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Abstract. Heterogeneous ice nucleation is thought to be the primary pathway for the formation of ice in mixed-phase clouds, with the number of active ice-nucleating particles (INPs) increasing rapidly with decreasing temperature. Here, molecular-dynamics simulations of heterogeneous ice nucleation demonstrate that the ice nucleation rate is also sensitive to pressure and that negative pressure within supercooled water shifts freezing temperatures to higher temperatures. Negative pressure, or tension, occurs naturally in water capillary bridges and pores and can also result from water agitation. Capillary bridge simulations presented in this study confirm that negative Laplace pressure within the water increases heterogeneous-freezing temperatures. The increase in freezing temperatures with negative pressure is approximately linear within the atmospherically relevant range of 1 to −1000 atm. An equation describing the slope depends on the latent heat of freezing and the molar volume difference between liquid water and ice. Results indicate that negative pressures of −500 atm, which correspond to nanometer-scale water surface curvatures, lead to a roughly 4 K increase in heterogeneous-freezing temperatures. In mixed-phase clouds, this would result in an increase of approximately 1 order of magnitude in active INP concentrations. The findings presented here indicate that any process leading to negative pressure in supercooled water may play a role in ice formation, consistent with experimental evidence of enhanced ice nucleation due to surface geometry or mechanical agitation of water droplets. This points towards the potential for dynamic processes such as contact nucleation and droplet collision or breakup to increase ice nucleation rates through pressure perturbations.
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Microstructure and crystal order during freezing of supercooled water drops
Supercooled water droplets are widely used to study supercooled water [1,2], ice nucleation [3-5], and droplet freezing [6-11]. Their freezing in the atmosphere impacts the dynamics and the climate feedback of clouds [12,13], and can accelerate cloud freezing via secondary ice production [14-17]. Droplet freezing occurs at multiple time and length scales [14,18], and is sufficiently stochastic to make it unlikely that two frozen drops are identical. Here we use optical microscopy and X-ray laser diffraction to investigate the freezing of tens of thousands of water microdrops in vacuum after homogeneous ice nucleation around 234–235 K. Based on drop images, we developed a seven-stage model of freezing and used it to time the diffraction data. Diffraction from ice crystals showed that long-range crystalline order formed in less than 1 ms after freezing, while diffraction from the remaining liquid became similar to the one from quasiliquid layers on premelted ice [19,20]. The ice had a strained hexagonal crystal structure just after freezing, which is an early metastable state that likely precedes the formation of ice with stacking defects [8,9,18]. The techniques reported here could help determine the dynamics of freezing in other conditions, such as drop freezing in clouds, or help understand rapid solidification in other materials.
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- Award ID(s):
- 2123634
- PAR ID:
- 10489656
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature
- Volume:
- 620
- Issue:
- 7974
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 557 to 561
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41586-023-06283-2
