Viscoelastic Fingering of Shear-Thinning Drops Impacting on Superhydrophobic Surfaces.

When water droplets impact solid surfaces at high velocity, they often develop radial protrusions, known as fingering instabilities, that subsequently break up during spreading and retraction, a process termed splashing. Here, we investigate the fingering dynamics of shear-thinning viscoelastic droplets impacting superhydrophobic surfaces. At low polymer concentrations, liquid elasticity promotes the emergence of elongated fingers while simultaneously stabilizing them against breakup, thereby suppressing splashing. In contrast, an increasing polymer concentration enhances viscous damping, reducing the number of fingers and ultimately suppressing the fingering instability. Our results indicate that the onset of fingering is governed by the interplay of inertia, surface tension, and viscous stresses, while the number of fingers scales robustly with the Weber number. This highlights the dominance of inertia-capillary dynamics in our range of Weber numbers once the instability is triggered. Remarkably, all impact outcomes resulted in complete rebound, in contrast to a previous observation for viscoelastic droplets. Finally, we employ a theoretical framework to predict the temporal evolution of the mean ligament length across polymer concentrations, providing quantitative insight into how elasticity modifies drop retraction dynamics.