Parameter optimization of interdigital transducers for high-performance acoustofluidic devices

Microfluidic actuators based on surface acoustic waves (SAWs) typically operate at frequencies below 100 MHz, often neglecting the effect of interdigital transducer (IDT) parameters on device size, cost, and actuation performance. In this paper, we aim to optimize IDT parameters to improve fluidic actuation performance while maintaining a compact size, thereby enhancing the space utilization and reducing manufacturing cost. We systematically investigate the effects of IDT parameters, including wavelength, acoustic aperture, and the number of electrode pairs on fluidic actuation and explore the underlying mechanisms. Results show that increasing the number of electrode pairs and wavelength, or reducing the acoustic aperture width, enhances fluidic actuation performance. Additionally, fluid behaviors vary significantly with frequency. Above 80 MHz, the droplet pumping exhibits a jumping motion that requires a higher power, while jetting resembles the launch of liquid droplet projectile. Below 62 MHz, the droplet pumping combines a rolling and sliding motion, with jetting following a continuous water column along the Rayleigh angle. Moreover, the ejected liquid column size is determined by the acoustic aperture width when the droplet size exceeds the aperture width. Based on these findings, we propose an optimized IDT design guideline: a wavelength range of 64 to 80 µm, 40 to 60 electrode pairs, and an acoustic aperture width of 4 to 6 mm, to achieve optimal fluidic actuation performance while maintaining a compact size for most biomedical applications.