Twisting and Untwisting Crystals

Chemists from New York University (NYU) and St. Petersburg State University in Russia have discovered a wholly new phenomenon for crystal growth--a crystal that continually changes its shape as it grows.

Crystals grow in the mind's eye by the addition of small units to a monolith, each part of which is in fixed translational relation to every other part. Here, it is shown that growth can induce dynamic twisting and untwisting of macroscopic crystals on the scale of radians. This reversibility has been observed in helical crystals of hippuric acid, a simple derivative of the amino acid glycine. Growing crystals in undercooled melts of hippuric acid twist about the axis of elongation. At the same time, the twisting is undone by new elastic stresses that build up as the crystal thickens. The dynamic interplay of twisting and untwisting ultimately fixes the crystalline morphology. The interference colors that the crystals display when sitting between crossed polarizers tells the whole story of the complex morphology in the modulated array of interference colors.

This research was performed by Bart Kahr, a professor of chemistry in the Molecular Design Institute at New York University. The research--supported by a grant from the National Science Foundation (CHE 08-45526)--focuses on the optical properties of polycrystalline patterns. Kahr says he found that some of the bizarre interactions of polycrystalline hippuric acid formations and light can be well explained by analyzing individual needles that show, in their rainbows of Newton's interference colors, helical twisting of the lattice.

Credit: John Freudenthal and Alexander Shtukenberg, New York University