Refractive-Index-Matched Fluorescent Particle Image Velocimetry for Investigation of Flow in Immersed Granular Materials

Abstract

Refractive-Index-Matched Fluorescent Particle Image Velocimetry for Investigation of Flow in Immersed Granular Materials The ubiquity of granular materials and their importance in everyday life, industry, and nature make their study imperative for proper manipulation of the flow. As granular materials are opaque, optical imaging experiments are limited to studies at the free surface or transparent boundaries of the flow. Though techniques like magnetic resonance imaging and X-ray tomography have been used to obtain information from the interior of the flow system, these studies are limited to a small class of granular materials. Therefore, a modified yet simple optical imaging technique that can access the bulk of the flow is desirable. Our study employs refractive-index-matched fluorescent particle image velocimetry (RIM FPIV) to obtain information about the bulk of the flow. In RIM FPIV, the system of dry granular materials is made transparent by adding a suitable liquid whose refractive index is close to that of the particles (glass beads). The addition of liquid results in immersed granular materials, in which the particles are completely submerged in the liquid. Reported literature indicates that immersed granular materials exhibit shear banding and rate-independent rheology at slow shear rates, just like dry granular materials. A recent study using discrete element method (DEM) simulations and experiments reports a dilation-driven vortex in dry granular materials in the slow flow regime. However, the opacity of the granular materials restricted the experimental studies to visualizing only the free surface of the flow. By utilizing the similarities between the two media, our study on an immersed granular medium could give us insight into the flow behaviour in the bulk of dry granular systems. In this thesis, we experimentally study the flow of immersed granular materials and compare our observations with that of dry granular media. We perform experiments on slowly sheared immersed granular materials in a cylindrical Couette and present velocity profiles for the free surface using conventional PIV and the bulk of the flow using RIM FPIV. We observe broadening of the shear band in immersed granular materials and explain how the presence of fluid in interstitial spaces is responsible for such behaviour. We also report increased radial flow in the free surface of immersed granular materials due to enhancement of dilation by the fluid. In our RIM FPIV experiments, the velocity profiles in the bulk of the flow show radially inward flow close to the free surface and radially outward flow close to the bottom of the Couette cell. This observation is consistent with the simulations previously performed on a system of dry granular materials, indicating a circulating axial flow. Therefore, this study is a good starting point for experimentally validating the secondary axial flow in the bulk of both immersed and dry granular materials.