Deposition and Adhesion of Polydopamine on the Surfaces of Varying Wettability

Mussel-inspired chemistry, particularly the versatile coating capability of polydopamine (PDA), has received much research interest as a promising strategy for fabricating functional coatings in numerous fields. However, the understanding of deposition mechanisms and adhesion behaviors of PDA on different substrates still remains incomplete, significantly limiting the related fundamental research and its practical applications. In this work, a colloidal probe atomic force spectroscopy technique was employed to quantify the interaction forces and adhesion between the PDA coatings and the substrate surfaces with different wettabilities. The surface force measurements and thermodynamic analysis of interaction energy indicate that the surface wettability has a significant influence on the adhesion, deposition behaviors, and morphologies of PDA coatings. Compared with the hydrophilic surfaces, the hydrophobic surfaces exhibit stronger adhesion with the PDA coatings. Furthermore, for the first time, this work demonstrates that ethanol has the capability of effectively displacing the trapped air/vapor layer or the so-called "hydrophobic depletion layer" on the hydrophobic substrate to allow the intimate contact between PDA and the substrate, thus enhancing the adhesion and facilitating the PDA deposition. This work provides new insights into the fundamental PDA deposition mechanism as well as the design and development of versatile mussel-inspired coatings on the substrates of varying hydrophobicity.