Lattice Boltzmann simulations of droplet impact on hydrophobic surfaces: analysis of air entrapment dynamics

The formation and evolution of the air entrapment constitute a critical physical mechanism during droplet impact on solid surfaces, directly affecting the performance of systems such as fuel spray in aero engines, spray cooling, and aircraft anti-icing/de-icing. Most existing studies are based on the assumption of ideal spherical droplet impact, ignoring the decisive effect of the actual dynamic state of the droplets from formation to impact on the air entrapment. In this study, the pseudopotential lattice Boltzmann method is employed to investigate the complete dynamic process from droplet formation to impact by means of a combined analysis of morphological evolution and energy conversion. The results reveal that the pre-impact oscillatory state of the droplet influences the onset of air entrapment. The occurrence of air entrapment requires simultaneous fulfillment of both critical morphological and energy conditions, namely that the aspect ratio ( AR ) is above 1.067 and the kinetic-to-surface energy ratio ( E _k / E _s ) exceeds 4.63. Three distinct evolution patterns emerge across the Weber number range, where the inverse correlation between entrapment size and spreading factor confirms competitive energy allocation between radial expansion and normal cavity formation. The energy released during the entrapment collapse substantially modifies the receding flow structure. These findings establish a quantitative relationship between the initial droplet state and impact behavior and elucidate the fundamental mechanism of air entrapment. This study provides a direct theoretical basis for optimizing droplet impact processes in aerospace applications via active control of injection parameters.