EFFECT OF SURFACE-ROUGHNESS ON THE RESPONSE OF THICKNESS-SHEAR MODE RESONATORS IN LIQUIDS

The effect of surface microstructure on the response of thickness-shear mode resonators in contact with liquids has been examined. Resonators were fabricated with different degrees of random surface roughness by polishing AT-cut quartz crystals with various abrasive particle sizes and then depositing conformal Cr/Au electrodes. The electrical response of liquid-contacted resonators was measured over a range of frequencies near resonance and fit to an equivalent-circuit model. A method is described for determining the complex shear mechanical impedance (the ratio of shear stress to particle velocity at the solid/liquid interface) from the equivalent-circuit elements. This impedance is sensitive to the influence of surface microstructure on the solid/liquid interaction. The surface mechanical impedance was measured while surface roughness, contacting liquid properties, device operating frequency, and liquid contact angle were varied. Results show that, for roughness features much less than the liquid decay length, the surface may be considered hydrodynamically smooth and the responses depend only on the density-viscosity product. As features become comparable to or larger than the decay length, new mechanisms, including liquid trapping and compressional wave generation, arise for energy storage and power dissipation. Surface treatments that vary the liquid contact angle show no significant effect for liquid coupling by a smooth surface; however, the extent of liquid trapping by a rough surface appears to diminish with increasing liquid contact angle, explaining results previously attributed to the occurrence of slip at the solid/liquid interface.