In this work, the interaction between a sessile droplet’s contact angle and a quartz crystal microbalance (QCM) is elucidated. We differentiate the QCM’s frequency response to changes in the droplet contact area from variations in the dynamic contact angle. This is done by developing a computational model that couples the electrical and mechanical analysis of the quartz substrate with the visco-acoustic behavior of the sessile droplet. From our analysis, we conclude that changes in the contact angle have an effect on the frequency response of the QCM when the droplet height is on the order of the viscous decay length or smaller. On the other hand, changes in the interfacial contact area of the sessile droplets have a significant impact on the frequency response of the QCM regardless of the droplet size.

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This study quantifies the evaporation rate of sessile droplets using a quartz crystal microbalance (QCM). Specifically, we analyze the evaporation of water droplets on a gold-coated flat surface exposed to dry nitrogen at different temperatures. In this approach, we use the QCM as a radius sensor and determine the contact angle by droplet imaging, which allows calculating the instantaneous volume and the evaporation rate. For comparison, we quantify evaporation using computational modeling and an experimental technique based on droplet imaging alone. In general, the QCM-based approach was found to provide higher accuracy and a better agreement with the model predictions compared to the approach using imaging only. With modeling and experiments, we also elucidate the role of droplet self-cooling, vapor advection, and diffusion on the net rate of evaporation of sessile droplets. For all the conditions analyzed in this study, the evaporation rate was found to decrease monotonically. We found this reduction to take place even in the presence of a steadily increasing droplet temperature due to a shrinking evaporation area. Considering the vapor transport mechanisms occurring in the ambient, we find diffusion to be the rate-limiting process controlling the net evaporation rate of the droplet.

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