Molecular dynamics study on bubble nucleation over surfaces with convex roughness under different wetting conditions

Bubble nucleation plays a pivotal role in microscale heat conduction, boiling heat transfer, and liquid-vapor phase change processes, as it not only governs heat transfer efficiency but also strongly regulates bubble dynamics. The nucleation processes are highly sensitive to the surface morphology and wettability of solid substrates. However, due to the inherent limitations of conventional experiments in terms of spatial resolution and observation times, revealing the microscopic mechanisms of bubble nucleation at the nanoscale remains a significant challenge—particularly under conditions involving complex surface structures and diverse wettability states. In this study, molecular dynamics simulations were employed to systematically investigate the mechanisms by which surface roughness and wettability influence bubble nucleation behavior on nanostructured surfaces at the atomic scale. Five copper substrates featuring sinusoidal protrusions were designed to represent different degrees of surface roughness. The sinusoidal profile, characterized by mathematical continuity and smoothness, not only facilitates the observation of bubble coalescence and contact angle evolution but also ensures comparability among models by maintaining identical protrusion height and overall width, thereby keeping the protrusion volume constant. This design allows direct comparison of bubble growth rates and other physical quantities across different models. In addition, three distinct wettability conditions, namely hydrophobic, neutral, and hydrophilic, were achieved by modifying the interaction potential between oxygen and copper atoms. During the simulations, a constant heat flux was applied to the bottom copper substrate to trigger spontaneous bubble nucleation, local low-density regions were identified using density distribution analysis to track bubble nucleation sites; and a piston-like pressure control mechanism was introduced via the top copper plate, and the displacement of this plate over time was used to quantify bubble growth rates under varying roughness and wettability. Additionally, the Kapitza resistance between solid and liquid phases was calculated to evaluate interfacial heat transfer efficiency. The results demonstrate that increasing surface roughness significantly promotes the formation of local low-density cavities, thereby accelerating the bubble nucleation and subsequent growth. As the surface wettability transitions from hydrophobic to hydrophilic, the solid-liquid interfacial thermal resistance decreases, leading to earlier bubble nucleation. Moreover, under hydrophilic conditions, the contact angle of the bubbles increases significantly, indicating enhanced detachment and growth behavior. Overall, the findings of this work advance the fundamental understanding of the microscopic mechanisms of bubble nucleation and provide theoretical guidance and technical references for the design of high-efficiency heat transfer structures and tunable fluid-solid interfaces at the nanoscale.