Study of Frequency Response of Quartz Crystal Microbalance to Different Wetting States of Micropillar Surfaces

Abstract

Enhanced wettability, known as superhydrophobicity or superhydrophilicity has drawn extensive attention in the past for wide range potential applications such as superhydrophobic surfaces for self-cleaning, anti-icing, dropwise condensation, and drag reduction. This research focuses on the investigation of the frequency responses of quartz crystal microbalance (QCM) devices coated with micropillars to the different wetting states of drops. A theoretical model was developed to correlate the resonant frequency shifts of QCMs with the penetrated (Wenzel state) and suspended (Cassie state) states based on the Euler-Bernoulli beam theory. In the experimental validation of the theory, Poly(methyl methacrylate) (PMMA) micropillars were fabricated on the QCMs using nanoimprint lithography (NIL) method and the different wetting states were generated by plasma treatment and chemical coating. The frequency shifts of the QCM device were measured by a network analyzer. A good agreement between experimental measurements and theoretical predictions was obtained. It was found that the micropillars operating in the penetrated state results in one order of magnitude higher frequency shift of QCM than the micropillars in suspended state. There exists a highly nonlinear vibrating behavior of micropillars with different heights in both penetrated and suspended states. The QCM based technology is a valuable tool for studying the wettability of different superhydrophobic or superhydrophilic surfaces.