Acoustically Driven Droplet Centrifugation Enabled by Frequency Operations Beyond Phononic Bandgaps

Phononic crystals (PnC) can help shape acoustic fields to enhance microfluidic functions in acoustofluidic devices. But operating within the defined phononic band gap (PBG) limits the scope for optimizing microfluidic functions. We show, for the first-time, efficient droplet centrifugation that works outside the range of the PBG. Instead of using deep reactive ion etching (DRIE) to create the PnC, we here use laser machining with the aim to demonstrate low cost and rapid prototyping of complex superstrates that can enhance particle concentration greatly. The results of this work are significant given that the center frequency of a PBG is tied to the size of the PnC. In the case of particle concentration, the efficiency increases with frequency. As such, working in the confines of a PBG requires a high level of precision in fabrication. The ability to work outside PBGs removes such restrictions, opening a range of possibilities. Besides, the results are based on a two-chip approach that yields a low-cost solution for disposable single-use acoustofluidics.