FEM modelling of a SAW microfluidic sensor based on the photoacoustic effect

Surface acoustic wave (SAW) integrated with microfluidics has been used for various acoustofluidic applications including sensing mechanical properties of fluids such as viscosity and density of fluids. We have recently demonstrated the excitation of a surface acoustic wave using the photoacoustic (PA) effect in a microfluidic channel. The opto-mechanical properties of the liquids in the microchannel can be detected using the mode-converted SAW. In this work, we investigated the response mechanism of the sensor using detailed numerical FEM simulation. The simulation study has shown that the microfluidic channel on the substrate acts as an acoustic resonator with multiple eigen modes. PA signals generated inside the microfluidic channel resonates at these modes. The displacement profile on the piezoelectric surface confirmed that the pressure mode propagating parallel to the piezoelectric substrate has the highest mode conversion efficiency (longitudinal wave to SAW). This mode is sensitive to the microchannel dimensions. The study has shown that a SAW device matched to this mode-frequency could be used for improving the sensitivity of the sensor. Experimental observation confirmed the theory.