Frequency and dissipation-factor responses to localized liquid deposits on a QCM electrode

We have investigated how a quartz-crystal microbalance (QCM) responds in resonance frequency, f, and dissipation factor, D, to local liquid deposits (droplets) at different positions on the QCM electrodes. The changes in resonance frequency and dissipation factor versus position of the deposited droplet are well described by Gaussian curves that peak at the centre of the electrode. The widths of the f and D Gaussian curves are not significantly different. The shape of the frequency-shift versus position curve is nearly identical to the corresponding classical sensitivity curve reported by Sauerbrey for solid deposits. An increase in surface roughness of the electrode film, onto which the droplets are deposited, affects f more than D, in agreement with results reported by Martin et al. We demonstrate for a water-glycerol mixture how this effect can be utilized to measure independently the density and viscosity of the contacting liquid or, alternatively, to obtain a measure of the surface roughness of a film deposited on the QCM. Varying the conductivity of the liquid (by adding salt) does not affect the resonance frequency when the liquid is deposited as droplets. This is in contrast to when one entire side of the crystal is completely covered by a liquid. In the latter case,f and D are sensitive to the conductivity of the liquid.