Unapodization: a Method to Produce Laterally Uniform Surface Acoustic Waves for Acoustofluidics

Surface acoustic waves (SAW) have underpinned many acoustofluidic applications. But the typical straight, full-width interdigital transducers used to generate these waves are problematic. Fresnel diffraction of the propagating SAW causes strong nonuniformity in the wave's amplitude as it propagates across the substrate, leading to flow variations, recirculation, and peculiar wetting phenomena in fluids driven by such waves. These have led to many research projects and publications, but, ultimately, uniform fluid flow from SAW-driven streaming remains elusive. By adopting a form of apodization, the shaping of the IDT, we demonstrate the ability to produce laterally uniform SAW for acoustofluidics. Unlike traditional apodization, which shapes the IDT to produce a desired electrical signal, this new approach-unapodization-instead inverts the governing equations to produce a desired spatial distribution of SAW at a defined position along the propagation direction of the SAW. Theoretical analysis and experiment demonstrate a 50% improvement in the SAW uniformity over a straight IDT for three example unapodized designs, all while maintaining the same frequency, input power, quality factor, and electromechanical coupling. The sole downside is a 50% increase in IDT area to accommodate the unapodization. A design tool is provided with tabulated unapodized IDT configurations. To illustrate the potential of unapodization in acoustofluidics, the irregular dewetting and anomalous flows of a thin fluid film induced by traditional straight IDT SAW devices is shown to become regular and uniform with the adoption of unapodized SAW devices.