Molecular dynamics insights into bubble nucleation of liquid sodium on hybrid wettability surfaces: Temperature dependence and scale-induced site inversion

Molecular dynamics simulations were performed to investigate the bubble nucleation characteristics of liquid sodium on hybrid-wettability surfaces composed of alternating hydrophilic and hydrophobic stripes. Wall temperature T wall and stripe width w were varied independently to separate thermal driving from geometric-scale effects. Four distinct temperature-dependent nucleation regimes were identified. At T wall = 715 K, nucleation occurred exclusively on the hydrophobic stripe, and vapor coverage of the hydrophilic region was achieved solely through lateral bubble propagation after a prolonged delay of 505 ps. At T wall = 735 K, independent secondary nucleation events emerged on the hydrophilic surface with a reduced inter-region delay of 140 ps. At T wall = 865 K, nucleation onset became simultaneous across both wettability regions, rendering the wettability contrast irrelevant. At T wall = 1100 K, a reversal of nucleation site density was observed, with more vapor embryos forming on the hydrophilic stripes. Analysis of local energy landscapes revealed a consistent decoupling between kinetic and potential energy distributions governing nucleation site selection. At T wall = 735 K, reducing the w from 20.4 nm to 4.08 nm monotonically decreased the nucleation onset time due to enhanced lateral thermal coupling. A complete inversion of nucleation site preference was discovered at the finest w of 2.04 nm, where nucleation first occurred on the hydrophilic stripes. This scale-induced site inversion originates from geometric confinement that suppresses hydrophobic nucleation while the superior thermal conductance of the hydrophilic interface eventually overcomes the higher free energy barrier. These findings provide molecular-level insights for designing hybrid-wettability surface architectures in high-temperature liquid metal cooling systems.