Response of a quartz crystal microbalance to a liquid deposit

The response of a quartz crystal microbalance (QCM) to a liquid deposit is less than to a solid deposit of the same mass per unit area because of viscous effects in the deposit. In this paper the relationship between the loss of QCM response and the deposit parameters is derived analytically, using the Navier–Stokes equation to determine the velocity distribution in the deposit, and a Rayleigh perturbation analysis to determine the response. The theoretical model considers droplet type as well as uniform film deposit morphology, and includes the influence of droplet size and distribution parameters. To examine the validity of the theory two series of experiments have been conducted using DC 704 silicone oil deposited on a Celesco Model 700A QCM of 10 MHz frequency. In one series the deposit mass was held constant and the QCM temperature was varied, while in the other series these conditions were reversed. Satisfactory agreement between the measured data and the theoretical predictions for QCM response loss was obtained. By making a more detailed interpretation of the data, with the help of the theory, the droplet deposit area coverage and number density were estimated.