Non-Newtonian fluid droplet impact dynamics on thin liquid films

Droplet impact on liquid films is a ubiquitous phenomenon in nature and many industrial applications. The present study highlights the impact dynamics of viscoelastic droplets on thin films of water and the same viscoelastic fluid as the droplet. In this experimental study, we have highlighted the variations in the impact dynamics that arise due to the non-Newtonian effects. The ejection of secondary droplets from the crown rim normally observed in the case of Newtonian droplet impacts on water films is suppressed, in the case of non-Newtonian droplet impacts on water films. Due to the fluid elasticity, the Rayleigh–Plateau instability-induced ejection of secondary droplets from the crown rim is inhibited. Long-lasting slender liquid filaments resembling beads-on-a-string structures are observed in the case of viscoelastic droplet impact on water films. However, when the drop and film are of the same viscoelastic fluid, such filaments are not observed during the crown formation stage. Subsequently, we have characterized the geometrical features of the crown and the regime maps of various outcomes of the droplet impact dynamics. It is observed that the elasticity of the liquid suppresses the crown growth and secondary droplet formation. The dependence of the impact dynamics on Weber number (We), polymer concentration in terms of elasticity number (El), Bond number (Bo), and film thickness (h*) are also highlighted in the form of regime maps. It is observed that secondary droplet ejection does not take place on an increase in Bond number and film thickness for both water and viscoelastic films.