Surface acoustic wave sensors detect chemical and biological species by monitoring the shifts in frequency of surface acoustic waves generated on piezoelectric substrates. These devices are conveniently small, relatively inexpensive and quite sensitive. Considerable attention has been focused on the development of response models to understand the characteristics of surface acoustic waves generated in SAW devices. Most of the analytical techniques require simplification of second order effects such as backscattering, charge distribution, diffraction and mechanical loading. Our previous investigations involved development of structural models for SAW gas sensors based on LiNbO3 substrate. In this work, we report the development of a fluid-structure interaction model of a SAW biosensor based on LiTaO3 substrate.

In the current work, a SAW device based on 36° YX-LiTaO3 with a liquid loading was modeled to gain insights into the acoustic streaming phenomenon (Fig. 1). The dimensions of the piezoelectric substrate were 400μm width x 500μm propagation length x 200μm depth was simulated to gain insights into the acoustic streaming in SAW devices. Two IDT finger pairs in each port were defined at the surface of 36° YX LiTaO3 substrate. The fingers were defined with periodicity of 40 μm and aperture width of 200 μm. The IDT fingers were modeled as mass-less conductors and represented by a set of nodes coupled by voltage degrees of freedom (DOF). The model was meshed with tetragonal solid elements with four degrees of freedom, three of them being the three translations and the fourth being the voltage.

Fluid is modeled as incompressible, viscous, and Newtonian using the Navier-Stokes equation. In modeling fluid-solid interaction, a purely Lagrangian frame is incapable of dealing with strong distortions of the fluid mesh. A purely Eulerian frame for the fluid domain introduces complexity in fluid-solid coupling. Therefore, mixed Lagrangian-Eulerian or Arbitrary Lagrangian Eulerian (ALE) methods are used for kinematical description of the fluid domain. The Eulerian description is used for ‘almost contained' flows and Lagrangian description is used for regions where the mesh would be highly distorted if required to follow fluid motion. The mesh is constantly updated without modifying the mesh topology. To achieve bidirectional fluid structure coupling, stress and displacement continuity are maintained across the fluid-structure interface. To achieve this, displacements are transferred from solid to fluid and pressure from fluid to solid. The fluid mesh is continuously updated as the piezoelectric substrate undergoes deformation. The Standard k-ε Model is used to study flow in the turbulent regime. The simulation was carried out for 100 nanoseconds (ns), with a time step of 1 ns. The excitation of the piezoelectric solid was provided by applying an AC voltage (with a peak value of 2.5 V and frequency of100 MHz) on the transmitter IDT fingers.

The shear horizontal acoustic wave propagation characteristics as well as its interaction with the liquid medium in a SAW biosensor are studied in detail. The power consumption and device sensitivity in SAW biosensors with guiding layers, groves and microcavities would be calculated based on the simulation data. The devices are optimized to produce the best sensitivity and least power consumption and are expected to contribute to the next generation of the combined removal/sensing device.