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.