Abstract

Ice clouds can form at low and moderate supercooling through heterogeneous ice nucleation on atmospheric particles. Typically, the nucleation requires active sites with special chemical and physical properties, including surface topology and roughness. This paper investigates microscopic mechanisms of how combinations of confinement by the surface topology and lattice match induced by the surface properties can lead to enhanced ice nucleation. We perform molecular dynamics simulations using both atomistic and coarse-grained water models, at very low supercooling, to extensively study heterogeneous ice nucleation in slit-like and concave wedge structures of silver-terminated silver iodide (0001) surfaces. We find that ice nucleation is greatly enhanced by slit-like structures when the gap width is a near-integer multiple of the thickness of an ice bilayer. For wedge systems we also do not find a simple linear dependence between ice nucleation activity and the opening angle. Instead we observe strong enhancement in concave wedge systems with angles that match the orientations of ice lattice planes, highlighting the importance of structural matching for ice nucleation in confined geometries. While in the slit systems ice cannot grow out of the slit, some wedge systems show that ice readily grows out of the wedge. In addition, some wedge systems stabilize ice structures when heating the system above the thermodynamics melting point. In the context of atmospheric ice nucleating particles, our results strongly support the experimental evidence for the importance of surface features such as cracks or pits functioning as active sites for ice nucleation at low supercooling.

Highlights

  • Heterogeneous ice nucleation is ubiquitous and important in the atmospheric processes at temperatures above 235 K (Tabazadeh et al, 2002; Djikaev et al, 2002)

  • We have shown that in general, confining water can significantly enhance ice nucleation even at very low supercooling, and lead to ice nucleation at higher temperatures than any known flat surfaces

  • We find enhanced nucleation for gap widths accommodating integer multiples of the ice bilayer width

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Summary

Introduction

Heterogeneous ice nucleation is ubiquitous and important in the atmospheric processes at temperatures above 235 K (Tabazadeh et al, 2002; Djikaev et al, 2002). We employ molecular dynamics (MD) simulations using both TIP4P/Ice (Abascal et al, 2005) and mW water models to extensively study heterogeneous ice nucleation in slit-like and concave wedge structures of silver iodide in the wurtzite structure (β-AgI), exposing the Ag-terminated (0001) surface. It has been shown both experimentally and computationally that silver iodide is an effective material in promoting ice nucleation (Marcolli et al, 2016; Fraux and Doye, 2014; Shevkunov, 2016; Zielke et al, 2014; Prerna et al, 2019; Roudsari et al, 2020), and it has been used as a rain seeding agent for many decades (Vonnegut, 1947).

Methods
Molecular Dynamics simulations
Construction of slit and wedge systems
Impact of temperature and ice growth dynamics
AgI wedge simulations with mW model at 263 K
Nucleation of ice on flat AgI(0001) with mW model
AgI wedge simulations with TIP4P/Ice model at 263 K
Microscopic ice growth mechanism
Ice cubicity in wedge simulations
Nucleation and growth at lower supercooling
Ice persisting in wedges above the melting point
Conclusions
Full Text
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