Controllable wetting characteristics of hierarchically structured surfaces

Surfaces decorated with two-level topography that greatly promotes hydrophobicity are of considerable importance in controlling wetting features to meet ice repellency or self-cleaning demands. However, most of the current researches only focus on the phenomenology on such surfaces, and the underlying mechanism remains unsatisfactorily described. Here, we have studied wetting characteristics of nano-scale water films on hierarchically structured surfaces from static to dynamic via molecular dynamics (MD) simulations. Using mW (monoatomic water) model, the dilemma of observing free evolution at solid-liquid interface can be well addressed. This helps us to identify wetting regimes at different Young contact angles (θ_Y), including metastable nonwetting regime at θ_Y = 84.6°, coexisting one/two-level wetting regime at θ_Y = 107.4°, and stable nonwetting regime at θ_Y = 135.3°. With the help of indirect umbrella sampling (INDUS) calculations, we obtain dynamic wetting transitions together with precise quantization of free-energy pathways to elucidate the mechanisms underlying different wetting regimes. Particularly, the coexisting one/two-level wetting regime remarkably changes the conventional wetting feature and forms a metastable one-level wetting state. It is a consequence of an asymmetric energy barriers with dewetting barrier (ΔF_d1 = 118.3 k_BT) being less than wetting barrier (ΔF_w1 = 127.1 k_BT). Moreove, the two-level topography can create the stable nonwetting regime to achieve the spontaneous dewetting by completely eliminating ΔF_d1 at θ_Y = 135.3°. Inspired by this, we design a novel wetting-functional surface through adding dispersed nanoparticles within hierarchically structured surfaces. The wetting-functional surface can promote the self-dewetting even for a rough surface with θ_Y = 107.4°. The current work greatly improves understanding of complex textured surfaces and guides practical applications that require switchable wetting states between nonwetting states and highly wetting states.