Rheology and shear banding behavior of soft hydrogels packings in the quasistatic flow regime

The flow of granular materials is strongly affected by boundaries and thus the overall flow geometry. Granular flow can be either concentrated in narrow zones or in broad shear bands. To understand the microscopic origin of such heterogeneous flow behaviour, it is important to use a boundary-driven flow geometry in which shear bands can be well characterized. A standard setup for steady-state slow flow experiments is the split-bottom geometry with a free surface. Shear zones in such a shear cell are wide and can be well captured by an empirical non-local flow formalism. One question is how the local pressure and particle properties affect the observed shear zones and the dissipative processes inside the shear zones. We investigate here how shear bands emerge from soft, low-frictional spherical grains whose interactions and gravitational stress gradients can be tuned.

We use magnetic resonance imaging (MRI) as tomographic technique to characterize the shape of the shear zones. For these experiments, hydrogel spheres (2 mm diameter) are swollen in water. They are mixed with tracer particles: a few percent of similar spheres swollen in a dilute CuSO₄solution. The copper sulphate doped particles provide a strong MRI signal and thus serve as probes for flow velocimetry. We determine the flow field after repeated stepwise rotation of the flow-inducing boundary plate. We observe a significant difference in flow fields for slippery and sticky hydrogel spheres. Additionally, the different hydrogel particles have a very different rheological response, especially when compressed. We discuss our results in the context of existing theory for dry granular split bottom flows.