100-year-old hair ice mystery solved

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Researchers have finally solved the 100-year-old hair ice mystery proving that Alfred Wegener, the first scientist to study this type of ice growing on rotten branches of certain trees, was indeed correct.

Hair ice is a type of ice that is shaped as fine, silky hairs and looks a lot like white candy floss. It grows on the rotten branches of certain trees when the weather conditions – humid winter nights with air temperature below 0°C – are available.

An international team of scientists from Germany and Switzerland set out to identify the ingredients that result into the formation of this ice and found that the fungus Exidiopsis effusa gave the ice its hairy shape. The study has been published in Biogeosciences, an open access journal of the European Geosciences Union (EGU).

Alfred Wegener, of tectonic-plate fame, was the first to study hair ice in 1918. He suggested a possible relation between the ice and the fungus in the wood. Some 90 years later, Gerhart Wagner, a retired Swiss professor who has been researching hair ice for decades, found evidence of this relation: treating the wood with fungicide or dunking it in hot water suppressed the growth of hair ice. But the fungus species and the mechanism that drives the growth of hair ice was yet to be identified.

Christian Mätzler from the Institute of Applied Physics at the University of Bern in Switzerland joined hands with Diana Hofmann, a chemist, and Gisela Preuß a biologist in Germany to understand the secrets of hair ice. Preuß studied samples of hair-ice-bearing wood collected in the winters of 2012, 2013 and 2014 in forests near Brachbach in western Germany and using microscopic techniques identified eleven different species of fungi.

“One of them, Exidiopsis effusa, colonised all of our hair-ice-producing wood, and in more than half of the samples, it was the only species present,” she says.

Mätzler, on the other hand, started looking at samples he collected in a forest at Moosseedorf, Switzerland, and his analysis confirmed that the driving mechanism responsible for producing ice filaments at the wood surface is ice segregation. Liquid water near the branch surface freezes in contact with the cold air, creating an ice front and ‘sandwiching’ a thin water film between this ice and the wood pores. Suction resulting from repelling intermolecular forces acting at this ‘wood-water-ice sandwich’ then gets the water inside the wood pores to move towards the ice front, where it freezes and adds to the existing ice.

“Since the freezing front is situated at the mouth of the wood rays, the shape of the growing ice is determined by the wood rays at their mouth,” says Mätzler.

Without the fungus, the ice forms a crust-like structure, the researchers say. The fungus is the key to thin hair like structures with a diameter of about 0.01 mm.

“Our hypothesis includes that the hairs are stabilised by a recrystallisation inhibitor that is provided by the fungus”, the researchers added.

Hofmann then studied the hair ice itself. Her chemical analyses of the melted ice showed the water to contain fragments of the complex organic compounds lignin and tannin. Since these are metabolic products of the fungal activity, this finding further confirms the biological influence on hair ice.

“These components may be the ones preventing the formation of large ice crystals at the wood surface,” says Hofmann.

The researchers say a reason why it took almost 100 years to confirm Wegener’s hypothesis is that hair ice is a somewhat rare and fleeting phenomenon, spotted mainly in broadleaf forests at latitudes between 45 and 55°N. “Hair ice grows mostly during the night and melts again when the sun rises. It’s invisible in the snow and inconspicuous in hoarfrost,” says Preuß.