Enhancement of Acoustic Streaming Induced Flow on a Focused SAW Device: Implications for Biosensing and Microfluidics
Reetu Singh, Department of Chemical and Biomedical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620, Subramanian K.R.S. Sankaranarayanan, School of Engineering and Applied Sciences, Harvard University, 9 & 15 Oxford Street, Cambridge, MA 02138, and Venkat Bhethanabotla, Department of Chemical Engineering, University of South Florida, ENB 118, 4202 E. Fowler Ave., Tampa, FL 33620.
Abstract: I. Motivation/Background Surface acoustic wave (SAW) devices (Fig. 1) can be used for detection of multiple biomarkers in body fluids. Our focus is on detection of ovarian cancer biomarkers. However, when used for biomarker detection in fluid media, these SAW devices suffer from the problem of non-specific protein binding which severely limits their sensitivity and bio-analyte discrimination capabilities. Fluid motion induced from high intensity sound waves, known as acoustic streaming, can be used to remove these non-specifically bound proteins to allow their reuse. Focused interdigital transducers (FIDTs) based on concentric wave surfaces can excite surface acoustic waves (SAW) with high intensity, high beam-width compression ratio and small localized area. The excited waves can be utilized to enhance the streaming induced removal of fouling proteins leading to highly sensitive biosensors. In the present work, we have developed a 3-D finite element fluid solid interaction (FE-FSI) model to investigate and analyze the streaming velocity fields and forces induced by SAW device with FIDTs based on concentric wave surfaces. The acoustic streaming enhancement brought about by the focused SAW device is analyzed by comparison with a conventional SAW device having uniform IDTs. II. Computational details The focused SAW (F-SAW) device was constructed by adopting a pair of concentrically shaped FIDTs on piezoelectric substrates such as LiNbO3 and LiTaO3 as shown in Fig 1. The 3-D structural model describes two-port FIDT structures on the surface of YZ LiNbO3 and consists of three finger pairs in each port. The fingers are considered as mass-less electrodes to ignore the second-order effects arising from electrode mass, thereby simplifying computation. The periodicity of the finger pairs is 40 microns and the aperture width of the fingers varies depending on their radial distance from device center as shown in Fig 2. The transmitting and receiving IDT's are spaced 90 microns or 2.25λ apart. The simulated F-SAW device dimensions are 800 microns in propagation length, 500 microns wide and 400 microns deep. Fluid is modeled as incompressible, viscous, and Newtonian using the Navier-Stokes equation. In modeling fluid-solid interaction, fluid is described with reference to an Eulerian frame while the Lagrangian frame is more suited for structural/solid domain. However, the two frames are incompatible. This incompatibility is overcome by using the arbitrary Lagrangian-Eulerian (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 a peak value of 2.5 V and frequency of 100 MHz) on the transmitter IDT fingers. III. Results/Discussion We have investigated the effect of transducer design parameters such as degree of arc (Da), geometric focal length (fL), and wavelength (λ) on the propagation characteristics (displacement and voltage profiles) of F-SAW and the induced streaming velocity fields. Additional parameters studied in this model include intensity, frequency, fluid density, and viscosity. The transient solutions generated from the model are used to predict trends in acoustic streaming velocity. The calculated streaming velocity and forces obtained for an F-SAW are compared with those from a conventional SAW device with the same wavelength. In comparison with the conventional SAW devices fabricated with uniform interdigital transducers, we find that the focused SAW devices lead to increased induced acoustic streaming velocity, thereby facilitating increased removal of non-specifically bound fouling proteins. Efforts are underway to simulate F-SAWs based on LiTaO3 which serves as a suitable substrate for biosensor elements in liquid phase applications. The results will be discussed in detail.