Predicting how fast crystals grow at the free surface of molecular glasses.

Organic glasses can crystallize at their free surfaces far more rapidly than in the bulk, a phenomenon that has challenged our understanding of glass dynamics for more than a decade. This behavior is commonly attributed to the enhanced molecular mobility present at interfaces, yet the microscopic origin of such fluctuations has remained elusive. Here we show that surface crystal growth can be understood in terms of collective small displacements (CSD), local rearrangements that enable reshaping of the amorphous packing leading to equilibration in liquids and glasses. Within this framework, crystal growth is governed by the slow Arrhenius process (SAP), the experimental manifestation of CSD. Building on this concept, we develop a minimal model that quantitatively predicts crystal growth rates at free surfaces from thermodynamic and energetic considerations. The model accurately reproduces both the magnitude and temperature dependence of growth rates for 14 different organic molecules, spanning a large range of glass transition temperatures and intermolecular interactions, with direct implications for pharmaceuticals and organic electronics. More broadly, our results establish CSD as the microscopic mechanism underlying fast surface crystal growth, offering a predictive framework to anticipate and ultimately control the properties of glasses and crystals.