Self-Assembly of Chiral Menthol Molecules from a Liquid Film into Ring-Banded Spherulites

We investigate the growth of spherulites in the wake of a solidification front in a thin film of liquid menthol. We observed that the two enantiomeric forms of menthol ((-)-menthol and (+)-menthol)) form ring-banded spherulites, in which needle-like crystals self-assemble into high- and low-density crystal regions. Interestingly, the racemic mixture produces nonbanded spherulites consisting of closely packed plate-like crystals. In the ring-banded spherulitic growth, we could clearly identify the curvature effect of the solidification front on the pattern formation on a millimeter spatial scale. We developed a numerical model based on the Cahn-Hilliard equation, which qualitatively describes the main features observed in experiments, namely, the formation of periodic ring-banded structures and the curvature effect of the propagating solidification front.