Air entrapment dynamics during droplet impact: Effects of size ratio and confinement ratio investigated via lattice Boltzmann method

Air entrapment accompanying droplet impact on solid surfaces is a core factor restricting performance enhancement of advanced technologies such as inkjet printing and spray cooling. Existing studies generally decouple the droplet formation and impact stages, neglecting the role of nozzle structural parameters in the initial control. Moreover, the intrinsic energy-driven mechanism governing air entrapment dynamics has rarely been clarified. This work employs the pseudopotential lattice Boltzmann method to systematically investigate the complete dynamic process of the droplet. The regulatory effects of size ratio ( SR ) and confinement ratio ( CR ) on air entrapment behavior are explored. The energy distribution and conversion characteristics before and after impact are quantified. A dynamic phase diagram for air entrapment formation is established. The results indicate that the size ratio and confinement ratio directly determine the onset and collapse of air entrapment by controlling the relaxation state of the droplet before impact. Within moderate parameter ranges, a flattened droplet morphology combined with moderate kinetic energy facilitates the formation of stable air entrapment. During the collapse of air entrapment, the rapid expulsion of high-pressure gas induces characteristic peaks in internal kinetic energy and viscous dissipation. The formation of air entrapment requires strict coupling conditions: under the influence of the size ratio, AR > 1.1 and 26.95 < We < 63.21; under the influence of the confinement ratio, AR > 1.03, We > 48.17, and CR > 0.14. This work clarifies the physical mechanisms by which droplet geometric parameters regulate air entrapment, providing important theoretical support for optimizing processes in related applications.