Evaporation of a sessile liquid droplet from the shear or flexural surfaces of a quartz tuning fork

Abstract

A piezoelectric quartz tuning fork (QTF) has been used to investigate the evaporation of a sessile water droplet with nanogram sensitivity. A water droplet is placed on one facet of a QTF tine and the changes in the resonance frequency and dissipation factor, which are related to the changes in mass and viscous damping, respectively, are measured in situ during the evaporation. Depending on the facet of the QTF tine on which the water droplet is placed, the changes in the frequency and dissipation factor arise in different vibration modes: the flexural or shear modes. The shear mode measurement is affected by the penetration depth, so changes in the frequency and dissipation factor are observed only when the water droplet is sufficiently thin, whereas the changes in the flexural mode measurements are observed during the entire evaporation process. When a droplet of polystyrene nanoparticle suspension is evaporated from the flexural surface, the concentration of the nanoparticle suspension can be determined from the difference in mass between the initial droplet and the dry nanoparticles. In contrast, changes in the physical properties of the suspension are obtained in situ from the evaporation from the shear surface. These results demonstrate that QTFs are useful tools for the investigation of the evaporation of suspensions from solid surfaces. (C) 2013 AIP Publishing LLC.