Acoustogeometric streaming for manipulation and mixing of 100 femtoliter droplets in nanoslit channels

Surface and viscous forces make manipulation and mixing of fluids difficult at the nanoscale. Finding broad adoption in microfluidics, MHz-order vibration produces even more powerful effects at the nanoscale, with the use of a new form of acoustic streaming---acoustogeometric streaming---that leads to an ability to split, merge, transport, and mix 200-fL droplets of water and other fluids. By using 40-MHz surface acoustic devices in lithium niobate bonded to a second lithium niobate layer, we are able to produce a channel from 120 nm to as little as 5 nm in height extending over 5 mm in length. The width is shaped to produce 130 micron wide droplet traps along 20 micron wide channels, Y-channels and other features. By forming traps as locally widened regions along such a channel, individual fluid droplets may be propelled from one trap to the next, split between them, mixed, and merged. A simple theory is provided to describe the mechanisms of droplet transport and splitting.