Molecular dynamics study of droplet condensation and movement on a wedge-shaped continuous gradient composite surface

Insufficient condensation and inefficient droplet removal on functional surfaces limit the performance of water harvesting and thermal management systems. To address these challenges, molecular dynamics (MD) simulations were performed to investigate the condensation and self-driven transport of water molecules (WMs) on wedge-shaped composite copper surfaces with continuous wettability gradients (WGs). The results reveal that condensation efficiency and droplet mobility are jointly governed by the WGs, the degree of surface hydrophilicity, and the wedge vertex angle. Expanding the hydrophilic region and increasing the vertex angle significantly enhance condensation efficiency, whereas droplet transport is regulated by the balance among the WG-induced driving force, Laplace pressure, and adhesive resistance. Notably, a direct comparison between continuous WGs and fixed CA designs inside the wedge demonstrates that continuous WGs provide superior condensation and drainage performance. In particular, the 150/120–0° gradient at α = 14° increases the number of condensed WMs by 26.3% and the drainage number by 64.5% compared with the corresponding fixed CA counterpart. Moreover, the droplet migration velocity exhibits a non-monotonic dependence on the WG, and the condensation enhancement by the gradient diminishes rapidly with increasing vertex angle. The continuous gradient improves drainage far more than condensation, indicating that droplet transport is more sensitive to continuous surface energy variation than condensation. These enhancements are attributed to reduced energy barriers and the coordinated driving effect of Laplace pressure and the surface energy gradient. Overall, this study provides valuable molecular-level insights into droplet dynamics on structured WG surfaces, offering theoretical guidance for designing surfaces with optimized condensation and drainage properties.