Three-Dimensional Numerical Modeling of Surface-Acoustic-Wave Devices: Acoustophoresis of Micro- and Nanoparticles Including Streaming

Abstract

Surface-acoustic-wave (SAW) devices form an important class of acoustofluidic devices, in which acoustic waves are generated and propagate along the surface of a piezoelectric substrate. Despite their widespread use, only a few fully three-dimensional (3D) numerical simulations have been reported in the literature. In this paper, we present a 3D numerical simulation taking into account the electromechanical fields of the piezoelectric SAW device, the acoustic displacement field in the attached elastic material, in which a liquid-filled microchannel is embedded, the acoustic fields inside the microchannel, and the resulting acoustic radiation force and streaming-induced drag force acting on micro-and nanoparticles suspended in the microchannel. A specific device design is presented, for which numerical predictions of the acoustic resonances and the acoustophoretic response of suspended microparticles in three dimensions are successfully compared with experimental observations. The simulations provide a physical explanation of the observed qualitative difference between devices with acoustically soft and hard lids in terms of traveling and standing waves, respectively. The simulations also correctly predict the existence and position of the observed in-plane streaming-flow rolls. The simulation model presented may be useful in the development of SAW devices optimized for various acoustofluidic tasks.

Acoustic Characterization of Polydimethylsiloxane for Microscale Acoustofluidics

“…Having boomed over the past decade, acoustofluidics are characterized by high energy efficiency and good biocompatibility [12], and are therefore considered promising tools in biomedical and chemical analysis [13,14] and particle manipulation [7,15]. The driving mechanism of acoustofluidics relies on the actuation of surface [10,13,16,17] or bulk [9,18,19] acoustic waves (SAWs or BAWs). In both cases, PDMS has proved to be flexible and reliable.…”

“…For example, Moiseyenko and Bruus [18] demonstrated whole-system ultrasound resonance in an all-PDMS BAW device, providing an all-polymer choice for point-of-care applications. In SAW chips, PDMS can be easily bonded to the piezoelectric substrates, such as lithium niobate (LiNbO 3 ) and is favored by many [13,[15][16][17].…”

“…The design, evaluation, and optimization of such devices require dealing with wave-PDMS interactions, e.g., wave absorption, reflection, propagation, resonance, and acoustic impedance matching [17,18,20,21]. In the prediction of resonance frequencies in all-PDMS BAW devices, accurate parameters describing the elasticity and viscosity of PDMS are required [18].…”

“…The L-wave velocity is typically approximately 1100 m/s, whereas the moderate damping is around 200 m −1 [22][23][24]. The S waves show a low phase velocity of approximately 100 m/s, inducing much shorter wavelengths [25], which leads to very large computer memory requirements in numerical simulations resolving the full physics of PDMS [17]. On the other hand, the attenuation of S waves in PDMS is very high, approximately 10 5 m −1 [25]; therefore, neglect of S-wave propagations seems reasonable [26].…”

“…On the other hand, the attenuation of S waves in PDMS is very high, approximately 10 5 m −1 [25]; therefore, neglect of S-wave propagations seems reasonable [26]. However, recent studies showed that S waves also cause differences inside acoustofluidic channels, especially the acoustic streaming and the motion of small particles dominated by that [17,20].…”