Microfluidic mixing by micropost-driven acoustic microstreaming: effects of micropost shape, actuation voltage, and fluid flow rate

Abstract

We describe an approach to enhancing microfluidic mixing by generating acoustic microstreaming flows around microposts in a microfluidic device. Specifically, we synthesize microposts with various cross-sectional shapes (i.e., circles, triangles, and stars) using photocrosslinkable polymers, allowing for precise control over their geometry. We also ensure unobstructed micropost vibration via carefully designed gaps between the microposts and the channel ceiling. Experimental findings reveal that the shape of microposts is critical in influencing microstreaming patterns and mixing efficiency. Circular microposts generate semi-symmetrical circular vortices, resulting in superior mixing performance (86.7%). In contrast, star-shaped microposts, despite having sharper edges and forming pairs of microvortices around their vertices, produce the lowest mixing performance (56.5%). This trend correlates with the microposts’ moment of inertia (MOI); circular posts exhibit the lowest MOI and thus oscillate more readily, whereas star-shaped posts are geometrically more resistant to bending, limiting vibration amplitude and reducing streaming strength. Further characterization of the microstreaming flow patterns in a static aqueous solution reveals that the lower mixing performance of star-shaped micropillars is likely due to the impact of the spacing between the microposts and the emergence of counter-rotating pairs of microvortices, leading to destructive interference. Triangular microposts exhibit moderate mixing performance, generating a pair of opposing vortices around each vertex. Increasing the actuation voltage and reducing the flow rates further improves mixing across all micropost shapes. These findings highlight the significance of micropost design and arrangement in enhancing the performance of microfluidic acoustic mixers.