Some limitations on the use of leaky SAW mode sensors in liquids

Surface acoustic wave (SAW) sensors suffer severe attenuation when they are immersed in fluids because the predominantly surface-normal components of particle displacement excite fluid compressional waves. This effect is minimized in the leaky SAW (LSAW) sensor, where the predominant displacement is in the plane of the surface. The measurements have shown that the most significant loss mechanism for an unshielded LSAW sensor is electrical dissipation, induced by the conductivities and dielectric loss tangents of many fluids. This kind of loss can be eliminated by shielding the sensor with a metal film. The sensitivity of a shielded sensor is eventually limited by the effects of mechanical damping, due to fluid viscosity. Methods for characterizing the dielectric properties of fluids are described, experimental results are presented, and a sensitivity limit for liquid-loaded LSAW sensors is postulated