Fast Inertial Microfluidic Actuation and Manipulation Using Surface Acoustic Waves

Though uncommon in most microfluidic systems due to the dominance of viscous and capillary stresses, it is possible to drive microscale fluid flows with considerable inertia using surface acoustic waves (SAWs), which are nanometer order amplitude electro-elastic waves that can be generated on a piezoelectric substrate. Due to the confinement of the acoustic energy to a thin localized region along the substrate surface and its subsequent leakage into the body of liquid with which the substrate comes into contact, SAWs are an extremely efficient mechanism for driving fast microfluidics. We demonstrate that it is possible to generate a variety of efficient microfluidic flows using the SAW. For example, the SAWs can be exploited to pump liquids in microchannels or to translate free droplets typically one or two orders of magnitude faster than conventional electroosmotic or electrowetting technology. In addition, it is possible to drive strong microcentrifugation for micromixing and bioparticle concentration or separation. In the latter, rich and complex colloidal pattern formation dynamics have also been observed. At large input powers, the SAW is a powerful means for the generation of jets and atomized aerosol droplets through rapid destabilization of the parent drop interface. In the former, slender liquid jets that persist up to centimeters in length can be generated without requiring nozzles or orifices. In the latter, a monodispersed distribution of 1–10 micron diameter aerosol droplets is obtained, which can be exploited for drug delivery and encapsulation, nanoparticle synthesis, and template-free polymer array patterning.