A numerical study of dynamic meniscus

Air entrainment is one of the bottlenecks during high speed coating process. Numerous studies concerned with the mechanics of air entrainment in coating process have been reported during the last four decades. The difficulty in visualization of three-phase contact point due to small dimension made it a hard task to unveil the mechanism of air entrainment experimentally. In most numerical studies on free surface flow, the effect of air upon dynamic meniscus was usually neglected, which induced the coarse accuracy in air entrainment study. In the present study, the numerical simulation of free surface coating flow as well as airflow upon dynamic meniscus was given. The effect of several parameters, such as coating material viscosity, and coating cup geometry, especially airflow upon free surface on the dynamic meniscus, was studied. The results showed that the dynamic effects of air in the vicinity of dynamic meniscus should not be disregarded. The pressure near the dynamic meniscus, especially at the dynamic contact point in the coating flow simulation, was much smaller than the atmosphere pressure, while in the gas phase calculation, the value was larger than the atmosphere one at the same position. These results indicated another explanation of the mechanics of air entrainment: the low pressure in the vicinity of dynamic contact point in the liquid makes the gas dissolve easily into the coating material, and the gas flow will even break the dynamic meniscus when the pressure difference between the gas and liquid phase is large enough. The effects of several factors on pressure distribution at the dynamic meniscus were also presented