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D-Mannitol belongs to a large and growing family of crystals with helical morphologies (Yu, L. J. Am. Chem. Soc.2003, 125, 6380). Two polymorphs of D-mannitol, α and δ, when grown in the presence of additives such as poly(vinylpyrrolidone) (PVP) or D-sorbitol, form ring-banded spherulites composed of handed helical fibrils, where the helix axes correspond to the radial growth directions. The two polymorphs form helices with opposite senses in the presence of PVP but the same sense in the presence of D-sorbitol. The characteristic dimensions of the fibrils, including thickness, aspect ratio, and pitch, were determined by scanning probe and electron microscopies. These values must form the basis of any theory that presupposes what forces give rise to crystal twisting, a problem that has been broached but unsettled in the literature of polymer crystallization. The interdependence of the rhythmic variations of both linear and circular birefringence, as determined by Mueller matrix microscopy, informs the cooperative organization of mannitol fibers. The microstructure of mannitol ring-banded spherulites compares favorably to that of high polymers and is evaluated within the context of current theories of crystal twisting.

Ferdinand Bernauer proposed in his monograph, "Gedrillte" Kristalle (1929), that a great number of simple, crystalline substances grow from solution or from the melt as polycrystalline spherulites with helically twisting radii that give rise to distinct bull's-eye patterns of concentric optical bands between crossed polarizers. The idea that many common molecular crystals can be induced to grow as mesoscale helices is a remarkable proposition poorly grounded in theories of polycrystalline pattern formation. Recent reinvestigation of one of the systems Bernauer described revealed that rhythmic precipitation in the absence of helical twisting accounted for modulated optical properties [Gunn, E. et al. J. Am. Chem. Soc. 2006, 128, 14234-14235]. Herein, the Bernauer hypothesis is re-examined in detail for three substances described in "Gedrillte" Kristalle, potassium dichromate, hippuric acid, and tetraphenyl lead, using contemporary methods of analysis not available to Bernauer, including micro-focus X-ray diffraction, electron microscopy, and Mueller matrix imaging polarimetry. Potassium dichromate is shown to fall in the class of rhythmic precipitates of undistorted crystallites, while hippuric acid spherulites are well described as helical fibrils. Tetraphenyl lead spherulites grow by twisting and rhythmic precipitation. The behavior of tetraphenyl lead is likely typical of many substances in "Gedrillte" Kristalle. Rhythmic precipitation and helical twisting often coexist, complicating optical analyses and presenting Bernauer with difficulties in the characterization and classification of the objects of his interest.

Organic glasses can grow crystals much faster on the free surface than in the interior, a phenomenon important for fabricating stable amorphous materials. This surface process differs from and is faster than the glass-to-crystal (GC) growth mode existing in the bulk of molecular glasses. We report that similar to GC growth, surface crystal growth terminates if glasses are heated to gain fluidity. In their steady growth below the glass transition temperature T, surface crystals rise above the amorphous surface while spreading laterally and are surrounded by depressed grooves. Above T, the growth becomes slower, sometimes unstable. This damage is stronger on segregated needles (α indomethacin, nifedipine, and o-terphenyl) than on crystals growing in compact domains (γ indomethacin). This effect arises because the onset of liquid flow causes the wetting and embedding of upward-growing surface crystals. Segregated needles are at greater risk because their slow-growing flanks appear stationary relative to liquid flow at a low temperature. The disruption of surface crystal growth by fluidity supports the view that the process occurs by surface diffusion, not viscous flow. Compared to the bulk GC mode, surface crystal growth is disrupted less abruptly by fluidity. Nevertheless, to the extent that fluidity damages them, both processes are solid-state phenomena terminated in the liquid state.

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