  Surface acoustic wave (SAW) devices form an important class of acoustofluidic
devices, in which the acoustic waves are generated and propagate along the
surface of a piezoelectric substrate. Despite their wide-spread use, only a few
fully three-dimensional (3D) numerical simulations have been presented 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 the
liquid-filled microchannel is embedded, the acoustic fields inside the
microchannel, as well as 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 the
numerical predictions of the acoustic resonances and the acoustophoretic
repsonse of suspended microparticles in 3D are successfully compared with
experimental observations. The simulation provides a physical explanation of
the the observed qualitative difference between devices with an acoustically
soft and hard lid 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 presented simulation model may be useful in
the development of SAW devices optimized for various acoustofluidic tasks.
