Simulation of binary droplet collisions from bouncing to shattering regime

Binary droplet collisions exhibit four primary outcomes at Weber number (We) below 100, i.e., bouncing, coalescence, reflexive separation, and stretching separation. At We exceeding 300, shattering becomes prominent, characterized by the catastrophic disintegration of the merged mass and ejection of satellite droplets. Previous numerical studies have largely focused on isolated regimes, with limited overlap across the full impact parameter space, and typically require regime-specific models. In this work, an integrated simulation approach is presented based on Smoothed Particle Hydrodynamics (SPH) to capture all major collision regimes from We=10 to 1000. This mesh-free method successfully reproduces both head-on and off-center collision outcomes for water, tetradecane, and heptane under near room conditions. The bouncing regime is modeled using an interfacial curvature-based approach tailored to account for hydrocarbon behavior, accurately replicating the transition to coalescence and stretching separation. The present SPH simulations capture the deformation morphologies and satellite droplet formation for stretching separation, while the critical We for reflexive separation is observed to be higher than experimental values. The shattering regime is validated for head-on collisions using the breakup diameter ratio, which shows strong agreement with experimental data in the range We=200-1500. This regime is further extended to off-center collisions, revealing subregimes. A comprehensive regime map is presented to illustrate the collision outcomes over the entire We range (0-1000).