Phase diagram for nanodroplet impact on solid surfaces

The impact dynamics of water nanodroplets on flat solid surfaces was studied by molecular dynamics simulations over a wide range of Weber numbers (We) and surface wettability (θ₀), where θ₀ is the Young contact angle. A phase diagram in the parameter space of We vs θ₀ was established accommodating eight impact outcomes noted in the final stage of impact, with three of them, holes rebound, partial-rebound splash, and rebound splash, for the first time being identified and reported. The eight impact outcomes were classified into three categories, i.e., non-bouncing, bouncing, and splash. The results show that the splash is triggered only when We_cr > 140. The boundaries separating bouncing from non-bouncing were determined based on the phase diagram. When θ₀ > 160°, the boundary is described as We_cr = a 1; when 110° < θ₀ < 160°, the boundary depends on both We and θ0, with a larger We required to trigger bouncing on a less hydrophobic surface, expressed as We_cr = b + ccosθ₀; when θ₀ < 110°, bouncing never takes place, and hence, the boundary is determined only by the critical contact angle, expressed as θ_0,cr = 110°. Here, a, b, and c are constants.