Low-order lateral transducer modes enabling millimeter-scale acoustofluidics.

Bulk acoustic wave devices commonly use thickness modes of piezoelectric plates to generate standing waves in microfluidic cavities, typically at 2 MHz and above, with corresponding wavelengths well below the millimeter scale. However, handling larger mesoscopic objects-such as organoids and small model organisms, including nematodes-requires millimeter-scale fluidic structures. Single-node alignment in such cavities demands half-wavelength fields and thus sub-megahertz actuation. Conventional sub-megahertz operation requires thicker piezoelectric plates, resulting in bulky designs or requiring plate thicknesses that are not readily available commercially. Here, we demonstrate a practical yet elegant alternative based on a low-order lateral extensional mode of the transducer plate. Compared with thickness-mode devices of identical footprint, the approach lowers the operating frequency approximately fourfold and increases the channel cross section sixteenfold. Unlike typical Lamb-wave devices based on multiwavelength propagation at several megahertz and above, the present approach uses a low-order standing S0 Lamb mode. The numerically optimized devices operate at approximately 540 kHz and achieve rapid particle focusing in millimeter-wide channels, while vibrometry confirms the predicted vibration patterns and admittance measurements show that the lateral resonance is well preserved upon chip loading, with only a small, predictable shift in resonance frequency. The present study focuses on modal characterization and particle-focusing validation. Practical limitations include heating at high actuation voltages and the finite lifetime of the liquid acoustic coupling layer.