Insights into transducer-plane streaming patterns in thin-layered acoustofluidic devices

While classical Rayleigh streaming, whose circulations are perpendicular to the transducer radiating surfaces, is wellknown, transducer-plane streaming patterns, in which vortices circulate parallel to the surface driving the streaming, have been less widely discussed. Previously, a four-quadrant transducer-plane streaming pattern has been seen experimentally and subsequently investigated through numerical modelling. In this paper, we show that by considering higher order threedimensional cavity modes of rectangular channels in thin-layered acoustofluidic manipulation devices, a wider family of transducer-plane streaming patterns are found. As an example, we present a transducer-plane streaming pattern, which consists of eight streaming vortices with each occupying one octant of the plane parallel to the transducer radiating surfaces, which we call here eight-octant transducer-plane streaming. An idealised modal model is also presented to highlight and explore the conditions required to produce rotational patterns. It is found that both standing and travelling wave components are typically necessary for the formation of transducer-plane streaming patterns. In addition, other streaming patterns related to acoustic vortices and systems in which travelling waves dominate are explored with implications for potential applications.