Frozen Patterns\nin Viscoelastic Droplets Impacting\non a Subcooled Surface

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