Quartz crystal microbalance and evaporation of sessile droplets

A quartz crystal microbalance and dissipation simultaneously measuring the frequency and the dissipation over more than one decade, from fundamental to overtone 15, is used to investigate the evaporation mechanisms of small water sessile droplets onto a gold surface. An unexpected weak increase in frequency is observed during the first stage of evaporation where the contact line is pinned while the dissipation is unchanged. The increase in frequency has a linear dependence with the overtone order so it is related to a very thin amount of water. A droplet deposited inside a circular barrier does not exhibit such a variation suggesting that the observed effect is related to the region in the vicinity of the edge of the droplet.