Nondestructive Testing of Surface Defects Based on the Droplet Pinning Effect

Current nondestructive testing (NDT) technologies have developed along diversified trajectories. In this study, a novel surface-defect nondestructive testing (NDT) method based on the pinning effect is proposed, which systematically elucidates the behaviors of three-phase contact line pinning, elastic deformation, and potential energy accumulation of droplets on microstructured surfaces, thus providing a theoretical basis for surface-defect characterization. A custom-built high-precision solid-liquid interfacial friction measurement apparatus was employed to achieve real-time acquisition and analysis of frictional signals during microdroplet sliding. Experimental results indicate that when droplets traverse surface defects, the friction force curves exhibit a characteristic periodic "rise-sudden drop" pattern, corresponding to the droplet pinning-depinning process, which can serve as a direct quantitative indicator of defect presence. By optimizing parameters such as droplet volume and sliding velocity, detection efficiency can be enhanced without compromising sensitivity or reproducibility. Further integration with multiphysics simulations clarified the critical influence of wetting states on defect recognition: under the Cassie state, the method can detect pit defects with widths no smaller than 5 μm, whereas under the Wenzel state, the lower detection limit is approximately 75 μm. Additionally, linear correlation models relating defect width to pinning duration were established for both wetting states.