Multifactorial Numerical Analysis on Microdroplet Impact Hydrodynamics and a Model for Maximum Spreading Coefficient across Broad Wall Wettability

In this work, we employ a coupled level set and volume of fluid (CLSVOF) method to systematically investigate the hydrodynamic characteristics of microdroplet impact on a solid surface under low-Weber-number conditions. The droplet impact regime is elucidated by quantitatively analyzing the dynamic evolution of droplet morphology. The effects of initial velocity, surfactant concentration, glycerol concentration, surface wettability, and droplet diameter on spreading width and rebounding height are emphasized, respectively. The results reveal three distinct temporal stages during the impact process: spreading, receding, and stabilizing stages. Increasing droplet initial velocity is found to significantly promote droplet spreading but simultaneously suppress rebound dynamics. Enhanced surfactant concentration extends both the time required to attain maximum spreading and the maximum rebound height. However, improved viscosity induced by glycerol addition demonstrates dual inhibitory effects on spreading and rebounding behaviors, thereby accelerating the stabilization process. Reduced wettability promotes rebound dominance over spreading, with a complete rebound phenomenon observed on the least wettable surfaces in this work. Droplet diameter exhibits proportional effects on impact dynamics, where larger droplets demonstrate enhanced spreading and rebounding magnitudes accompanied by an extended duration in all three characteristic stages. Finally, a novel semiempirical model is developed to predict the maximum spread coefficient of the microdroplet across broad wall wettability and demonstrates a superior prediction accuracy by comparing with the experimental data sets using existing theoretical models.