Ice formation on surfaces enhanced via a non-classical process 17 August 2021

approach to better predict and control crystallization processes.

A common model used to quantify nucleation kinetics based on a thermodynamic framework, classical nucleation theory (CNT), suggests that molecules must form an nucleus of critical size before a crystallization process occurs. The formation of the critical ice nucleus is associated with a single free energy barrier, which needs to be overcome to trigger further ice growth. However, over the years, both experiments and simulations have revealed that CNT is often insufficient to Ice nucleation on foreign surfaces can proceed via both describe some complex nucleation processes. one-step (magenta arrow) and two-step (orange arrows) Consequently, CNT has been a subject of immense pathways, facilitated by the balanced synergetic, debate, and non-classical nucleation theories have entropic effects of hexagonal (green spheres) and been alternatively proposed. rhombic (purple spheres) ice structures. Credit: The Hong Kong University of Science and Technology Different from CNT, which is based on overcoming a single free energy barrier, non-classical nucleation theories suggest that nucleation processes consist of two or more steps separated Ice is omnipresent and profoundly impacts our by multiple free energy barriers. Although non- daily life, influencing areas such as climate classical nucleation theories may be a more change, transportation, and energy consumption. sustainable model, the atomistic mechanisms and Understanding the process of ice formation can structural evolutions during nucleus formation in decelerate the rate at which melt and sea non-classical nucleation pathways are not well levels rise and alleviate other major environmental known; and remains a challenge for experimental concerns. techniques to unravel.

Since ice formation is mainly governed by ice Now, for the first time, a group of researchers at nucleation followed by the growth of the nuclei, HKUST led by Prof. Xuhui Huang from the scientists have put in a great effort to understand Department of Chemistry combined Markov State the thermodynamics and kinetics behind the Models (MSMs) – which model long-timescale nucleation processes. Ice nucleation can occur in dynamics of chemical molecules—and transition two distinctive ways: homogeneously in bulk water path theory (TPT) – which describes the reaction or heterogeneously on the surface of a pathway of rare events—to elucidate the ensemble material, where heterogeneous ice nucleation pathways of HIN. MSMs identify intermediate states (HIN) is the predominant mode of ice formation on of disordered ice mixtures and compare parallel earth. However, unlike homogeneous ice pathways (classical vs. non-classical). This nucleation, the water-surface interactions present advantage helped unravel the underlying in HIN make the nucleation process sensitive to mechanisms of non-classical nucleation processes surface properties. Understanding how surfaces and the co-existence of the two pathways. impact the nucleation process is a promising

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These researchers show that the disordered mixing of ice stabilizes the critical nucleus and makes the non-classical nucleation pathway as accessible as the classical pathway, whose critical nucleus mainly consists of potential energy-favored ice. They also discovered that at elevated temperatures, the nucleation process prefers to proceed via the classical pathway since the potential energy contributions, which favor the classical pathway, prevail.

"Not only does our work uncover the mechanisms of non-classical nucleation processes, but it also demonstrates how the combination of MSMs and TPT offers a powerful framework to study structural evolutions of ice nucleation processes," said Prof. Huang. "More importantly, this method can be extended to other crystal nucleation processes that are challenging to study, which will open new doors for scientists attempting to predict and control crystallization processes."

The findings were recently published in the scientific journal Nature Communications. The first author of this work: Dr. Chu Li is a long-time HKUST affiliate who completed his Ph.D., and currently conducts his post-doctoral training at HKUST.

More information: Chu Li et al, Temperature- dependent kinetic pathways of heterogeneous ice nucleation competing between classical and non- classical nucleation, Nature Communications (2021). DOI: 10.1038/s41467-021-25267-2

Provided by Hong Kong University of Science and Technology APA citation: Ice formation on surfaces enhanced via a non-classical nucleation process (2021, August 17) retrieved 29 September 2021 from https://phys.org/news/2021-08-ice-formation-surfaces-non- classical-nucleation.html

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