MECHANICAL RESPONSE OF THICKNESS-SHEAR MODE QUARTZ-CRYSTAL RESONATORSTO LINEAR VISCOELASTIC FLUIDS

A mechanically based description of the response of thickness-shear mo de quartz-crystal resonators to contact with linear viscoelastic fluid s is presented. The calculation describes the effect of viscoelasticit y in terms of a dimensionless correction to the Newtonian result, chi, which is primarily dependent on the product of the dominant relaxatio n time of the overlying medium and the oscillation frequency of the re sonator. The results show that the effect of a finite relaxation time is a lower frequency shift compared to a viscous reservoir with identi cal density and a viscosity matching the low-frequency limit of the vi scoelastic reservoir. This is due to reduced viscous dissipation in th e boundary layer adjacent to the crystal surface and an effectively sm aller inertial load on the resonator. The model is shown to reduce app ropriately to previously published results in the low-and high-frequen cy limits, in addition to predicting correctly experimental results fo r polymeric fluids assuming Rouse-like behaviour. (C) 1998 Elsevier Sc ience S.A.