Finite element modeling of acoustic streaming in SAW devices

Fluid motion induced from high intensity sound waves is called acoustic streaming. SAW devices used in biological species detection suffer from fouling that results from binding of non-specific protein molecules to the device surface. The acoustic streaming phenomenon can be used to remove these non-specifically bound proteins to allow reuse of SAW devices. A finite element model of acoustic streaming phenomenon is presented in this work. 2-D FE models of SAW device based on YZ-LiNbO3 with a liquid loading are modeled. Solid domain based on a micron-sized piezoelectric substrate with dimensions (1600μm propagation length x 500μm depth) was simulated to gain insights into the acoustic streaming in SAW devices. Two IDT finger pairs in each port with periodicity of 34.87 μm were defined at the surface of Y-cut, Z-propagating LiNbO3 substrate. The IDT fingers were modeled as mass-less conductors and represented by a set of nodes coupled by voltage degrees of freedom (DOF). Fluid domain is modeled as an incompressible, viscous, and Newtonian using the Navier-Stokes equation. The incompatibility of the Lagrangian frame of reference for solid modeling and Eulerian frame of reference for the fluid is overcome by using the arbitrary LagrangianEulerian (ALE) method where the mesh is constantly updated without modifying the mesh topology. To account for the fluid-solid interaction, an interface is defined across which 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 structure was simulated for a total of 100 nanoseconds (ns), with a time step of 1 ns. The excitation of the piezoelectric solid was provided by applying an AC voltage (with varying peak value and frequency of 100 MHz) on the transmitter IDT fingers. The above models are utilized to investigate methods for increasing induced acoustic streaming velocity while minimizing the effect on antibody sensing layer in immuno-SAW sensors. Parameters studied in this model include voltage intensity, and fluid viscosity. The transient solutions generated from the model are used to predict trends in acoustic streaming velocity.