Visualisation of individual polymer molecules inside impacting drops

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 new chemical additives that greatly improve the spray application. For instance, it has been shown that tiny amounts of polymer additives can completely suppress drop rebound on hydrophobic surfaces, as well as the emission of secondary droplets during evaporation on heated surfaces. Thus, using judiciously chosen dilute polymer solutions, one can simultaneously improve the spray characteristics, droplet deposition, and product retention onto the targeted surface, with obvious benefits for both industrial and domestic applications. However, the physical mechanisms behind these phenomena are poorly understood at present, so that we have only a qualitative picture of them.In order to better understand how the dynamics of polymer molecules is related to the macroscopic behaviour of the drop during impact, we propose here to visualise fluorescent polymer molecules inside impacting drops. Direct molecular visualisation techniques have been developed in recent years, and allow one to visualise by optical microscopy the conformation of polymer molecules to which special fluorescent dyes have been attached. By grabbing image sequences with a high speed camera, we will study the dynamics of polymer molecules dissolved in the fluid during the spreading of a drop impacting onto a surface. The information collected during these experiments is expected to improve our understanding of the effect of polymer additives on drop impact phenomena, but will also shed light on the more fundamental aspects of polymer molecule dynamics, which have never been investigated in similar conditions.In order to successfully carry out this research project, we are seeking financial support for one post-doctoral Research Associate, and for purchasing the necessary laboratory equipment.