Acoustofluidic polymer micropillars for efficient micromixing and nanoparticle synthesis

Abstract

Acoustofluidics has emerged as a promising field for particle manipulation and micromixing in lab-on-a-chip technologies. Among the various materials utilized in microfluidics, cyclic olefin copolymer (COC) is increasingly employed due to its excellent physical and chemical properties, low cost, and versatile fabrication methods. In this work, we demonstrated a unique approach to acoustic mixing by integrating polydimethylsiloxane (PDMS) and COC microchannels with oscillating COC micropillars. These micropillars, fabricated via a COC-hydrocarbon solvent swelling microfabrication process, protrude from the COC surface and oscillate out-of-plane when actuated by a piezoelectric transducer, generating vortices that enhance mixing. A 3D numerical model was employed to optimize micropillar geometry through a parametric study of diameter and height. Both simulation and experimental results showed that smaller diameters and greater heights enhance mixing performance, with the 20 µm diameter and height configuration outperforming other designs. This geometry achieved effective mixing up to 900 µL/h (15 µL/min) and maintained observable effects even at 6000 µL/h (100 µL/min). The numerical model was validated against experimental data, with <5 % error. Finally, the PDMS-COC micropillar microchannel was successfully employed for the synthesis of gold nanoparticles (AuNPs) using acoustically assisted mixing. The innovative system enabled the synthesis of high-concentration AuNPs with average sizes ranging from 15 to 35 nm and polydispersity index (PDI) <0.36. These results demonstrate the effectiveness of the micropillar-based microchannel for nanoparticle synthesis and highlight its potential for broader applications in lab-on-a-chip mixing technologies.