Although water covers over 70% of the Earth’s surface, it remains one of the world’s most mysterious liquids. Understanding how the individual water molecules interact and behave with each other at the molecular level is a highly active area of research and this curiosity extends to the behavior of water as it freezes.
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When a liquid undergoes a phase transition to become solid, the average interparticle distance is decreased and the molecular structure becomes much more densely packed. The behavior of water does not become any less puzzling when it is a solid – ice has 17 experimentally confirmed polymorphs, which have the same chemical structure but differ in the arrangement of the atoms and molecules in the lattice.1
Most liquids will freeze when the ambient temperature is reduced below their freezing point but the propensity of a liquid to freeze can be altered by the presence of any impurities. The same is true for the melting behavior of a solid, where impurities can have a significant impact on how the melting process occurs.2
Many industries such as the food industry and aviation have a great need to understand and control the behavior of ice formation. Ice can be used as a way of freezing produce to inhibit microorganism growth and extend the shelf life of food, but avoiding the formation of large ice crystals is important to avoid destroying the food quality. The aviation industry also needs to avoid the accumulation of ice on sensitive structural components which can result in structural failures and hindered flight dynamics.3
Understanding how ice forms is crucial for disrupting its growth process. Ice often forms at a nucleation site, which could be the presence of a chemical impurity or surface roughness on a solid.4
Nucleation can occur via heterogenous or homogenous mechanisms, where the latter happens away from the surface in the bulk solution.
With heterogenous nucleation mechanisms, the impurity can either trigger the nucleation and freezing process via a collision, deposition of the ice on the surface, immersion of the impurity into the ice, or a more complex condensation mechanism.5
The exact mechanism of nucleation and the environmental conditions can affect exactly what polymorph of ice is formed as well as crystal sizes. Some of the smallest ice crystals ever measured contain just 275 water molecules.6 More average ice crystals are around 10 µm in diameter but the initial ice nuclei that are formed can be as small as 10 nm.5
Using experimental techniques to image the nucleation and freezing processes of water is incredibly challenging. The relatively small size of the crystals means techniques need to have a reasonable spatial resolution but also need to avoid melting the crystals during observation.
The strong infra-red absorption bands in water and their sensitivity to the local bonding environment of the water has made infra-red spectroscopy a powerful tool for investigating the dynamics and structure of water.6 For true imaging of the ice structures, in particular how the ice interacts with the surface or other nucleation sites, microscopy techniques are highly desirable.
Scanning probe microscopy methods with their excellent spatial resolution have proved popular. However, there are often interactions between the tip and substrate being studied which can perturb the measurement.
One technique that has become very important for our understanding of ice and nucleation dynamics is environmental scanning electron microscopy (SEM).5 Environmental SEM was developed to overcome the issues many samples experience under the high vacuum conditions of standard SEM.
In an SEM experiment, an electron beam is produced from an electron gun, and focused onto a sample.
The scattered electrons (usually the backscattered and secondary electrons in SEM) are then detected and an image of the sample can be reconstructed.
High vacuum conditions are required for such experiments as the electrons from the gun could otherwise be scattered by particles in the atmosphere and perturb the measurement.
The high voltage on the electron guns also means that arcing and discharges are a risk, which could ultimately damage the equipment.
The requirement for high vacuum conditions is also problematic as many samples cannot withstand the high vacuum conditions or require special preparation in order to do so.
In environmental SEM, electron guns and detectors have been designed to cope with higher ambient pressures and can be used for the study of certain liquid samples, including looking at ice formation in water.
Recent work with environmental SEM on water samples has made it possible to visualize the ice nucleation process itself and achieve 10 nm spatial resolution.5 This has made it possible to disentangle how environmental conditions affect the freezing process and how this is also dependent on the presence of particular mineral species.
Being able to probe such small nucleation sites with environmental SEM is of particular interest to atmospheric research in the troposphere, which plays a key role in the chemistry of the atmosphere. The ability to see how certain nucleation pores contribute to ice formation is a step closer to creating realistic models to capture the physics of how these processes unfold.
References and Further Reading
- Salzmann, C. G. (2019). Advances in the experimental exploration of water’s phase diagram. Journal of Chemical Physics, 150(6). https://doi.org/10.1063/1.5085163
- Mitsui, T., & Aoki, K. (2019). Fluctuation spectroscopy of surface melting of ice with and without impurities. Physical Review E, 99(1), 5–9. https://doi.org/10.1103/PhysRevE.99.010801
- Bragg, M., Hutchison, T., & Merret, J. (2000). Effect of ice accretion on aircraft flight dynamics. 38th Aerospace Sciences Meeting and Exhibit (p. 360). https://doi.org/10.2514/6.2000-360
- Li, K., Xu, S., Shi, W., He, M., Li, H., Li, S., Zhou, X., Wang, J., & Song, Y. (2012). Investigating the effects of solid surfaces on Ice nucleation. Langmuir, 28(29), 10749–10754. https://doi.org/10.1021/la3014915
- Pach, E., & Verdaguer, A. (2022). Studying Ice with Environmental Scanning Electron Microscopy. Molecules, 27, 258. https://doi.org/10.3390/molecules27010258
- Pradzynski, C. C., Forck, R. M., Zeuch, T., Slavíček, P., & Buck, U. (2012). A fully size-resolved perspective on the crystallization of water clusters. Science, 337(6101), 1529-1532. https://doi.org/10.1126/science.1225468
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