How macrostructures enhance droplet coalescence jumping: A mechanism study

Macrostructures exhibit excellent performance in coalescence-induced droplet jumping enhancement. Previous velocity prediction models were mainly established for flat plain surface, which usually set up the balance relationship between excess surface energy, kinetic energy and dissipation. But these models are no longer appropriate in macrostructure cases due to great variation of oscillational kinetic energy and gravitational potential energy caused by textures. In this work, coalescence-induced droplet jumping on macrostructures was investigated numerically on flat, sidewall, string, ridge and egg shape substrates. An energy conversion model based on energy efficiency rather than energy balance was established for departure velocity by proposing three sub-efficiencies to evaluate jumping enhancement. In this model, oscillational kinetic energy was distinguished from dissipation, gravitational potential energy induced by macrostructures were considered, and the real droplet morphology were used to calculate surface energy. By dividing the whole droplet jumping process into merge and contact stage, the influence of macrostructures on duration of each stage was closely connected to the macrostructure effects on dissipation and kinetic energy conversion. Pressure and velocity distributions near the macrostructure were also discussed to further explore the mechanism for droplet jumping enhancement. Further, the significant increase of initial surface energy for egg case points out a possible new approach to enhance jumping velocity. This model helps us understand the determinants of jumping enhancement with macrostructures and it also applies to flat plain.