Quasi-Scholte wave-based acoustofluidics: Trapping, levitation, and movement of microparticles.

Conventional acoustofluidic devices typically utilize high-frequency surface acoustic waves or Lamb waves to manipulate particles in fluid. However, since these waves possess a supersonic phase velocity (i.e., greater than the speed of sound in fluid), they tend to leak energy into the surrounding liquid, causing undesirable acoustic streaming and compromising manipulation stability. In this work, we introduce a compact acoustofluidic platform that harnesses both a non-leaky quasi-Scholte wave and a leaky Wood's anomaly mode, excited by a piezoceramic plate with a periodic electrode array. The quasi-Scholte wave-arising from the fluid-loaded plate interface-is characterized by its subsonic phase velocity and an evanescent field that is strongly confined near the surface, decaying exponentially into the fluid. This localized field generates a strong negative vertical acoustic radiation force, making it highly effective for particle trapping. In contrast, the Wood's anomaly mode, excited by periodic structural diffraction, produces a weaker but spatially periodic field that exerts a positive vertical force, enabling stable particle levitation. Through frequency tuning, our system enables dynamic switching between trapping and levitation, along with particle movement via phase modulation. Experimental results demonstrate precise, stable, and programmable control over polystyrene microparticles, making this platform energy-efficient, high-throughput, and well-suited for acoustofluidics.