Advanced Characterization of Plastron on Cassie–Baxter Superhydrophobic Surfaces by Drop Adhesion Force

Cassie-Baxter superhydrophobic surfaces repel aqueous media by trapping a thin air layer known as plastron. However, current methods to estimate plastron characteristics on such surfaces, either directly through digital still imaging/confocal microscopy or indirectly through contact angle goniometry, are technically challenging and often imprecise. Previously, we demonstrated that the solid-liquid area fraction, which quantifies the fraction of the solid surface that is in direct contact with the surrounding liquid, can be accurately measured by reflectance optical microscopy, provided that the material surface has approximately uniform local reflectivity. To eliminate the complexity associated with optical reflectance measurements, we recently demonstrated that the drop adhesion force correlates linearly with the solid-liquid area fraction. In this study, we advance the drop adhesion force measurements obtained at different environments, compression distances, and during pressurization-hold experiments to reveal whether the plastron sustains external pressure or undergoes gradual collapse. Here, we demonstrate that measuring drop adhesion force is a robust, rapid, and technically simple method for assessing plastron properties under a wide range of environmental and surface conditions. The proposed method provides information on plastron stability within minutes and thus could aid in the design of ultrarepellent surfaces with long-term stability in demanding applications.