Controlling drop impact by additives

Whether painting, printing, cooling or simply cleaning surfaces, spraying a fluid onto them is an event we encounter daily. However, in many cases the processes involved are very complex and a multitude of different factors need to be considered. According to the fluid properties, the impact velocity, and the surface roughness, drops may either stick to the wall, or hit the wall and bounce off, or break-down upon impact into smaller droplets; on the other hand, the size and size distribution of drops in sprays is often determined by design or environmental constraints.

Recent advances in our understanding of 'complex fluids' have led to the development of chemical additives that greatly improve spray applications. In this seminar the effects of additives on drop impact phenomena are reviewed, with special attention to the effects of flexible polymer additives. The results can be interpreted in terms of a coil-stretch transition of polymer molecules driven by the hydrodynamic action of the surrounding fluid: at rest, polymer molecules are coiled in a minimum energy conformation, whereas if the hydrodynamic forces are sufficiently large they unfold, opposing an increasing resistance to deformation as they are stretched.