Drop Adhesion Force as a Measure of the Solid–Liquid Area Fraction on Cassie–Baxter Superhydrophobic Surfaces

Superhydrophobic surfaces have attracted considerable attention due to their remarkable ability to repel aqueous media by trapping an air layer. However, estimating the solid-liquid area fraction is challenging due to the complexity of their intrinsic properties and the limited resolution of methods for assessing contact area. We demonstrated that solid-liquid area fraction can be measured using reflectance optical microscopy, which provides a millimeter field of view with a micrometer lateral resolution. However, this method depends on the surface's optical characteristics. To simplify these measurements, this study examines the drop adhesive force as a means of quantifying solid-liquid area fraction. Utilizing a controlled experimental setup, we investigated the relationship between drop adhesive forces and solid-liquid area fraction. After considering various surface geometries obtained by laser and electrochemical treatments, we engineered 23 surfaces with solid-liquid area fractions ranging from 0.1% to ∼50%. Our results show that the adhesive force exerted by droplets on these surfaces increases linearly with the degree of air entrapment beneath the liquid. This provides a measurable indicator of their Cassie-Baxter superhydrophobicity. This research advances our fundamental understanding of plastron on superhydrophobic surfaces, facilitating the development of highly repellent, ultraslippery surfaces.