  We study by numerical simulation in two and three dimensions the coupling
layer between the transducer and the microfluidic chip in ultrasound
acoustofluidic devices. The model includes the transducer with electrodes, the
microfluidic chip with a liquid-filled microchannel, and the coupling layer
between the transducer and the chip. We consider two commonly used coupling
materials, solid epoxy glue and viscous glycerol, as well as two commonly used
device types, glass capillary tubes and silicon-glass chips. We study how
acoustic resonances in ideal devices without a coupling layer is either
sustained or attenuated as a coupling layer of increasing thickness is
inserted. We establish a simple criterion based on the phase of the acoustic
wave for whether a given zero-layer resonance is sustained or attenuated by the
addition of a coupling layer. Finally, we show that by controlling the
thickness and the material, the coupling layer can be used as a design
component for optimal and robust acoustofluidic resonances.
