Trapping of Aqueous Droplets under Surface Acoustic Wave-Driven Streaming in Oil-Filled Microwells

Microwell arrays are ideal platforms for cellculturing, cell separation, and low-volume liquid handling. Theability to manipulate droplets in microwells could open up theopportunity for developing new biochemical assays. Here, we studythe trapping of aqueous droplets in an oil-filled microwell drivenby the application of nanometer amplitude vibrations called surfaceacoustic waves (SAW). We elucidate the dynamics of the dropletwithin the vortex toward thefinal trapping location and the physicsof the trapping phenomenon using a theoretical model byconsidering the relevant forces. Our study revealed that thecombined effect of acoustic radiation and hydrodynamic forces leads to droplet migration and trapping. We demarcate the trappingand nontrapping regimes in terms of the minimum critical input power required for the trapping of droplets of different sizes anddensities. Wefind that the critical power varies as the square of the droplet size and is higher for a denser droplet. The effects ofinput power and droplet size on the trapping location and trapping time are also studied.