Surface Properties Determine Ice Crystal Shape

Scientists in China have demonstrated that ice crystals grow up and away from water repellent surfaces, forming six-leaf clover and sunflower shapes.

AsianScientist (Oct. 19, 2017) – In a study published in the Proceedings of the National Academy of Sciences, scientists in China have demonstrated how the shape and direction of ice crystal formation is affected by the properties of the solid surface it is grown on.

When water droplets suspended in the air freeze, they generate snowflakes—ice crystals with six-fold symmetry. But when ice grows along a solid surface, like frost growing on windows, it can take on an almost infinite range of different shapes.

In this study, a group of researchers led by Dr. Liu Jie of the Chinese Academy of Sciences Institute of Chemistry in Beijing demonstrated that the crystalline patterns of ice are affected by whether a surface repels or absorbs water. The researchers showed that when a surface tends to repel water, ice crystals can be cultivated to grow away from the surface at an angle, resembling a clover with six leaves.

Using a high-speed camera attached to a microscope, the team captured images of ice forming on aluminum that had been covered with a hydrophobic, or water-repellent, coating. Water drops sprayed on the surface remained taut and spherical instead of spreading out.

The researchers triggered ice formation across the entire surface by spraying it with silver iodide nanoparticles, which acted as seeds for ice growth. As the ice developed, the crystals grew outwards and up from the nanoparticle, forming a symmetrical, six-leafed clover with only a single point of contact with the surface.

On hydrophilic, or absorbant, surfaces, water spread out quickly, and so did ice—forming a sunflower-shaped crystal in full contact with the surface.

Furthermore, when the team prepared a hybrid surface with both hydrophilic and hydrophobic parts, ice spreading on the hydrophilic side came to a halt at the boundary with the hydrophobic side.

The researchers also observed that the clover-like ice crystals growing away from a hydrophobic surface could be removed by wind more easily than crystals on a hydrophilic surface.

The article can be found at: Liu et al. (2017) Distinct Ice Patterns on Solid Surfaces with Various Wettabilities.


Source: Chinese Academy of Sciences.
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