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.