Nanoacoustofluidics: Unprecedented fluid manipulation via controlled MHz-order vibration at the nanoscale

Abstract

Manipulation of fluids and colloids at the nanoscale is made exceptionally difficult by the dominance of surface and viscous forces. The use of MHz-order vibration has dramatically expanded in microfluidics, enabling fluid manipulation, atomization, and microscale particle/cell separation. We find even more powerful results at the nanoscale, with the key discovery of a new mechanism of acoustic wave-fluid motion interaction. We show that MHz-order surface acoustic waves (SAW) can manipulate fluids and fluid droplets within fully transparent, high-aspect ratio 5–150 nm tall, 10–100 μm wide, 5 mm long nanoslits fabricated via a direct, room temperature bonding method for lithium niobate (LN). The application of SAW causes fluid draining and pumping, developing ∼1 MPa pressure through the nanoslit structure. The mechanism of specific draining pattern is well explained by a detailed theoretical model based on the nonlinear acoustic vibration phenomenon in nanoscale channels. We also show how individual fluid droplets of only 1–10 fl are propelled by incident SAW in the nanoslit structure, entrapping them at locally widened regions along the nanoslit due to the capillary pressure difference. Our results indicate MHz-order SAW as a powerful tool for fluid manipulation in nanoscale.