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Microscopic salt exclusion dynamics of directional freezing in brine

Published online by Cambridge University Press:  15 October 2025

Zhaohao Wu
Affiliation:
College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, Zhejiang, 310058, PR China School of Engineering, Westlake University , Hangzhou, Zhejiang, 310030, PR China
Xiangbo Gao*
Affiliation:
College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, Zhejiang, 310058, PR China School of Engineering, Westlake University , Hangzhou, Zhejiang, 310030, PR China
Jinbo Jia
Affiliation:
School of Engineering, Westlake University , Hangzhou, Zhejiang, 310030, PR China
Yingbo Jiang
Affiliation:
College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, Zhejiang, 310058, PR China School of Engineering, Westlake University , Hangzhou, Zhejiang, 310030, PR China
Herbert E. Huppert
Affiliation:
Institute of Theoretical Geophysics, King’s College, University of Cambridge, King’s Parade, Cambridge, CB2 1ST, UK
Liang Lei*
Affiliation:
School of Engineering, Westlake University , Hangzhou, Zhejiang, 310030, PR China Research Center for Industries of the Future, Westlake University , Hangzhou, Zhejiang, 310030, PR China Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou, Zhejiang, 310024, PR China
*
Corresponding authors: Liang Lei, leiliang@westlake.edu.cn; Xiangbo Gao, gaoxiangbo@westlake.edu.cn
Corresponding authors: Liang Lei, leiliang@westlake.edu.cn; Xiangbo Gao, gaoxiangbo@westlake.edu.cn

Abstract

Directional freezing of brine is widely found in numerous environmental and industrial settings. Despite extensive studies, the microscopic evolution of ice-brine structures remains unclear. By combining in situ micro-computed tomography visualisation and theoretical analyses, we reveal new details inside the porous ice structure and its evolution towards a cleaner ice layer. We identify three distinct stages characterised by different brine exclusion rates during solidification: a rapid initial stage possibly lasting seconds from nucleation to local equilibrium without long-range heat or mass transfer; a second stage where the system reaches global thermal equilibrium, involving brine expulsion by volume expansion and convection associated with gravity; and a final prolonged stage dominated by diffusion. Comparison between analytical solutions and the migration rates of microstructural features such as brine stripes, columns and pockets extracted from photographic images confirms these understandings. Morphologically, we capture the formation of random striped patterns together with brine columns during downward freezing and brine skirts during upward freezing, all of which gradually transform into vertically aligned polygonal patterns. The volume fraction of brine pockets in porous ice near the cold end reduces to less than 10 % after 22 h in most experiments. The residual brine pockets, however, are not rejected out of the porous ice as fast as predicted by diffusion and remain persistent. Our findings provide new insights into the brine freezing dynamics, with implications ranging from sea ice formation to freeze desalination and general solidification of binary melts.

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Type
JFM Papers
Copyright
© The Author(s), 2025. Published by Cambridge University Press

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Supplementary material: File

Wu et al. supplementary movie 1

Time-lapse animation of vertical section during downward freezing for the 0.6 mol/l case.
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Wu et al. supplementary movie 2

Time-lapse animation of vertical section during upward freezing for the 0.6 mol/l case.
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File 4.9 MB
Supplementary material: File

Wu et al. supplementary movie 3

3D morphology of brine columns and skirts and their evolution with time.
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File 8.9 MB
Supplementary material: File

Wu et al. supplementary material 4

Wu et al. supplementary material
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