This study investigates the evolution mechanism of elastic instability in a bilateral double-cavity channel through numerical simulations and microfluidic experiments. Depending on the Weissenberg number ($\text{Wi}$), the flow is categorized into three regimes: stable symmetric vortex structures at low $\text{Wi}$ ($\text{Wi}\le 4.5$); periodic corner vortices with horizontal symmetry breaking at intermediate $\text{Wi}$ ($5\le \text{Wi}\le 7.5$); and strongly asymmetric flow at high $\text{Wi}$ ($\text{Wi}\ge 8.5$). Power spectral density and correlation analyses reveal that the periodic evolution of corner vortices indicates a potential breathinglike mechanism driven by hydrodynamic interactions between cavities. At high $\text{Wi}$, enhanced elastic stresses lead to pronounced asymmetry, as corner vortices gradually expand to fill the cavities, exacerbating the flow instability. Experimentally, polyacrylamide solutions with concentrations of 100, 200, and 300 ppm were employed for validation. Fluorescent particle tracking results qualitatively support the numerical predictions, consistently revealing the instability dynamics associated with symmetry breaking. Furthermore, a two-level multiresolution dynamic mode decomposition was conducted to uncover the multiscale dynamics of the instability. Level 0 modes capture low-frequency, large-scale structures, while Level 1 modes resolve localized high-frequency oscillations in the shear layers. At $\text{Wi}=2.5$, the flow remains fully symmetric and stable, with cavity modes exhibiting high coherence. At $\text{Wi}=5$, symmetry breaking arises between the left and right cavities, and the amplitudes of unstable modes in both levels increase to $O\left({10}^{-1}\right)$, indicating that both large-scale structures and high-frequency fluctuations play significant roles. At $\text{Wi}=25$, the flow becomes strongly unstable with an increased number of modes exhibiting positive growth rates. The dominant structures display marked asymmetry in both streamwise and transverse directions, while localized high-frequency oscillations are superimposed on large-scale recirculation patterns, revealing multiscale interactions between global vortex dynamics and localized shear-layer instabilities.

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• Instability control
• Non-Newtonian fluids
• Vortex dynamics