Liquid droplets have been studied for decades and have recently experienced renewed attention as a simplified model for numerous fascinating physical phenomena occurring on size scales from the cell nucleus to stellar black holes. Researchers from Duke University have developed an acoustofluidic centrifugation technique that leverages an entanglement of acoustic wave actuation and the spin of a fluidic droplet to enable nanoparticle enrichment and separation. By combining acoustic streaming and droplet spinning, rapid (<1 min) nanoparticle concentration and size-based separation are achieved with a resolution sufficient to identify and isolate exosome subpopulations. The underlying physical mechanisms have been characterized both numerically and experimentally, and the ability to process biological samples (including DNA segments and exosome subpopulations) has been successfully demonstrated. Together, this acoustofluidic centrifuge overcomes existing limitations in the manipulation of nanoscale (<100 nm) bioparticles and can be valuable for various applications in the fields of biology, chemistry, engineering, material science, and medicine.
(A) Illustration of the acoustofluidic centrifuge system. The droplet is placed on a PDMS ring that confines the fluid boundary and is located between two slanted IDTs. As the SAWs propagate into the droplet, the liquid-air interface is deformed by the acoustic radiation pressure, and the droplet starts to spin. Particles inside the droplet will follow helical trajectories (inset) under the influence of both induced vortex streaming and the spinning droplet. (B) A sequence of images showing the side view of a 30-μl rotating droplet. The SAW is activated at 0 s. The sequence shows that as the droplet starts spinning, it stretches out to a concave ellipsoid shape, as illustrated in (A). Yellow arrow indicates the reference position that rotates along with the spinning droplet.