Microscale flows of viscoelastic fluids through complex geometries exhibit rich spatiotemporal dynamics, with significant implications for microfluidic applications and porous media transport. However, current understanding of such complex flow behaviors remains limited. In this study, we investigate the unsteady flow of polymer solutions, ranging from dilute to entangled regimes, in microcavity arrays by employing streak imaging and micro-particle image velocimetry. We report a robust “multistable flow” phenomenon characterized by spontaneous, stochastic switching between a “full-vortex” and a “partial-vortex” state. By mapping the global spatiotemporal dynamics, we reveal a mechanism of “topological decoupling,” where the contraction channels effectively decorrelate the local flow dynamics, rendering the switching dynamics of neighboring cavities statistically independent. Furthermore, a quantitative phase diagram allows us to delineate two distinct instability mechanisms governed by fluid rheology. Specifically, we identify an inertio-elastic instability that follows a cooperative empirical scaling law between the Reynolds (Re) and Weissenberg numbers (Wi), Wi∝Re−0.5. Rooted in boundary-layer dynamics, this scaling reveals that inertia facilitates symmetry breaking by amplifying the local shear rate and elasticity at the channel constrictions. Conversely, when elasticity becomes highly dominant, the system transitions to a purely elastic instability, governed by a dynamic competition between elastic destabilization and shear-thinning stabilization. This competition explains the threshold paradox, where pronounced shear-thinning raises the critical energy barrier for instability. These findings provide a comprehensive physical framework linking molecular dynamics to macroscopic flow topology, offering new theoretical insights into porous media transport and microfluidic control.
Xu et al. (2026) studied this question.