[O-07] Kitchen Earth Science: its potential for producing diverse goals by hands-on experiments

[O07-P04] Thermal convection of strong shear-thinning fluids

★Invited Papers

*Takatoshi Yanagisawa^1,2, Yuji Tasaka^2,1, Kohei Ohie², Daisuke Noto³, Masanori Kameyama^4,1 (1.Japan Agency for Marine-Earth Science and Technology, 2.Faculty of Engineering, Hokkaido University, 3.University of Pennsylvania, 4.Geodynamics Research Center, Ehime University)

Keywords:thermal convection, magma, rheology, shear-thinning, flow field

The behavior of thermal convection of fluids with temperature dependent viscosity is very important in the Earth science, such as mantle convection, and has been studied over a century. In case of Rayleigh-Benard convection driven by the temperature difference between the top and bottom, the dominant effect of temperature dependency of viscosity is to induce the asymmetry of top and bottom thermal boundary layers, resulting the increase of the internal temperature. Non-Newtonian effects of fluids on convection are also important, such as for magma flows, however, they are so complicated and not fully understood yet. Here we studied thermal convection of shear-thinning fluid, whose viscosity decreases with the increase of shear rate. We compared the behaviors in details with the cases of Newtonian fluid, in narrow rectangular boxes. The working fluid is xanthan gum – water solution. Xanthan gum is a kind of polysaccharide and widely used for foods. Prior to the experiment, we obtained shear rate – viscosity relations (viscosity curves) by using a high-performance rheometer. We found that even a dilute (~0.01wt%) water solution shows two orders of viscosity decrease in two orders of shear rate increase. In the experiments of convection, we observed several interesting behaviors of the flow field when the shear rate reaches the range. The dominant one is that the up- and down-welling flows between the convection cells get narrower, and the central regions of cells remain relatively at rest in the convection. We also performed numerical simulations of convection including the viscosity corves, and succeeded in reproducing the behaviors we observed in experiments.