26/03/2021
Crystal nucleation is one of the most fundamental processes in the physical sciences, and almost always proceeds heterogeneously (promoted by a foreign substance) in nature. In conjunction, changing a material’s polymorph is arguably the most fundamental change one can make to its properties and applications. Despite the importance to nature (e.g. cloud formation) and technology (e.g. drug design), the interplay of heterogeneous nucleation and polymorphism is poorly understood, especially at the molecular level. No example of this is more ubiquitous and impactful than the formation of ice, which is vital to fields as diverse as geochemistry, microbiology, and climate chemistry. Ice has two distinct polytypes: stable hexagonal ice, and metastable cubic ice (Ic). Ic is believed to be important in the atmosphere where it affects key properties of clouds, as well as vital to technological applications such as cryopreservation. Yet it has only been achieved in the lab for the first time this year, and the processes use high-pressure ice phases under rather complex controlled conditions not readily encountered in nature. Now with its confirmed existence there is a heightened focus on routes to its formation as might occur in nature, namely heterogeneous nucleation.
With this in mind we tackle a pivotal unanswered question: is it possible to nucleate desired polytypes of ice I through heterogeneous nucleation? We do so by both developing a general methodology to design substrates to promote desired polymorphs/types, and performing a high throughput computational screening study using molecular dynamics simulations to directly observe nucleation on over 1,100 model substrate systems. Considerable new insight is gained from this, and we find that it is indeed possible to nucleate desired polytypes of ice. Excitingly, we discover many novel designs to form pristine Ic and show its promotion is highly robust and reliable. Moreover, new metrics introduced unravel the interplay of heterogeneous nucleation and polymorphism and clear guidelines for experiments to achieve Ic are presented.
We believe this work makes a significant step towards achieving and understanding the formation of Ic via heterogeneous nucleation; an elusive route vital to myriad natural phenomena and a burgeoning number of technological applications. The work is also generalisable and can be extended to polymorphism and stacking disorder in materials in general.
We hope this work helps to bridge the gap between simulations and experiments by providing new routes to polymorphs/types and the microscopic insight required to predict a substrate’s suitability.
The paper can be found in the following link: Routes to cubic ice through heterogeneous nucleation | PNAS