Dynamic Repellency of Water-Proof Surfaces

Dynamic wettability is essential for various applications in biology and technology. Conventional optical-based contact angle methods, such as sessile-drop goniometry and tilting-plate techniques, suffer from difficulties in baseline determination and inconsistencies in surface topography. Moreover, they frequently neglect the critical influence of micro/nanostructures, thereby hindering the advancement of liquid-repellent surfaces. Here, we demonstrate a normal force-based method to accurately quantify dynamic wettability via a characteristic parameter, <i>K</i>, which is derived from force curves during the dynamic receding state of the contact line. This force-derivation method avoids optical distortion, accounts for droplet compression, and accurately classifies surfaces into Wenzel, Cassie, and combined states. We demonstrate that <i>K</i> arises from the interplay of normal force, surface energy, and adhesion work, being exclusively dependent on the liquid-solid interface. Validated across diverse artificial and natural substrates, it is crucial for applications in the self-cleaning industry and agricultural spray and related to impact dynamics, enabling the prediction of rebound ability with high resolution.