All transducers used in biological sensing suffer from fouling resulting from nonspecific binding of protein molecules to the device surface. The acoustic-streaming phenomenon, which results from the fluid motion induced by high-intensity sound waves, can be used to remove these nonspecifically bound proteins to allow more accurate determinations and reuse of these devices. We present a computational and experimental study of the acoustic-streaming phenomenon induced by surface acoustic waves.A coupled-field fluid-structure interaction (FSI) model of a surface-acoustic-wave (SAW) device based on a micrometer-sized piezoelectric substrate (YZ-LiNbO3) in contact with a liquid loading was developed to study the surface-acoustic-wave interaction with fluid loading. The fluid domain was modeled using the Navier-Stokes equation; the arbitrary Lagrangian-Eulerian approach was employed to handle the mesh distortions arising from the motion of the solid substrate. The fluid-solid coupling was established by maintaining stress and displacement continuity at the fluid-structure interface. A transient analysis was carried out by applying a time-varying voltage to the transmitter interdigital transducer (IDT) fingers. Simulation results predict strong coupling of ultrasonic surface waves on the piezoelectric substrate with the thin liquid layer causing wave mode conversion from Rayleigh to leaky SAWs, which leads to acoustic streaming. The transient solutions generated from the FSI model were utilized to predict trends in acoustic-streaming velocity for varying design parameters such as voltage intensity, device frequency, fluid viscosity, and density. The induced streaming velocities typically vary from 1 mum/s to 1 cm/s, with the exact values dictated by the device operating conditions as well as fluid properties. Additionally, the model predictions were utilized to compute the various interaction forces involved and thereby identify the possible mechanisms for removal of nonspecifically bound proteins. Our study indicates that the SAW body force overcomes the adhesive forces of the fouling proteins to the device surface and the fluid-induced drag and lift forces prevent their reattachment. The streaming velocity fields computed using the finite-element model in conjunction with the proposed mechanism were used to identify the conditions leading to improved removal efficiency. Predictions of the model are in good agreement with those of simple analytical theories as well as the experimentally observed trends of nonspecific protein removal in typical SAW biosensing operations.
This dissertation is dedicated to my parents, G. Srinivasan and Lakshmi Srinivasan and to my sister Kavita Srinivasan. Their love, support and patience have always been the foundations of my inspiration. for his valuable guidance and support during this thesis work. Thanks to Dr Paris H. Wiley, Dr. Thomas Weller and Dr. Babu Joseph for agreeing to serve on my supervisory committee. I would like to thank Stefan Cular and Randy Williams for helping me at every step. I also appreciate Dr. Michael T Harris and Sang-Yup Lee of Purdue University for providing the Pd-TMV materials. Special thanks are extended to Sensors Research Group and my friends for their support and encouragement throughout this research. i
Shear-horizontal surface acoustic wave (SAW) sensors with microcavities in the delay paths of 36° YX-LiTaO3 substrate were studied using finite element methods. Microcavities of square cross sections of sizes λ∕4 and λ∕2 and of different depths were located in the middle of the delay path. Simulation results for nonfilled and polystyrene-filled microcavity devices were compared with standard delay line shear-horizontal SAW, optimized Love-wave, and etched grating sensors. We found that the best case microcavities studied reduce insertion loss by 19.25dB from 33.28dB and exhibit velocity sensitivity 4.83 times larger than that of the standard SAW sensor simulated.
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