The role of extensional rheology on droplets bouncing on air

While it is conventionally understood that droplet rebound is suppressed on hydrophobic surfaces upon the addition of polymers, the exact cause has been open for debate. In particular, the role of the extensional viscosity on droplet rebound has been neglected due to the fact that the spreading dynamics are uninfluenced by the polymer additives as well as the fact that droplet rebound is actually increased under Leidenfrost conditions. Here, utilizing a high-speed total internal reflection microscopy (TIRM) setup, we measure in-situ the air film formation and collapse for Weber numbers (We) between 2 < We < 40 when drops impact onto smooth, viscous thin films, micrometers in thickness, for various concentrations and molecular weights of dilute polymers in the droplet. Under ambient conditions, the maximum We for rebound increases with increasing polymer molecular weight and concentration. We report that not only does the addition of polymer additives help in the rebound process when there is a non-wetting viscous air film, but the extensional rheology significantly influences the critical condition under which the air film fails and promotes contact with the surface below.