Speaker
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Abstract
Wetting dynamics plays a central role in applications such as coating processes, microfluidics, and green energy technologies. Despite extensive theoretical work, several aspects of the influence of droplet size on wetting dynamics remain unclear, particularly how nanoscale dynamics evolve toward macroscopic behaviour as the system size increases.
In this contribution, we present a detailed molecular dynamics study of droplet wetting on silica surfaces, focusing on the role of surface wettability and finite-size effects on macroscopic scaling laws.
Initially spherical droplets with varying radii are placed in contact with silica substrates and simulated to capture their spreading behaviour. The substrate wettability is tuned by introducing hydroxyl ions at different concentrations, resulting in equilibrium contact angles spanning the hydrophilic regime from $0^\circ$ to approximately $90^\circ$. The surface ion density, $C \approx 0$--$9.4\,\mathrm{nm^{-2}}$, is varied at the silica--water interface. Complete wetting, corresponding to an equilibrium contact angle of approximately $0^\circ$, is observed at $C \approx 4.7\,\mathrm{nm^{-2}}$.
For each wettability condition, the temporal evolution of the droplet morphology is analysed in detail, with the final equilibrium configurations found to be consistent with spherical cap geometries. At early times, the simulations recover the well-established inertial scaling behaviour associated with neck formation, following a $t^{1/2}$ power law. At later times, the dynamics transition towards a viscous-dominated spreading regime characterized by Tanner's law, with a $t^{1/10}$ scaling, which emerges when the system size is sufficiently large.
Our results demonstrate how finite-size effects influence the emergence of macroscopic wetting dynamics from nanoscale molecular processes. The findings provide quantitative guidelines for designing molecular simulations across length scales and contribute toward a systematic multiscale framework for understanding wetting phenomena.