Plastically-induced enhancement and suppression of inertialess viscoelastic instabilities

Abstract Under certain complex flow conditions, long-chain flexible polymer solutions can simultaneously display elastic behavior under both solid and liquid states ( “elastoviscoplasticity”). When exposed to significant elastic effects and negligible plasticity (“viscoelasticity”), it is well-known that a purely elastic turbulent-like flow state emerges even in the absence of inertia (“elastic turbulence”) due to the stretching and aligning of polymer molecules. Recent numerical and experimental studies have enhanced our fundamental understanding of purely elastic instabilities under low-inertia conditions; however, the combined effects with plasticity remain poorly understood. Here, we present direct numerical simulations of inertialess elastoviscoplastic instabilities across a broad parameter space using the well-known Kolmogorov forcing problem. We discover that increasing the plastic tendencies have a dramatic effect on the qualitative flow features, whereby traveling coherent structures in the polymer stress field become destabilized and form longer streaks due to growing unyielded regions. The initial effect is an increase in the elastic flow response, which subsequently enhances flow fluctuations and promotes further mixing. However, as the unyielded regions expand to a critical limit, we observe the recently discovered jamming transition [From et al. arXiv (2025)], characterized by slow and rigid-like behavior. Here, plasticity and elasticity dominate, absorbing all of the energy, marked by a distinct change in the dissipation regime at smaller scales. Remarkably, our work demonstrates that even a small fraction of unyielded regions can “plastically induce” inertialess elastic instabilities not experienced in purely viscoelastic fluids at the same flow conditions, providing a time-and-cost-effective passive approach to enhance microfluidic mixing.