ADVANCES
24 Scientific American, December 2021Nancy MorrisonAlamy Stock PhotoThe “hair” sprouting off this branch
resembles a downy feather duster or fluffy
tuft of cotton—maybe even fragments
of a bad white wig. But each strand is in
fact made of hard, cold ice.
For these manes of silky-smooth ice
crystals (aptly termed “hair ice”) to appear,
a particular fungus called Exidiopsis effusa
has to colonize rotting wood from a broad-
leaf tree, and temperatures must hover
just below freezing in a sufficiently humid
environment. Such a specific convergence
of conditions might seem rare, but it
happens often enough that scientists had
been puzzling over this bizarre ice forma-
tion’s cause for more than a century—
including Alfred Wegener, the scientist
who proposed in 1912 that Earth’s contin-
ents were once a single mass—before
researchers pinpointed the role of E. effusa
in 2015.
By observing hair ice in the wild (and
growing some in a home garden),
investigators have learned a few things
about its formation. Under the right
temperature and humidity conditions, ice
forms on a branch surface while water
stays liquid inside the wood’s pores. The
temperature difference between the two
states of water creates a suction that draws
liquid water to the freezing front—gradually
extending the reach of the growing “hair.”Studies suggest that as the fungus
digests part of the wood, it provides
fragments of larger molecules that serve
as a scaffold on which the ice can grow.
This process can churn out hairs stretching
to a length of 20 centimeters (almost eight
inches). Each strand can be as thin as 0.02
millimeter in diameter, and some curl or
wave. They might last for days, even as
temperatures fluctuate near the freezing
point. Some researchers hypothesize that
the thin crystals are able to endure such
changing conditions be cause something
within the plant material acts like an
antifreeze, which can keep ice crystals
from changing shape even as the temp-
erature varies. When it gets cold outside
every year, scientists continue to docu-
ment this icy mystery.
For more, visit http://www.ScientificAmerican.com/
science-in-imagesBIOGEOCHEMISTRYScience
in Images
By Leslie Nemo