Theory and applications of quartz resonators as sensors for viscous conductive liquids

Results are reported on the use of quartz-crystal resonators as a conductivity and/or viscosity sensor in dilute-liquid chromatography. A general theory is presented for AT-cut crystal with one surface exposed to a viscous conductive liquid. A characteristic equation which described the resonance condition and accounts for all acoustoelectric interactions was obtained in terms of crystal and liquid parameters. For viscous nonconductive liquids, the resonance condition is reduced to a simple relationship for the frequency change of the crystal in terms of the liquid density and viscosity. For dilute conductive liquids, the change in resonant frequency is derived in terms of the solution conductivity and dielectric constant. A miniature liquid cell containing an 11 MHz quartz crystal was designed and fabricated. Several viscous and conductive solutions were tested, resulting in frequency-shift data which are in excellent agreement with the theoretical model