Identify the general physical mechanism of the elasto-viscoplastic instability

Identify a simple mechanism for the short-wavelength instability of elasto-viscoplastic flows under general physical conditions, including both circular Couette and pressure-driven configurations.

Background

The study shows that instability can occur in inertialess elasto-viscoplastic circular Couette flow, with the most unstable modes localized near pseudo-plug boundaries or near the outer wall in fully yielded low-shear regions. The authors note that this localization is also observed in their earlier work on elasto-viscoplastic Poiseuille flow.

Despite the common features across these geometries, the paper does not establish a general physical explanation for the instability. Such a mechanism would help determine whether the short-wave behavior is intrinsic to elasto-viscoplastic materials or instead reflects details of the Saramito constitutive law.