Response of quartz crystal resonators possessing a superhydrophobic surface

A quartz crystal resonator immersed in a water-glycerol solution has a systematically reducing resonant frequency and an increase in bandwidth of the resonance as the concentration of glycerol increases. This behavior is well-known and is accurately described by the Kanazawa and Gordon model, which provides a clear proportionality to the square root of the density-viscosity product of the mixture. It is also well-known that a hydrophobic surface having high aspect ratio protrusions can become superhydrophobic so that the liquid no longer retains contact with all points on the surface. In such a situation for a quartz crystal surface, it is expected that a decoupling of the acoustic wave will occur and the response may not then conform to the Kanazawa and Gordon model. In this work, we report on the behavior of quartz crystal resonators fabricated with 5-18 micron tall micro-post structures both before and after they have been treated with a fluorochemical to make them hydrophobic. We report contact angle data showing that the hydrophobic surfaces are super-repellent for the entire range of water-glycerol mixtures. We obtain and fit the impedance spectra to the Butterworth-van-Dyke model and show, in this extreme case, a complete change in acoustic response occurs. The impedance spectra show a rich mixture of behavior including resonances that become sharper as the concentration of glycerol increases and surfaces showing a decoupling of the acoustic wave response.