Flow visualization of microscale mixing enhancement in a liquid–liquid mixing Y-junction micromixer with acoustic streaming induced by a triangular-shaped sharp-edge spine structure

This study presents a novel Y-junction micromixer configuration that enhances mixing of high-viscosity liquids through 3D acoustic streaming flows. Micro-scale flow visualization experiments were conducted to investigate the impact of viscosity variations on liquid-liquid mixing behavior, specifically the challenges posed by high-viscosity laminar flows. The acoustic micromixer's mixing performance was evaluated using water-ethanol and ethylene glycol-water liquid mixtures, yielding a range of viscosity ratios from 1 to 8.21 between the two distinct miscible liquids. A parametric study was also conducted to explore the influence of various operating parameters, including fluid viscosity, volume flow rate, oscillation frequency, and amplitude. This study employed the digital in-line holographic microparticle tracking velocimetry (DIH-µPTV) technique to visualize the streaming flow and mercury lamp-induced epi-fluorescence mixing, and to measure scalar transport concentrations using an in-house MATLAB code. The findings indicate that the combined effects of acoustic streaming and molecular diffusion induced by concentration gradients within the liquid significantly enhance the mixing efficiency. The optimal conditions were identified at a frequency of 12 kHz, an amplitude of 34 V, a flow rate of 80 L/min, and a liquid viscosity ratio of 8.21, resulting in a significant improvement in the mixing index to 96.26% with the acoustic field, compared to 49.86% without the field. This represents 46.4% improvement in liquid mixing efficiency. These findings highlight the micromixer's potential applications in areas such as flow visualization, drug delivery, chemical synthesis, rapid fluid mixing, and material synthesis.