Trapping nanoscale particles via quasi-Scholte mode in acoustofluidics

Non-contact and label-free acoustic manipulation of particles is crucial for various applications ranging from cell separation and tissue engineering to micromachining and nanofabrication. Surface acoustic waves (SAWs) have been widely used for microscale particle manipulation; their leaky nature in liquid often generates significant bulk acoustic streaming that undermines stable trapping of nanoscale particles. To address this challenge, we introduce an acoustofluidic device comprising a zinc oxide (ZnO) thin film deposited on aluminum foil with one-sided water loading. This design excites quasi-Scholte waves, a specialized nonleaky mode confined to the fluid-solid interface, which effectively suppresses bulk streaming and enables stable nanoparticle trapping. Both theoretical modeling and experiments confirm that the resulting strongly evanescent field operated at 5.11 MHz generates negative vertical forces and strong lateral (in-plane) trapping forces, successfully trapping 250 nm-radius particles on the foil surface. As the particle radius decreases to 150 nm, streaming-induced drag becomes the dominant manipulation mechanism. Operable at low frequencies with a simple and scalable design, our platform offers a versatile route for precise nanoscale particle trapping, with significant potential for bioengineering and nanofabrication applications.