation were controlled by the pull length and pull rate of the extension, respectively. The shapes of bubbles in the solidified polyurethane foam were analyzed in three dimensions by X-ray computed tomography. The experimental results demonstrate that strain and capillary number control the degree of bubble deformation. Highly deformed (tube-like) bubbles were found in the experiments with large strain and large capillary number. In each experimental sample, there is scatter in the relation between the degrees of bubble deformation and expected capillary numbers, which we interpret to be the result of the inhomogeneous shear field around individual bubbles and bubble coalescence. Despite the large scatter, the average deformation degree is predicted well by the dynamics of a single bubble, where the viscous stress acting on the bubble surface is taken as the product of bulk strain rate and effective viscosity. Using this approach, the average deformation degree of experimental bubbles in each sample was approximately calculated from the deformation model of a single bubble at least in the range of pure shear strain smaller than 1.5. Small bubbles in the samples extended at low viscosities tend to be less elongated than those calculated from the deformation model, due to shape relaxation. The effect of shape relaxation is quantified by a new dimensionless number, i.e., the quench number, which represents the ratio of the timescale of increasing viscosity and shape relaxation. Our experiment suggests that bubble shapes in highly vesicular pumice can be used to analyze their viscosity and strain rate during the elongation process. An analysis of the published data was presented as a test case. (C) 2020 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.jvolgeores.2020.106772

Event date： 2018.9

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Event date： 2018.9

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Event date： 2018.8

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Event date： 2018.5

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Event date： 2017.9

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Event date： 2017.8

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Event date： 2017.5

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Event date： 2016.10

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Event date： 2016.9

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Event date： 2016.6 - 2016.7

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Event date： 2016.5

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Event date： 2015.9

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Experimental study on drainage process of liquid film between bubbles during bubble growth

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Experimental study on the coalescence of two growing bubbles in three-dimensional space

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Textural implications for the magma mixing during the Waimihia eruption, Taupo