NSF/DMR-BSF: Growth Induced Crystal Curvature

New York University, New York NY

Investigators

Abstract

Non-Technical Abstract The great crystallographer Shubnikov said that "crystals flash forth their symmetry." He was emphasizing in this pithy remark the characteristic feature of crystals, their striking polyhedral shapes with sharp edges and flat faces; crystals, by definition, are straight. Witness facetted gemstones, or naturally grown quartz. However, it was recognized almost a century ago that a wide variety of crystals can be made to grow with twisted, helix-like morphologies -- forms that are decidedly not straight. Remarkably, this general feature of the crystal kingdom was largely forgotten, even though it is characteristic of common substances such as aspirin. With support from the Solid State and Materials Chemistry program, the research team aims to understand how crystals twist themselves as they grow. These processes are windows into the mechanical properties of crystals built from molecules, processes that underlie important industrial operations such as the tableting of pharmaceuticals. Twisted crystals are vivid in their rhythmic bands of optical contrast, promising a bridge between microscopic and macroscopic handed forms. The research team is partnered with several Bronx public high schools in order to provide science identity-building research experiences. Technical Abstract The origin of growth induced deformations of form presumably lie in the supramolecular chemistry of mixed crystals and in the manner in which these forces are manifest in the mechanical properties of solid bodies. The team has shown that additives induce twisting stereospecifically at growing crystal interfaces. Additives likely twist crystals by creating mixed domains with distinct metric properies. Mismatches at the molecular level produce strain that in turn can generate stress and twist moments. The strategy is threefold: 1. Establishing the conditions for twisting mixed or disordered crystals of substituted benzenes about which we already understand the origin of growth induced strain from long-standing studies of single crystals. 2. Using advanced microscopies to accurately describe the shapes of crystals with growth induced curvature -- this work is proceeding in collaboration with Professor E. Efrati at the Weizmann Institute of Science under the collaborative arrangement established by the Division of Materials Research and the US-Israel Binational Science Foundation. 3. Weizmann experts in differential geometry of twisted ribbons are analyzing growth deformation within the framework of geometric frustration encoded in local differential growth. These studies will be matched with atomistic force-field and molecular dynamics simulations.