Surface tension driven leveling of spray droplets

Spray coating has the potential to be a major non-contact coating method. However, to obtain good coverage and uniformity of the coating layer on paper, sufficient leveling of droplets must take place. Though there has been some work on the leveling of scratches in coating layers due to blades, little has been done to understand the leveling of droplets on paper. A model of surface tension driven leveling is developed that can start with the shape after the droplet impact event, and follow the leveling of the fluid layer as a function of time. The model accounts for surface tension of fluid, viscosity, and base paper absorption characteristics, such as void fraction, pore size and contact angle. The competing processes of leveling and absorption det. the final surface uniformity. The key parameters during leveling are droplet size and fluid viscosity. Other parameters such as coating solids content and filter cake flow resistance can also result in incomplete leveling and lead to poor quality coating layers. The basic predictions of the model are supported by the general trends reported on spray coating. [on SciFinder(R)]