Constitutive flow law for hydrogel granular rafts near the brittle-ductile transition

Spatially varying flow laws have been identified in dry granular flow, yet their applicability to unjammed suspensions remains unclear. This study demonstrates that the quasistatic suspension flow combines dry granular rheology with nonlocal effects in the shear band and damped viscous flow in the outer creep region. Through rotary shear experiments on a hydrogel granular raft, we observe that the flow decays from the interface in the quasistatic regime, where the particles remain mobile even below the yield stress. These findings suggest the universal flow law across the transition between jammed/brittle granular behavior and unjammed/ductile viscous flow.

💡 Research Summary

In this work the authors investigate the rheology of a quasi‑two‑dimensional granular raft composed of soft hydrogel spheres floating on a dense aqueous solution. By employing a transparent Couette cell, the inner cylinder is rotated at angular velocities ranging from 10⁻³ to 1 rad s⁻¹ while the torque is recorded at 2 Hz. Simultaneously, high‑speed imaging (1–10 fps) tracks the positions of every particle, allowing the construction of a full radial velocity field vθ(r). The torque data are converted into a shear stress distribution τ(r)=Γ/(2πr²) and the velocity gradients yield the local shear rate γ̇(r)=∂vθ/∂r−vθ/r.

The measured velocity profiles reveal a central shear band adjacent to the rotating wall where the tangential velocity decays exponentially, vθ∝exp