Manipulated wettability of a superhydrophobic quartz crystal microbalance through electrowetting

The liquid phase response of quartz crystal microbalances (QCMs) with a thin coating (similar to 9 mu m) of epoxy resin with and without a carbon nanoparticles top layer is reported. The nanoparticles convert the epoxy surface to a superhydrophobic one with a high static contact angle (similar to 151 degrees-155 degrees) and low contact angle hysteresis (similar to 1 degrees-3.7 degrees) where droplets of water are in the suspended Cassie-Baxter state. The frequency decrease of the fully immersed QCM with the superhydrophobic surface is less than with only epoxy layer, thus indicating a decoupling of the QCM response. A wettability transition to a liquid penetrating into the surface roughness state (for droplets a high contact angle hysteresis Wenzel state) was triggered using a molarity-of-ethanol droplet test (MED) and electrowetting; the MED approach caused some surface damage. The electrowetting-induced transition caused a frequency decrease of 739 Hz at a critical voltage of similar to 100 V compared to the QCM in air. This critical voltage correlates to a contact angle decrease of 26 degrees and a high contact angle hysteresis state in droplet experiments. These experiments provide a proof-of-concept that QCMs can be used to sense wetting state transitions and not only mass attachments or changes in viscosity-density products of liquids.