Effects of liquid relaxation time on S H surface waves liquid sensors

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Abstract
Effects of liquid relaxation time on the propagation characteristics of shear horizontal (SH) surface waves at the interface between a piezoelectric crystal and a fluid medium are theoretically analyzed for liquid sensor applications. General closed-form expressions are derived for both velocity change and attenuation coefficient for a Maxwellian viscoelastic fluid that can be treated as a Newtonian fluid or as an amorphous solid, depending on the liquid shear relaxation time. It is shown that relaxation time becomes important as the liquid viscosity and/or the wave frequency increases. The result is a saturation level in the propagation loss and the device sensor frequency shift as the liquid relaxation time approaches the wave period. The effect of liquid relaxation is thus to set detection limits on the range of liquid vicosity that can be measured. However, it is also shown that in actual sensor applications, a trade-off can be performed between the maximum measurable viscosity and the SH surface wave device frequency of operation, and thus the device sensitivity.

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