Developing a non-optical platform for impact dynamics analysis on nanostructured superhydrophobic surfaces using a quartz crystal microbalance

Quantitative analysis of water droplet behavior under dynamic conditions is one of the critical challenges for applications of wettability-controlled surfaces. Currently, various optical analysis techniques have been employed to analyze impact dynamics. Despite the convenience of direct observation of water droplets, most of these techniques have limited applicability to microscopic and quantitative investigations. In an effort to overcome these limitations, here, we suggest a complementary analysis platform using a quartz crystal microbalance (QCM) to study impact dynamics. A high-speed camera and QCM were applied together to study the behavior of water droplets that impact wettability-controlled surfaces with various We numbers (Weber number). For these experiments, ZnO nanowire surfaces were prepared and chemically modified by alkyl-thiol molecules with various carbon chain lengths (C0-C12) to control the surface energy. For nanowire surfaces with high surface energies (C0-C6) and for the lowest surface energy sample (C18), both methods exhibited highly consistent impact dynamics, showing stable wetting and dewetting properties, respectively. In addition to these apparent behaviors, QCM was further able to provide detailed microscopic information regarding the penetration and deformation of water droplets in a quantitative way based on acoustic sensing. More interestingly, QCM was able to determine the metastable water repellency of a C12-modified surface with a high We number, which could not be detected by the high-speed camera. These results suggest the significant potential of QCM as a new platform to analyze the impact dynamics of water droplets via quantitative, microscopic investigations.