Development of a Granular Tribo-rheometer for 4D Spatiotemporal characterization of Soft Earth suspensions under shear.

We present a new, unique design for a granular rheometer that enables 4D spatiotemporal characterization of particle rearrangement dynamics, enabling simultaneous characterization of both suspension rheology and tribology. The Couette setup design consists of two concentric plates, fixed along the axis of rotation, with a suspension system sandwiched between them. The bottom plate is rotated to tune the applied shear rate or strain. Meanwhile, the top plate allows application of constant normal load to the suspension and is free to move along the axis of rotation of the first plate while having a fixed rotation. We use model Soft Earth granular systems, an index matched mixture of PMMA particles (diameter = 1.5mm) and viscous oil. Using dye dispersed in the fluid medium, we extract inverse images of particle centroids under shear and characterize the rearrangement dynamics of the materials. The unique design coupled with optical tools enable us to estimate the effective friction coefficient of a suspension, while simultaneously measuring the rheology and particle-scale dynamics in the shear gradient direction. We present the initial results of experiments with model Soft Earth granular systems under varying load.