Evaporation of sessile drops: Application of the quartz crystal microbalance

The application of the quartz crystal microbalance (QCM) to the study of th e evaporation of sessile. droplets is reported. The evaporation of a homolo gous series of light alcohol, from the surface of an oscillating quartz cry stal, has been investigated. The droplet evaporation process is observed to cause reproducible, characteristic changes in crystal oscillation frequenc y that; are indicative of the complex thermophysical phenomena occurring at both the liquid-vapor and crystal-fluid interfaces. The influence of surfa ce morphology on the frequency responses during the evaporation process is understood in the framework of the perturbation theory of surfaces of sligh t roughness and random corrugation in the low viscosity Limit. The experime ntal data are understood in terms of the radial sensitivity of the QCM S(r, phi) via the deduction of the areal retreat speed v(r). The extreme modes of droplet evaporation, associated with constant contact area and constant contact angle, are identifiable from the observed frequency responses. The trend of characteristic frequency responses observed is consistent with sur face-tension driven convection effects, which are often responsible for flu id-vapor interface phenomena.