Acousto-viscoelastic synergy in sharp-edge micromixers: Hydrodynamic mechanisms and mixing intensification.

While microscale sharp-edge acoustic streaming is relatively effective in Newtonian fluids, its interaction with complex viscoelastic fluids and the intricate hydrodynamic mechanisms remain underexplored. To elucidate this complex hydrodynamics, we developed a 3D fully coupled acousto-viscoelastic multiphysics numerical model, corroborated by fluorescence and micro-particle tracking experiments. By integrating the Oldroyd-B constitutive model with acoustic perturbation and Generalized Lagrangian Mean theories, our model successfully achieves precise decoupling of the background flow, acoustic response, and acoustic streaming. The study reveals that the morphology and intensity of acoustic streaming are modulated by the viscoelastic background flow. Under strong acoustic fields, streaming vortices in Newtonian fluids are confined near the sharp edge. In contrast, viscoelastic fluids trigger an "elastic vortex enhancement effect" upon strong acoustic excitation, transforming the acoustic vortices from localized to full-field structures, thereby achieving mixing performance that surpasses that of Newtonian fluids. Simulation results demonstrate that the acoustic field heavily stretches the long polymer chains, generating an "elastic hoop stress" driven by the first normal stress difference (N1). This stress enables the local counter-rotating vortices to breach boundary-layer confinement and expand toward the channel center, yielding stronger mixing effects. To quantitatively characterize this multiphysics competition, we propose a dimensionless Acousto-Viscoelastic Coupling Factor (Kave) and construct a flow regime map. Ultimately, we deduce a strict quadratic scaling between driving voltage and flow rate (Vpp2∝U) for optimal mixing. This finding provides fundamental theoretical criteria for the structural design and operational control of advanced viscoelastic micromixers.