Flow induced by acoustic streaming on surface-acoustic-wave devices and its application in biofouling removal: a computational study and comparisons to experiment.

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.