Dynamical behaviors of nanodroplets impinging on solid surfaces in the presence of electric fields.

The collision of droplets with solid surfaces is a common phenomenon in nature. However, droplets exhibit interesting motion states when captured by surfaces. This work investigates the dynamical behavior and the wetting condition of droplets captured by different surfaces in electric fields via molecular dynamics (MD) simulations. By adjusting the initial velocity of droplets (V0), electric field intensity (E) and directions, the spreading and wetting properties of droplets are analyzed systematically. The results indicate that the electric stretching effect occurs when a droplet strikes the solid surface in electric fields and the stretch length (ht) of droplets continuously increases with the enhancement of E. In the low field strength regime, the direction of electric fields has an effect on ht: the value of ht is larger in the case of positive electric fields as compared to negative electric fields. In the high field strength regime, the direction of electric fields makes no difference to ht: the droplet is stretched observably, and the breakdown voltage U is calculated to be 0.57 V nm-1 under both positive and negative electric fields. Droplets impacting with surfaces at initial velocities display various states. The droplet bounces off the surface regardless of the direction of electric field at V0 ≥ 1.4 nm ps-1. The maximum spreading factor ßmax and ht both increase with V0 and are not affected by field directions. The simulation results are consistent with experiments, and the relationships between E, ßmax, ht and V0 are proposed, which provide the theoretical basis for large-scale numerical calculations such as computational fluid dynamics.

Molecular Dynamics Simulation on Behaviors of Water Nanodroplets Impinging on Moving Surfaces.

Droplets impinging on solid surfaces is a common phenomenon. However, the motion of surfaces remarkably influences the dynamical behaviors of droplets, and related research is scarce. Dynamical behaviors of water nanodroplets impinging on translation and vibrating solid copper surfaces were investigated via molecular dynamics (MD) simulation. The dynamical characteristics of water nanodroplets with various Weber numbers were studied at five translation velocities, four vibration amplitudes, and five vibration periods of the surface. The results show that when water nanodroplets impinge on translation surfaces, water molecules not only move along the surfaces but also rotate around the centroid of the water nanodroplet at the relative sliding stage. Water nanodroplets spread twice in the direction perpendicular to the relative sliding under a higher surface translation velocity. Additionally, a formula for water nanodroplets velocity in the translation direction was developed. Water nanodroplets with a larger Weber number experience a heavier friction force. For cases wherein water nanodroplets impinge on vibration surfaces, the increase in amplitudes impedes the spread of water nanodroplets, while the vibration periods promote it. Moreover, the short-period vibration makes water nanodroplets bounce off the surface.