Elastic amplification of the Rayleigh-Taylor instability in solidifying melts

The concomitant mechanical deformation and solidification of melts are relevant to a broad range of phenomena. Examples include the preparation of cotton candy, the atomization of metals, the manufacture of glass fibers, and the formation of elongated structures in volcanic eruptions known as Pele's hair. Usually, solid-like deformations during solidification are neglected as the melt is much more malleable in its initial liquid-like form. Here we demonstrate how elastic deformations in the midst of solidification, i.e., while the melt responds as a very soft solid ([Formula: see text] Pa), can lead to the formation of previously unknown periodic structures. Namely, we generate an array of droplets on a thin layer of liquid elastomer melt coated on the outside of a rotating cylinder through the Rayleigh-Taylor instability. Then, as the melt cures and goes through its gelation point, the rotation speed is increased and the drops stretch into hairs. The ongoing solidification eventually hardens the material, permanently "freezing" these elastic deformations into a patterned solid. Using experiments, simulation, and theory, we demonstrate that the formation of our two-step patterns can be rationalized when combining the tools from fluid mechanics, elasticity, and statistics. Our study therefore provides a framework to analyze multistep pattern formation processes and harness them to assemble complex materials.