2024 AIChE Annual Meeting
(40e) Process Variables That Control of the Morphology of Lamellar Crystals through Bending Elasticity
Author
Santore, M. - Presenter, University of Massachusetts Amherst
Ultra-thin materials are of great interest in the creation of flexible electronics, conformal coatings, and materials that could be used to repair the body. Control of crystallization and the role of processing is also critically important in the pharmaceutical industry. We have been exploring how nanometrically thin fluid sheets such as phospholipid lamella enable morphological control of 2D (ultrathin) crystals that nucleate and grow within the lamellae themselves, for instance achieving thicknesses of only a few nanometers. Ultrathin crystals are mechanically solid and rigid to shear, they often prefer to be flat rather than curved due to molecular symmetry across a bilayer, and when they bend they tend to do so only cylindrically. When forced to nucleate and grow on spherical templates for instance a droplet surface, they experience prohibitive strain energy cost as a result of their inability to shear or stretch. As a result, beyond a size that increases with the size of the droplet, the crystal morphology shifts from a compact to a striped shape as the crystal grows. We have discovered that crystals growing within fluid lamella of closed vesicles exhibit the opposite dependence on vesicle size, with crystals growing within larger, more gradually curved vesicles adopting morphologies with greater protrusions. We show here that while lamellar crystals, like those growing on droplets, do not tolerate finite Gaussian curvature, when incorporated into fluid membranes, the bending of the membrane plays an important role in the overall energy, thereby dictating morphology. Further, when nucleation and growth results from a thermal quench into a two phase region, thermal membrane stress plays an important role in setting the bending costs, and this stress is relaxed by water transport. Hence the cooling history translates to a vesicle-size dependent tension history that controls crystal morphology. Batch processing methods that regulate thermal stress enable small scale production of highly regular lamellar crystals of select regular shapes (flowers, stars, hexagons) that correlate with vesicle size, facilitating their separations