Love-mode surface acoustic wave liquid sensors using a polyimide waveguide layer

Love-mode surface acoustic wave devices with polyimide waveguide layers between 0.65 and 1.50 μm thick have been constructed on ST quartz. The devices incorporate a smooth delay line and a corrugated delay line to trap the liquid, preventing viscous entrainment. This could be used as a method to calculate the liquid density from the density-viscosity product. The smooth devices exhibit sensitivities up to 2.6 times greater than for SiO₂ waveguide devices, with a larger response from the corrugated devices. The frequency response is linear for small viscosities, but non linear with higher viscosity liquids, suggesting non-Newtonian liquid behavior. Temperature measurements show the temperature coefficient of frequency varies between +30 ppm°C^-1 and -53 ppm°C^-1 for different polyimide thicknesses, with a temperature independent response estimated for layers 0.95 μm thick.