Collision of nanoscale water droplets: A molecular dynamics study

Through molecular dynamics (MD) simulations, the collision of two unequal-size droplets with various diameter ratios (Δ) is elaborately scrutinized over a wide range of impact Weber numbers (We). The dynamic evolutions are carefully observed via visual software, which is found to be very different compared with equal-size collisions. The spreading hysteresis is observed over a full spectrum of We for unequal-size collisions because of the merged droplets experiencing wave-capillary propagation, which is commonly seen at the macro-scale. The merged droplet is always directionally moving toward the negative x-direction, resulting from the existence of an energy difference between two collisional droplets. The variation of Δ and We on unequal-size-induced directional movement is discussed in detail by recording the mass-center variation of merged droplets at different given conditions. The maximum spreading factor of merged droplets, βmax, co, has been discussed by involving Δ into the energy-balance equation. Although the predicted results are slightly lower than MD simulations, this is still the most suitable mode due to the different energy dissipation mechanisms at the nanoscale. How Δ affects the off-center collisions is examined by observing collisional dynamics and drawing phase diagrams with respect to three important characteristic parameters (We, B, and Δ). Finally, we establish a theoretical model for predicting the boundary between coalescence and stretched separation, which agrees well with data extracted from MD simulations. This work paves the way to understand the collision with two unequal-size nanodroplets and provides guidance for nanoscale applications.