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Since the discovery that the oscillations of resonating quartz crystals can be sustained in a liquid environment, such crystals have quickly found use as a sensitive microbalance in electrochemistry, making possible in situ measurements of mass changes at the electrochemical interface. The early contributions of the IBM Almaden Research Center to this exciting field of development are sketched. The principles of operation are detailed, with emphasis on an intuitive description to permit considerations of new applications. Mass density changes of the order of 10 nanograms per square centimeter (ng/cm2) are routinely detectable as changes in the resonant frequency of about a hertz. The mass density of a monolayer of material ranges from a few tens of ng/cm2 for polymeric materials to a few hundreds of ng/cm2 for metals. Detailed analysis of the electrical behavior of the resonator in liquid media shows that the resonant frequency, the quality factor of the resonance, and the admittance at resonance are all sensitive to the viscoelastic properties of the contacting liquid, having implications in the study of the behavior of non-Newtonian fluids, including polymeric films.
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