Surface acoustic wave (SAW) devices are finding increasing use in medical diagnostic applications, such as detection of specific proteins in bodily fluids for detection of pathologies. These devices can also be used in Lab-On-a-Chip devices for biological applications that utilize micro-fluidics for detection, transport, mixing, and biological assays. In applications aimed at biological sensing, the sensing medium such as blood exhibits a Non-Newtonian behavior. In biosensing applications of SAW devices, SAW induced acoustic streaming which refers to fluid motion induced by high frequency sound waves, is an important phenomenon that can be used for the removal of non-specifically bound proteins from the device surface. Acoustic streaming also finds use in a wide variety of other applications such as detection of ovarian cysts and detection of blood clotting via ultrasound and convective transport in microfluidic applications of SAW devices. This work reports on the influence of non-Newtonian fluid dynamics on the acoustic streaming and fluid velocity profiles in SAW devices, using a computational fluid-structure interaction finite element model.
II. Computational details
A SAW device, based on YZ Lithium Niobate substrate, in contact with a fluid film was modeled using a three dimensional bidirectionally coupled fluid-structure interaction model. A substrate with dimensions 400 μm width x 800 μm propagation length x 200 μm depth in contact with a 50 μm thick fluid film was modeled. Two interdigital transducer (IDT) finger pairs in each port were defined at the surface. The fingers were defined with periodicity of 40 μm and aperture width of 200 μm. The IDT fingers were coupled by voltage degree of freedom. Blood is modeled as a Non-Newtonian fluid whose viscosity is defined using the Carreau model. To elucidate the effect of non-Newtonian dynamics on acoustic streaming, results are compared with Newtonian fluid with viscosity at infinite shear rate. 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 of100 MHz) on the transmitter IDT fingers.
A transient analysis of the fluid flow profiles on the SAW device indicates significant differences between fluid velocity patterns, magnitudes of fluid velocities, and wall shear stresses for Non-Newtonian fluid loading on the device when compared to a Newtonian fluid. Our results indicate that the peak fluid velocities decreased for Newtonian fluid loading suggesting a significant viscous dissipation of energy as compared to the case of a non-Newtonian fluid (Fig 1). The extent of induced shear stresses at the piezoelectric device-fluid interface is almost two orders of magnitude higher for Non-Newtonian fluids (Fig 2). These results have implications in biosensing as well as micro-fluidic applications involving Non-Newtonian fluids. The results will be discussed in detail.
Figure 1: Fluid velocity profiles (µm/s) in (a) Newtonian fluid (b) Non-Newtonian fluid
Figure 2: Wall shear stress profiles (MPa) in (a) Newtonian fluid (b) Non-Newtonian fluid