UNC-CH Physics and Astronomy Colloquium

James De Yoreo, Pacific Northwest National Laboratory

“They Call it Free Energy, so, Hey, Why Pay?”

Nucleation is the seminal process in the formation of ordered structures ranging from simple inorganic crystals to macromolecular films. Recent observations have revealed a rich set of hierarchical pathways involving higher-order species ranging from multi-ion clusters to dense liquid droplets to transient amorphous or crystalline phases. Despite their complexity, a holistic framework for understanding such pathways based on classical concepts emerges when the effects of complexities in free energy landscapes and kinetic factors are considered. I illustrate that framework using in situ TEM and AFM studies of inorganic, organic, and macromolecular systems. The results show that introduction of size-dependent phase stability or high driving force coupled with the existence of metastable polymorphs leads to true two-step pathways characterized by the initial appearance of a bulk precursor. Creation of microstates representing local free energy minima stabi-lized by configurational factors also drives hierarchical pathways, but the intermediates can only exist as transient microscopic entities. Small changes in molecular structure can eliminate these transients and lead to direct nucleation pathways. In either case, reduction in molecular mobility can freeze in non-equilibrium states for kinetic reasons. The findings provide a common basis for understanding the development of order in diverse systems.