Molecular Dynamics Simulations of the Motions of Droplets with Embedded Nanoparticles on Wettability Gradient Surfaces

Tuning the spontaneous motion of pure nanodroplets on programmed surfaces has progressed notably. However, impurities exist in droplets more often than not. We present molecular dynamics simulations of the dynamic behaviors of a mixture of water droplets and particles at the nanoscale, as they are placed on a wettability gradient substrate. We uncover three different styles of motion, namely, sweeping, separating, and pinning, as interaction strengths among the substances vary. By investigating the potential energies between the droplet and the nanoparticle/ substrate, we explain the competition of the driving force and the impedance due to an imbalance of van der Waals force, which is responsible for differentiating the styles. We also find in some specific cases the phenomenon of tailing, where one side of the droplet is pinned by the nanoparticle as the other side moves forward to generate an extremely large difference of the contact angles. The results provide a fundamental understanding of the dynamic behaviors at the nanoscale and may inspire potential applications in a broad range, such as drug delivery, dust cleaning of nanomachines, liquid purification, etc.