(626f) Friction Force-Based Measurements for Simultaneous Determination of the Wetting Properties and Stability of Superhydrophobic Surfaces

Various techniques are used to fabricate superhydrophobic (SH) surfaces with effective non-wetting and water shedding properties which is desirable in several applications. SH surfaces are typically characterized in terms of their hydrophobicity (i.e., by static and dynamic contact angle measurements) and durability. Contact angle measurements have been demonstrated to lead to high uncertainties arising from inaccurately identifying the location of the three-phase contact line. In this research, a force-based technique, in which a water drop is sheared against a superhydrophobic surface, is used to predict and reduce the standard deviation of sliding angle measurements by 56-64% compared to optical-based measurements. In addition, the kinetic regime of the drop friction measurement is found to provide more consistent data compared to the threshold friction (i.e., peak force before the drop slides), which is attributed to the fact that the threshold friction is influenced by the resting time of the drop and localized inhomogeneities on the surface. To further demonstrate the capabilities of the force-based technique in characterizing the performance of SH, three SH surfaces are fabricated which have indistinguishable wetting properties based on optical-based contact angle measurements. However, by applying increased pre-loads to a water drop on these similar SH surfaces and measuring the sliding friction, the force-based technique identifies the critical loads at which a Cassie-Baxter to Wenzel state transition occurs. Force-based characterization, which includes measuring sliding friction and determining the critical force to transition from the Cassie-Baxter to Wenzel state, is proposed as a more accurate and sensitive technique to differentiate the wetting properties of SH surfaces.