Heterogeneous Ice Growth in Micron-Sized Water Droplets Due to Spontaneous Freezing

Niloofar Esmaeildoost,Jonas A Sellberg,Ne-Te Duane Loh,Trevor A Mcqueen,Olof Jönsson,Marjorie Ladd-Parada,Hartawan Laksmono

doi:10.3390/cryst12010065

Abstract

Understanding how ice nucleates and grows into larger crystals is of crucial importance for many research fields. The purpose of this study was to shed light on the phase and structure of ice once a nucleus is formed inside a metastable water droplet. Wide-angle X-ray scattering (WAXS) was performed on micron-sized droplets evaporatively cooled to temperatures where homogeneous nucleation occurs. We found that for our weak hits ice grows more cubic compared to the strong hits that are completely hexagonal. Due to efficient heat removal caused by evaporation, we propose that the cubicity of ice at the vicinity of the droplet’s surface is higher than for ice formed within the bulk of the droplet. Moreover, the Bragg peaks were classified based on their geometrical shapes and positions in reciprocal space, which showed that ice grows heterogeneously with a significant population of peaks indicative of truncation rods and crystal defects. Frequent occurrences of the (100) reflection with extended in-planar structure suggested that large planar ice crystals form at the droplet surface, then fracture into smaller domains to accommodate to the curvature of the droplets. Planar faulting due to misaligned domains would explain the increased cubicity close to the droplet surface.

Highlights

Our climate system relies heavily on the understanding of water and ice and the transition between these two phases [1,2]
It is clear that the Wide-angle X-ray scattering (WAXS) profile at 3.91 ms contains a strong contribution of residual water scattering, as macroscopic amorphous ice is not expected to be formed at these nucleation temperatures and cooling rates
The peak locations of our WAXS profiles are shifted compared to Ih recorded at 88 K [51]

Summary

Introduction

Our climate system relies heavily on the understanding of water and ice and the transition between these two phases [1,2]. Water and ice are significant heat drains or sources due to their considerable specific and latent heat and have a major impact on the climate when it comes to clouds [3,4,5]. It is predicted that all macroscopic ice crystals start from critical-sized nuclei that increase with temperature but are as small as 1 nm in diameter upon deep supercooling [8,9] These nuclei are theorized to start from a five-membered ring of H-bonded water molecules rather than the hexagonal or cubic six-membered rings [9,10]. When ice was deposited on a cryogenically cooled Pt(111) substrate, spiral growth around screw dislocations determined whether the structure of ice was cubic or hexagonal [26]

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