Impact, expansion and retraction of drops of complex fluids

In many situations, one wishes to control the outcome of a drop impact on a surface, in order to minize fouling, prevent rebound, or optimize deposition. For this purpose, surfactants and polymers are often added to the impacting liquid. However the details of the interaction of such complex fluid drops with surfaces remain incompletely understood. I will show that for the case of surfactants, it is the dynamic surface tension (DST) at very short time scales that controls the outcome of the drop impact event. I will show how the DST at the time scale of the impact can be measured, since this quantity is not normally measurable with exisiting techniques. Knowing this, we can relate it to the outcome of a spraying test using surfactants. For polymeric liquids, non-Newtonian effects become important. We provide a quantitative understanding of the expansion of the drops, that turns out to be similar to that of Newtonian fluids. However for the drop retraction, non-Newtonian normal stresses slow down the retraction and can be used to optimize deposition.