Collision of Nanoscale Water Droplets Towards Cylinder Surfaces Under Different Conditions

Molecular dynamics (MD) simulations are performed to investigate the collision dynamics of two water nanodroplets impinging from opposite directions on a solid fiber. The present work mainly focuses on the effects of some important characteristic parameters, including Weber number, diameter ratio, cylinder diameter, and eccentric distance. For central collision, we observe a total of four outcomes, which are separated droplet bouncing, random detachment, droplet split, and breakup. The maximum spreading factors can be well predicted using βmax~We2/3Re-1/3, with βmax~We1/2Re1/5 serving as an empirical alternative. The surface wettability cannot affect the scaling law, indicating inertial and viscous forces dominate the spreading process. At a fixed scaling variable, βy, max decreases systematically with increasing diameter ratio due to kinetic-energy and momentum imbalance. Such an imbalance redirects a larger fraction of the incident energy into directional translation rather than lateral spreading. Regime maps further show that, for equal-size collisions, separated droplet bouncing occurs at low Weber numbers, and increasing its value promotes the transition from separated bouncing to random detachment and droplet split. For unequal-size, directional and oblique split replace the random-detachment and droplet-split regimes. In addition, increasing e reduces the effective collision overlap and shifts regime transitions toward higher impact velocity, while droplet-size asymmetry and eccentric collision promote nonuniform film stretching, oblique rupture, and directional separation. These results clarify the coupled roles of inertia, dissipation, thermal fluctuations, and geometric asymmetry in nanoscale droplet transport on curved surfaces.