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

N. Zhang, O. Manor, J. Friend
University of California, San Diego,
United States

Keywords: nanofluidics,vibration,nanofabrication,acoustofluidics


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 truly hand-held devices for atomization and drug delivery, cytometry, point-of-care medical diagnostics, and other so-called "holy grail" applications. We find even more powerful results at the nanoscale, with the key discovery of a new mechanism of acoustic wave-fluid motion interaction. We will show that 10-250 MHz surface acoustic waves (SAW) can manipulate fluids, fluid droplets, and particles, and drive irregular and chaotic fluid flow within fully transparent, high-aspect ratio 5–100 nm tall, 1–10 micron wide, 4 cm long nanoslits fabricated via a direct, room temperature bonding method for lithium niobate (LN). The application of SAW causes continuous fluid pumping, developing 1 MPa pressure through the 4 cm nanoslit structure at flow rates of ~10 µL/min regardless of end conditions and the presence of meniscii outside the nanoslit channel. Switching the SAW direction causes flow reversal. Though the hydrodynamic Reynolds number is ~10^(-7), the extremely large acceleration developed in the fluid from the acoustic irradiation, 10^8 m/s^2 or more, causes mixing within the nanofluidic structure. We also will show how individual fluid droplets of only 1–10 femtoliters may be propelled in the nanoslit structure, entrapping them at locally narrowed regions along the nanoslit through a small yet consistent local increase in the capillary pressure due to the narrowing. In this way, shuttling of individual droplets among these narrowed regions can be performed with incident SAW, and we discuss strategies for mixing the droplet's contents and combining separate droplets. Because these droplets are within the nanoslit structure, their evaporation is arrested by the small gas volume in the nanoslit and the minimal exchange of gas with the external environment. witches the flow direction and drains the channel against 1 MPa capillary pressure, and can be used to controllably manipulate ≈10 fL droplets. Finally, we describe techniques for sieving entire 10 μL droplets through the nanoslit device, providing pumpless size exclusion separation, and our latest results on doing so with a gradient in the channel height to perform gradient separations of objects from 0.1 to 10 nm in size. The extraordinary capabilities of this technology in molecular diagnostics, medicine, and chemistry will be made clear through the examples of how this device is being utilized in its prototype form.