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Dynamic response of quartz crystal microbalances in contact with silicone oil droplets

Quartz crystal microbalances (QCMs) in contact with liquid droplets have been used to investigate the rheological properties of liquids and the diverse solid-liquid interfacial phenomena. In this article, we first report the experimental results of the QCM responses due to the deposition of microliter droplets of silicone oils. The silicone oils with viscosity in a range from 50cS to 103cS are tested. It has been found that the responses in the frequency and resistance measurements of the QCM in contact with the silicone oils are different from the Newtonian liquids. More importantly, it has been found that for the silicone oils the frequency of the QCM steadily increases for several hours and even exceeds the initial value of the unloaded QCM which has resulted in the positive frequency changes. The collaborative effects of the interfacial slip and viscoelasticity have been discussed to qualitatively interpret the positive frequency changes. Secondly, we discuss the cyclical variations in the frequency and resistance during the experiments of the silicone oils, which are attributed to the generation of the compressional wave in the droplets. The present work has shown some phenomena which need to be taken into consideration when using the droplet QCM as a rheological sensor and may stimulate the ongoing research on the related issues in the QCM community.