The vast majority of the population has probably never seen or even heard of it. But hair ice, a rare form of ice that takes the shape of fine, silky hairs and looks like white candy floss is impressive nonetheless.
Hair ice forms on rotten branches of particular trees, when the weather conditions are just right. This usually is during humid winter nights when the air temperature drops slightly below 0°C. A team of scientists in Germany and Switzerland have established the missing ingredient which gives hair ice its peculiar shape, and it is a fungus known as Exidiopsis effusa.
Christian Mätzler from the Institute of Applied Physics at the University of Bern in Switzerland, says:
“When we saw hair ice for the first time on a forest walk, we were surprised by its beauty. Sparked by curiosity, we started investigating this phenomenon, at first using simple tests, such as letting hair ice melt in our hands until it melted completely.”
Alfred Wegener, a polar researcher, geophysicist and meteorologist who pioneered the Continental drift theory, was the first to study hair ice. In 1918, he observed a whitish cobwebby coating on the surface of hair-ice-bearing wood, which his assistant identified as fungus mycelium, the mass of thin threads from where mushrooms grow.
Wegener suggested there was a link 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, Wagner found, suppressed the growth of hair ice. But the fungus species and the mechanism that drives the growth of hair ice remained un-identified until now.
Mätzler, a physicist, joined forces with Chemist Diana Hofmann, and biologist Gisela Preuß to conduct a set of experiments. They wanted to find out precisely what conditions are equired to grow this type of ice, and what its properties are.
Mätzler’s findings confirmed hypotheses by other researchers that the mechanism producing ice filaments at the wood surface is ice segregation.
Liquid water close to the branch surface freezes in contact with the cold air, forming an ice front and ‘sandwiching’ a thin water film between this ice and the wood pores. Suction caused by 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. “The same amount of ice is produced on wood with or without fungal activity, but without this activity the ice forms a crust-like structure. The action of the fungus is to enable the ice to form thin hairs – with a diameter of about 0.01 mm – and to keep this shape over many hours at temperatures close to 0°C. Our hypothesis includes that the hairs are stabilised by a recrystallisation inhibitor that is provided by the fungus.”
Biologist Preuß analyzed samples of hair-ice-bearing wood collected in the winters of 2012, 2013 and 2014 in forests near Brachbach in western Germany. She 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.
According to the researchers, the main reason why it took almost 100 years to confirm Wegener’s hypothesis is that hair ice is a fairly rare and fleeting anomoly, seen 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.
Hofmann, D., Preuss, G., and Mätzler, C.
Evidence for biological shaping of hair ice
Biogeosciences, 12, 4261-4273, doi:10.5194/bg-12-4261-2015, 2015
Photo: Gisela Preuß