Design of efficient focused surface acoustic wave devices for potential microfluidic applications

Focused interdigital transducers (F-IDTs) patterned on surfaces of piezoelectric substrates can be used to generate surface acoustic waves (SAW) with high intensity and high beam-width compression ratio. A three dimensional coupled field finite element model of a focused SAW (F-SAW) device with interdigital transducers shaped as concentric circular arcs based on a YZ LiNbO3 substrate is developed in this study. This model was utilized to investigate the effect of geometric shape of transducers on the focusing properties of F-IDTs to identify the optimal design for potential microfluidic applications. The transducer design parameters investigated in the current study include number of finger pairs, degree of arc, geometric focal length, and wavelength of F-SAW. The transient response of the device on application of impulse and ac electrical inputs at the transmitting FIDT fingers were utilized to deduce the device frequency response and propagation characteristics of F-SAWs, respectively. The influence of applied input voltage on the propagation characteristics is also investigated. The insertion loss calculated for the various F-IDT designs was used to identify the optimal transducer configuration for sensing and microfluidic applications. The focusing properties as well as the wave propagation characteristics for the various F-IDT designs were evaluated in terms of the amplitude field and displacement contours generated in regions close to and at the focal point. Comparison with a conventional SAW device operating at megahertz frequency range and uniform IDT design is also made. Our study indicates that the focusing property of the device is significantly influenced by the geometric shape of the F-IDTs. The streaming phenomenon induced by F-SAW propagation, when in contact with a fluid medium, is discussed in detail. The simulated amplitude fields generated using ac analysis for the various designs in conjunction with wave propagation parameters derived using perturbational techniques such as Campbell–Jones are utilized to calculate the streaming forces and velocities based on successive approximation technique applied to Navier–Stokes equation (Nyborg’s theory). The maximum streaming force and velocity are obtained at the focal point of the F-SAW device. The magnitude of the generated streaming force and induced streaming velocity are strongly influenced by the transducer configurations. Based on the simulation results of this study, we provide guidelines for designing various F-IDTs to suite desired applications. F-SAW devices operating with higher applied input voltages and at higher frequencies, with optimal geometric length and larger degree of arc, are best suited for actuation and fluid microtransport.