Investigation on air entrapment in paint drops under impact onto dry solid surfaces

The present annual report summarises the purpose of the project and the ongoing investigations performed at the Institut für Industrielle Fertigung und Fabrikbetrieb Universität Stuttgart (IFF) on the numerical study of paint drop impacting onto dry solid surfaces. Both Newtonian and yield-stress viscous droplets were applied. Detailed numerical observations of the drop impact dynamics with the focus of air entrapment were obtained. It has been found that at the early stage of the droplet spreading there is no contact line movement, but only direct contact of the droplet outline with the substrate, which results in the formation of an air disc under the impact point. The maximum air disc is reached, when the drop spreading is driven by the movement of the fully wetted contact line. Numerical results showed much more bubble entrapment at the interface between liquid and solid for Newtonian droplets. For shear thinning non-Newtonian fluids the created air disc and air bubbles during drop spreading are reduced tremendously because of the quite low liquid viscosity. The effects of the drop properties, impact velocity and static contact angles on the maximum air disc and on the air bubble release from the droplet film were analysed.