| Discussion of pattern formation in spherulites has been dominated for more than a century by the idea that concentric optical banding is caused by helical twisting of radial fibrils. Despite its success in describing polymer spherulites, this model is not easily extended to small molecules that lack the lamellar sheets of long-chain molecules. The ubiquity of banded spherulites belies the agency of crystal-specific twin laws in their formation, and micrographs of several substances described herein, including potassium dichromate, phthalic acid, and mannitol, show that rhythmic deposition is a sufficient condition for banding in these systems. Several mechanisms for spherulite growth are considered, and it is concluded that rhythmic precipitation, sometimes accompanied by helical twisting, is a general mechanism for banding in spherulites. Tightly controlled spherulite growth experiments establish that humidity is an important control factor in determining the morphology of phthalic acid spherulites from solution. Rapid evaporation favors formation of a kinetically stabilized, amorphous glass that precedes spherulite formation and provides the diffusion control necessary for rhythmic deposition. Presence of a glassy phase in several spherulite-forming substances indicates that it may be requisite for controlling diffusion in solution growth of spherulites. |
