Microfluidic Mixing Mediated by Acoustic Streaming around Microscale Obstacles*

Microscale mixing is an important chemical process that facilitates reactions for biomedical diagnostics and drug development.  In this work, we present the application of a microfluidic platform that utilizes acoustic streaming as a mechanism for fluid mixing.  The platform features a Y-channel configuration with circular obstacles embedded within the channel.  A piezoelectric transducer, driven at a frequency of 5.6 kHz and varying voltage levels (15 V – 70 V), is used to generate streaming about the circular obstacles that induce mixing within the channel.  Fluorescent polystyrene particles are used to visualize the mixing process and evaluate the local flow field via microscopic particle image velocimetry (uPIV).  The polystyrene particles are also used to assess the quality of the mixing process for flow rates ranging from 100 uL/hr to 400 uL/hr.  The mixing process mediated by the acoustic streaming about the circular obstacles is compared to the case where the obstacles are triangular in structure.  Although the triangular obstacles generate stronger vortices in comparison to the circular obstacles, we find that the circular obstacles are more efficient at mixing the fluid due to localized trapping at the triangle corners.  Lastly, we introduced a polymer solution consisting of Methoxy poly(ethylene glycol)-block-poly(ε-caprolactone), a block polymer to test if the mixing process will result in flash precipitation. When an organic stream of block polymer is mixed with water under acoustic conditions, ~200 nm diameter polymer nanoparticles are created. Thus, ultimately demonstrating the applicability of this platform as a microfluidic reactor.