ON THE MEASUREMENT OF THIN LIQUID OVERLAYERS WITH THE QUARTZ-CRYSTAL MICROBALANCE

We present a simple model that predicts the changes in resonance frequ ency and dissipation factor for a quartz-crystal microbalance (QCM) wh en it is coated with a viscous film that may or may not slip on the cr ystal. In this context, the validity of the Sauerbrey equation (change in resonance frequency alpha change in applied mass) is discussed The Sauerbrey equation gives an accurate estimate of the film thickness, t(f), only if (i) the film is thin compared to the shear-wave penetrat ion depth, delta, into the liquid, i.e., t(f) much less than delta and (ii) the film does not slide on the QCM electrode(s). We have shown t hat by measuring both the QCM resonance frequency and the dissipation factor simultaneously, the thickness range over which t(f) can be meas ured accurately can be extended to about 2 delta for non-slipping film s. If the film slips, which we have only observed far molecularly thin films, changes in dissipation factor can be used to calculate the coe fficient of friction between the film and tile substrate. We also show the usefulness of measuring the dissipation factor of the QCM when st udying solid to liquid phase transitions.