Role of extensional rheology on droplet bouncing

Recent experiments and theories have dismissed the role of increased extensional viscosity during impact of viscoelastic droplets. Here we show that for relatively low Weber numbers, We=ρU02D0/γ=O(100-101), where ρ is the density, U0 the impact velocity, D0 the droplet diameter, and γ the surface tension, droplets tend to bounce on an air film with a thickness, h, which sets off capillary waves that eventually focus into a single wave. This focusing causes rapid deformation of the droplet thus producing high strain rates, which we verified using a particle-tracking method. Without the addition of polymers, the capillary wave focusing generates droplet contact with the substrate; however, with the addition of polymers, contact is inhibited even for relatively small polymer concentrations (10 ppm). We attribute the inhibition of contact to the large increase in the extensional viscosity near the center of the droplet, which dissipates the kinetic energy of the droplet during impact and deformation.