Controlled microfluidic droplet acoustoinjection on one chip

We present an all-in-one acoustofluidics device for controlled acoustic field-mediated injection of surfactant stabilized water-in-oil droplets. The microfluidic channels and interdigitated transducer (IDT) channels are produced on the same master wafer and cast within one PDMS slab, making our acoustofluidics device simple to construct while retaining the same height for all channels. The IDTs with a curved, serpentine, paired and focusing geometry are easily embedded into the PDMS slab by filling the IDT channels with low melting point metal alloy. In this article, we propose the working mechanism of our embedded IDTs, which we call acoustoinjection, and carry out a precise characterization by laser doppler vibrometry (LDV) and infrared imaging to describe the injection of droplets within microfluidic channels. Although we observe that the device has acoustic resonance in the MHz frequency domain, we show that it operates most efficiently for acoustoinjection in the kHz frequency domain. In this frequency domain, our acoustofluidics device generates a pressure wave that causes destabilization of the surfactant-supported droplet interface enabling the injection of aqueous solution into the water-phase of the droplet with minimum heat generation. We show droplet injection for different surfactant concentrations, droplet passing speeds, and injection rates with high accuracy. This integrated device has the potential to serve as an alternative to electric field mediated picoinjection technologies by acoustic field-mediated and non-harmful manipulation of droplets with bio-content.