Frozen Patterns in Viscoelastic Droplets Impacting on a Subcooled Surface

The impact of droplets on a cold surface is ubiquitous in nature and various industrial applications, ranging from the icing of supercooled droplets on aircraft to the solidification of ink droplets in 3D printing. However, our understanding of the impact dynamics of droplets of complex fluids on cold surfaces is still very limited. Here, we experimentally study the spreading and frozen patterns of viscoelastic polymer droplets falling onto a subcooled substrate. We observe that the maximum spreading diameter of post-impact droplets decreases with increasing the subcooling temperature and the polymer concentration. Remarkably, all experimental data for spreading collapse into a universal curve, following the classic theory that accounts for inertial, capillary, and viscous forces. Unexpectedly, we find that, in contrast to the case of pure fluids, which exhibits three frozen modes, only two distinct modes, namely, freezing and hierarchical cracking, can be observed for polymer droplets. Finally, based on the undercooling temperature and polymer concentration, we construct a phase diagram for characterizing the morphologies of all frozen patterns. We expect that our findings may have implications in understanding the solidification of complex fluids on cold surfaces, for instance, in the fields of spray coating, inkjet printing, and additive manufacturing.