Whether nano hydrophilic spots control the condensation pattern: The mechanism of critical wettability

Reducing hydrophilic spot size on hybrid wetting substrates can decrease droplet pinning. When spot size is reduced to that of nucleation, whether condensation is still controllable remains to be explored. This work studies condensation patterns on substrates with nano-spots using molecule dynamics. Density fluctuation results in the uneven distribution of vapor, leading to vapor nucleation at partial nano-spots first. These nucleated clusters compete with bare spots to attract vapor, resulting in different condensation patterns. If clusters dominate in attracting vapor, nucleation on bare spots is hindered, and condensation patterns are disordered. When the nano-spot wettability exceeds critical θ_cr, vapor preferentially nucleation at bare spots and condensation patterns become uniform. This preference is attributed to the strong attraction of solids to vapor and more hydrogen bonds formed between adsorbed water molecules. The growth of individual clusters on substrates with different condensation patterns is evaluated by the power law of temporal-evolved cluster size (N_c = C₀t^b). The competition between formed clusters to attract vapor molecules hinders the cluster growth, and two hindrance effects are distinguished: (I) weak competition only reduces the coefficient C₀, whereas power b maintains 3/2, and (II) severe competition also reduces b, which finally shrinks to 1. © 2024 Elsevier Ltd