Shear localization in quasi-two-dimensional frictionless hydrogel layer controlled by packing fraction and system geometry

How do packing fraction and system geometry a↵ect shear banding, which controls the entire system’s rheological properties? In this study, we demonstrate that both increases in packing fraction and in shear channel width decrease shear band width, leading to shear localization. We conducted shear experiments on a hydrogel granular layer floating on a liquid surface in a Couette cell. A relationship between torque and rotation rate was measured and in-situ particle motions were recorded at packing fractions of 0.6–0.8 and channel width normalized with particle size of 9–33. A sudden formation of a shear band was captured at the initial yielding point. At the steady state, we tracked each particle’s motion and quantified the velocity field. Based on the velocity distribution, the measured shear band width exhibits a decrease with channel width. This systemgeometry- induced localization simply depends on channel width regardless of strain rate. High packing fraction also causes the shear localization. We also measured the flow law of the granular layer. It demonstrates the typical transition with strain rate from constant stress yielding to viscous flow. However, the shear band width is not a↵ected by the regime transition from yielding to viscously flowing.