  Motivated by the recent demonstration of drastically different turbulent flow
dynamics of an EVP fluid by Abdelgawad et al. (Nat. Phys., 2023, 1-5.), we
analyse the linear stability of the sliding Couette flow of an EVP fluid. After
yielding, the EVP fluid behaves as an Upper Convected Maxwell (UCM) fluid. In
the creeping-flow limit, in the absence of yield stress, two discrete linearly
stable Gorodtsov and Leonov (GL) modes exist for an arbitrarily high value of
the Weissenberg number. As yield stress effects, i.e., Bingham number
increases, the GL modes become unstable, leading to the novel instability.
Analysis reveals that these modes originate near the boundaries due to the
`extra normal stress' arising from the interplay between yield stress and
elasticity. The extra normal stress is an inherent feature of an EVP fluid.
Thus, the predicted novel instability is expected to be present in wall-bounded
flows.
