Localization and shaping of surface acoustic waves using PDMS posts and application for particle filtering and washing

We introduce a technique to control the position and effective area of the surface acoustic wave (SAW) acting on a fluid in a PDMS microchannel. The acoustic force can be localized at specific areas of the channels using a structured, ultra-thin PDMS foil beneath the microchannel. This setup enables to shape the standing surface acoustic waves (SSAW) only by changing the geometry of the PDMS foil. In this way the effect is independent of the design of the acoustic transducer and even single interdigitated transducer can be used to excite standing surface acoustic waves. The alignment of the assembly is very flexible allowing for different orientation of the microchannels with respect to the IDT. To demonstrate the significance and versatility of this technique we use a diagonal orientation to separate and wash particles of different size with a total deflection length of several wavelengths