Abstract
Analogue materials are commonly used in volcanology to perform scaled laboratory experiments. Analogue experiments inform on fundamental fluid dynamic, structural and mechanical processes that are typically very difficult to observe and quantify directly in the natural volcanic system. Here we investigate the suitability of an aqueous solution of hydroxyethyl cellulose polymer (HEC) for use as a lava/magma analogue, with a particular focus on its rheological behaviour. We characterize a range of physical properties as functions of the concentration and temperature of the solution: density; specific heat capacity; thermal diffusivity; thermal conductivity; surface tension; as well as rheology. HEC has a non-Newtonian, shear-thinning rheology that depends on the concentration and temperature of the solution. We show that the rheology is well described by the Cross model, which was originally developed for polymer solutions, but has also been applied to bubbly magmas. Using this similarity, an approach for scaling analogue experiments that use shear-thinning polymers, like HEC, to bubbly magma is presented. A detailed workflow and a spreadsheet are provided to allow experimentalists to investigate the effects of non-Newtonian behaviour in their existing laboratory set-ups. This contribution will allow for the more complex, but often more realistic case of bubble-bearing magmas to be rigorously studied in experimental volcanology.
Highlights
Laboratory experiments offer a way to systematically observe and quantify volcanologically relevant processes that are not accessible to direct observation in the field
We focus on quantifying the polymer rheology, since that is essential for scaling any analogue experiment in fluid dynamics
hydroxyethyl cellulose polymer (HEC) density is very similar to water, we suggest that HEC will be useful for experiments that commonly use water and in which buoyancy is important
Summary
Laboratory experiments offer a way to systematically observe and quantify volcanologically relevant processes that are not accessible to direct observation in the field. Surface tension is an important parameter that forms a part of many dimensionless groups that are commonly used in experimental scaling, such as the Morton number Mo 1⁄4 gμ04=ρΓ3, which can be thought of as a material property group that captures the balance of stresses arising from viscosity, inertia, and interfacial tension, and the Eötvös number (or Bond number) Eo = ρgd2/Γ, where d [m] is the bubble or droplet diameter, which is the ratio of stresses arising from buoyancy and interfacial tension These groups are important when performing analogue experiments on bubble/gas-slug rise and magma degassing η 1⁄4 η∞ þ 1ηþ0−Àcηγ∞Áp ð23Þ Γ [mN m−1]. Previous experimental studies have reported surface tension values of: 0.35–0.38 N m−1 for basalts and andesites (Walker and Mullins Jr, 1981); 0.1–0.2 N m−1 for basalts (Khitarov, 1979); 0.052–0.068 N m−1 for Na rich phonolite (Gardner, 2012); 0.083 to N0.135 N m−1 for rhyolite (Gardner and Ketcham, 2011) and 0.043–0.062 N m−1 for hydrous dacites (Mangan, 2005)
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