Evaluating Superhydrophobic Surfaces under External Pressures using Quartz Crystal Microbalance.

The performance of hydrophobic surfaces under hydraulic pressures is critical to a wide range of practical applications such as drag reduction of seaboard vessels and design of microfluidic devices. This research focuses on the evaluation of drag reduction and velocity slip of hydrophobic surfaces and coatings under external hydrostatic pressures using an acoustic wave device (i.e., quartz crystal microbalance, QCM). The correlation between the resonant frequency shift of a QCM device and drag reduction of hydrophobic surface coated on the QCM was theoretically developed and the model was validated by comparing the measurement results of the drag reduction of an epoxy-based superhydrophobic coating with those measured by a rheometer. The QCM device was further employed to study the wetting state transition and drag reduction of water on a micropillar array based superhydrophobic surface under elevated hydrostatic pressures. It was found that the transition from Cassie to Wenzel states occurred at a critical hydrostatic pressure which was indicated by a sudden frequency drop of the QCM device. In addition, the effective heights of the meniscus at the liquid/air interface increased with the external pressure before the transition took place. The drag reduction induced by the micropillar surface decreased with the increasing hydrostatic pressures. It was demonstrated that the developed QCM based technology provides a low cost, simple, and reliable tool for evaluating hydrophobic performance of various surfaces under external hydrostatic pressures.