(39e) Microcavity Induced Acoustic Streaming Enhancement in a Surface Acoustic Wave Device Based On Langasite

Acoustic streaming, which refers to fluid motion induced by high intensity sound waves, can be used for a variety of sensing and microfluidic applications such as biofouling elimination, piezoelectric actuation, droplet manipulation, and biosensing [1]. When an acoustic wave having a prominent surface normal component interacts with a fluid medium, the acoustic energy dissipation leads to a mode conversion from Rayleigh to leaky SAW [2] This mode conversion results in longitudinal wave propagation in the fluid domain which, if of sufficiently high intensity, results in a net pressure gradient along its direction of propagation thereby inducing fluid flow. This fluid motion resulting from the attenuation of sound wave induces acoustic streaming. The pressure gradient induced in the fluid domain, and therefore the intensity of acoustic streaming, can be manipulated by surface modifications in the SAW device. There are various modifications which can be done to the delay path as well as the transducer configurations which can alter the wave propagation characteristics and can be used to increase the SAW induced streaming phenomenon. In this work, we report acoustic streaming enhancement brought about by etching micro-cavities in the delay path of a SAW device based on Langasite having mutually interacting multidirectional interdigital transducers. Finite element models are used to gain insights into the device fluid interactions. FE models of a SAW device based on LGS substrate, with dimensions 1600 μm length x 1600 μm depth x 200 μm height, were developed. Figure 1a shows a schematic diagram of the simulated SAW device with orthogonal IDT configuration integrated with delay path modification, in the form of micro-cavities. IDT finger pairs, with width 2λ and periodicity 40 μm, were defined on the surface for each port along (0, 22, 0) and (0, 22, 90) Euler directions. The piezoelectric domain was meshed using coupled field tetragonal solid elements ensuring high mesh densities near the surface. 20 micro-cavities of dimensions λ/2 x λ/2 x λ/2 were etched in the delay path of the orthogonal SAW device (Fig. 1). Our simulation results indicate that the streaming velocity fields are strongly dictated by the SAW displacement amplitudes and the kind of acoustic mode. Our simulations indicate that PS filled micro-cavities with dimensions λ/2 x λ/2 x λ/2 exhibit the highest energy transmission and maximum energy entrapment near the device surface (Figure 2). Our analyses of displacement profiles for device with micro-cavity in the delay path indicate that these improvements are brought about by a larger coherent reflection of the incident Love wave and subsequent reduced conversions into bulk shear modes which radiate into the substrate We show in this work that improved acoustic energy entrapment arising from micro-cavities leads to a significant amplification of device displacement amplitudes which in turn leads to orders of magnitude increase in induced streaming forces. These results have tremendous significance and implications in the areas of biosensing and microfluidic applications. Fluid structure interaction models to understand fluid device interaction in the presence of microcavities, are underway. Results will be discussed at AIChE 2009 meeting. References 1 W. Nyborg, Acoustic Streaming (Academic Press, New York, 1965) 2 J. Kondoh and S. Shiokawa, Proceedings of IEEE Ultrasonics symposium (1995).