  We present a finite-element method modeling of acoustophoretic devices
consisting of a single, long, straight, water-filled microchannel surrounded by
an elastic wall of either borosilicate glass (pyrex) or the elastomer
polydimethylsiloxane (PDMS) and placed on top of a piezoelectric transducer
that actuates the device by surface acoustic waves (SAW). We compare the
resulting acoustic fields in these full solid-fluid models with those obtained
in reduced fluid models comprising of only a water domain with simplified,
approximate boundary conditions representing the surrounding solids. The
reduced models are found to only approximate the acoustically hard pyrex
systems to a limited degree for large wall thicknesses and not at all for the
acoustically soft PDMS systems.
