Reversible twisting during helical hippuric acid crystal growth

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 Reversible twisting and untwisting of macroscopic crystals of hippuric acid (N-benzoylglycine, C ₉H ₉NO ₃) on the scale of radians. Crystals growing 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 crystal morphology. A correspondence between the optical properties of hippuric acid single crystals and twisted needles measured with a Mueller matrix microscope is established. The measured crystalloptical properties are in complete accord with the optical indicatrix rotating helically along the axis of elongation at the growth front, or counter-rotating so as to unwind earlier growth. The Reversible morphological changes captured here in situ are likely also found in banded spherulites of high molecular weight polymers, optically modulated chalcedony minerals, elements, proteins, and other molecular crystals for which there is evidence of helical twisting. The analysis of such systems is usually predicated on the relationship of crystalline helical pitch to a power law exponent. However, in the absence of previous considerations of untwisting, the mechanistic content of such relationships is questionable.