Powerful Acoustogeometric Streaming from Dynamic Geometric Nonlinearity

Abstract

Past forms of acoustic streaming, named after their progenitors Eckart (1948), Schlichting (1932), and Rayleigh (1884), serve to describe fluid and particle transport phenomena from macro to micro scales. Governed by the fluid viscosity, traditional acoustic streaming arises from second-order nonlinear coupling between the fluid's density and particle velocity, with the first-order acoustic wave time-averaging to zero. We describe a form of acousto-geometric streaming that has a non-zero firstorder contribution. Experimentally discovered in nanochannels of a height commensurate with the viscous penetration depth of the fluid in the channel, it arises from nonlinear interactions between the surrounding channel deformation and the leading order acoustic pressure field, generating flow pressures three orders of magnitude greater than any known acoustically-mediated mechanism. It enables the propulsion of fluids against significant Laplace pressure, sufficient to produce 6 mm/s flow in a 130-150 nm tall nanoslit. We find quantitative agreement between theory and experiment across a variety of fluids and conditions, and identify the maximum flow rate with a channel height 1.59 times the viscous penetration depth.

Ultrasound-Responsive Systems as Components for Smart Materials

“…These observed flow rates are more than 10 times higher, and the flow pressures more than 10 3 times higher, than those predicted by any other mechanism. 394 This acoustogeometric streaming mechanism represents a unique new direction for applications requiring fast flows in nanoscale channels.…”

Sharp-edge-based acoustofluidic chip for programmable pumping, mixing, cell focusing and trapping

“…16 This streaming is a steady fluid motion due to the attenuation of acoustic oscillations near a boundary 17,18 or in the bulk of the fluid, 19 but can also stem from a dynamic geometric nonlinearity. 20 A special form of the boundary-driven streaming is the streaming near sharp edges, which can be traced back to the acoustic needle experiments of Hughes and Nyborg 21 in 1962, for the disruption of cells. Since then, acoustically excited sharp edges have proven to be a promising technology for streamingbased mixing [22][23][24][25][26][27][28] and pumping 9 of fluids, and even for acousticradiation-force-based trapping of particles and cells.…”

Manipulation and Mixing of 200 Femtoliter Droplets in Nanofluidic Channels Using MHz‐Order Surface Acoustic Waves

“…[ 32 ] We find even more powerful results at the nanoscale, with the key discovery of a new mechanism of acoustic wave‐fluid motion interaction. [ 33 ] It arises from nonlinear interactions between the surrounding channel deformation and the leading order acoustic pressure field, generating flow pressures three orders of magnitude greater than any known acoustically‐mediated mechanism.…”

“…However, a ∼100 nm height was chosen to obtain the best possible performance of the acoustic wave propagation in the nanoslit using acoustogeometric streaming. [ 33 ] Absorbers placed at the SAW LN device's ends absorb extraneous SAW and prevents undesirable reflections. c) Other configurations make it possible to explore drop splitting, mixing, and transport, such as this configuration with only one main channel of ten traps, two of which are connected to inlets at the side, and the x ‐axis‐oriented main channel is open to the outside close to the IDT while it is connected to an outlet at the distant end.…”

“…b) By etching several of these channels into an LN layer, cutting a 1 mm hole at the end distant from the SAW interdigital transducer (IDT), and then flipping the result and bonding it with room-temperature LN-LN bonding,[34] machined-side down onto the LN substrate that has the SAW IDT, it is possible to form and transport drops in these channels with heights to less than 10 nm (figures not to scale for clarity). However, a ∼100 nm height was chosen to obtain the best possible performance of the acoustic wave propagation in the nanoslit using acoustogeometric streaming [33]. Absorbers placed at the SAW LN device's ends absorb extraneous SAW and prevents undesirable reflections.…”