Enhancement in ultrasonic micro-transport using focused inter-digital transducers in a surface acoustic wave device: Fluid-structure interaction study

Biosensing and microfluidic applications of surface acoustic wave (SAW) devices, involving micro-transport, rely on acoustic streaming resulting from high intensity sound waves interacting with the fluid medium. In this work, we investigate the enhancement in the efficiency of acoustic streaming via the use of interdigital transducer (IDT) modifications, viz. focused IDTs (F-IDTs). We have developed, for the first time, a three dimensional bi-directionally coupled fluid-structure interaction finite element model of a focused SAW (F-SAW) device with F-IDTs based on concentric wave surfaces and subject to liquid loading. The simulated device displacement profiles indicate focusing and enhancement of surface displacement amplitudes, instantaneous fluid velocities, and streaming velocities in a F-SAW device compared to a conventional SAW device having uniform IDTs with a similar size, finger periodicity and applied input voltage. Furthermore, the F-SAW device brings about focusing of acoustic energy near the center of the device, thereby enhancing the device displacements and fluid velocities in the center of the delay path in contrast to the conventional SAW in which these quantities decay on moving away from input IDTs towards the device center. Thus, our results indicate enhancement in acoustic streaming induced flow in F-SAW devices compared to conventional SAW. The results have a general applicability for various biosensing and microfluidic actuation applications that rely on the acoustic streaming phenomenon.