Surrounding every power line with a heating element is one option. But Victor Petrenko, at Dartmouth College in New Hampshire, thinks he has a smarter idea – use the ice itself as the element.

Working with a consortium of US and Canadian power companies, he has developed a system that sends high-frequency electrical signals along the cables to create a current in the ice build-up and melt it.

It runs off small power units placed along the lines every 100 kilometres or so. The signal does not reduce electricity transmission through the cables, and because it uses around 50 watts per 100 kilometres of line it should cost a fraction of what it normally takes to keep the lines clear.

Protonic semiconductor

The key to the system is that frozen water is one of the few natural materials that act as a “protonic” semiconductor. In conventional semiconductors such as silicon, current flows as a stream of electrons, but in ice, current is carried by hydrogen ions – protons – which jump between water molecules in the ice lattice.

As well as transporting charge through the ice, the movement of protons can cause the water molecules to rotate within the lattice. This means that the flow of charge can alter ice’s mechanical properties.

The possible consequences of this are most obvious at the surface of an ice crystal, where the top layer of water molecules are exposed to the air. Here the boundary is coated with an extremely thin “quasi-liquid” layer of jostling water molecules. At about -157 °C this layer is only about one molecule thick, but as the temperature rises it grows thicker, reaching about one micrometre at 0 °C.

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Measurements have revealed something unexpected about this quasi-liquid layer: its conducts electricity 100 times better than solid ice. Petrenko describes it as a super-ionic state because the layer appears to contain a higher concentration of protons than the ice beneath. His crucial realisation was that he could use this layer as a heating element, by sending a current through it.

Faster and slower

Petrenko has also used the idea to making skis go faster – and slow down. To act as a brake, the electric field has to be pulsed across the electrodes on the underside of the skis for less than a millisecond.

This melts the surface of the snow but allows it to refreeze almost instantly. As the ice crystals re-form, they grip the bottom of the ski and create friction. Other applications for the ice brake include the soles of shoes or boots, and possibly even car tyres.

But Petrenko’s two-decade fascination with the physics and chemistry of frozen water did not get off to an encouraging start. When he declared his intention to dedicate himself to ice, he was employed at the Institute of Solid State Physics in Chernogolovka, north of Moscow.

“My boss’s reaction was to put ice to my head,” Petrenko recalls. “He thought I was crazy. But I was persistent and within two years I had built an ice laboratory.”

Ferrites are highly magnetic compounds, so the particles should be easy to
separate from the waste. They may also be commercially valuable; using them in
the magnetic coating of recording tape is one obvious application.

Iron compounds are always present in mine drainage, and there have been
previous attempts at using ferrites to extract pollutants from mine water. But
they have required the water to be heated to more than 60 °C for two days while
a precipitate formed. This made the process too expensive to be practical. Xu
says that his process takes less than half an hour at ambient temperatures.

In the laboratory, Xu speeded up the reaction by manipulating the chemistry
of the iron in solution. Iron ions can have two different electrical charges, or
valence states. The key to extracting ferrites is to control the ratio between
the ferric (Fe³⁺) and ferrous (Fe²⁺) ions.

Xu found that if he made the ratio of ferric to ferrous ions 2:1 and added
sodium hydroxide as the neutralising agent, a solid containing a large
proportion of ferrites started to form immediately. Xu separated the ferrites
using a magnetic filter. With real acidic mine water, says Xu, the ratio of
ferric to ferrous ions could be shifted by adding ferrous ions in the form of
iron sulphate, or by aerating the water to oxidise some of the ferrous ions to
the ferric form.

Ferrites are more complex compounds than those that are found in the usual
neutralised sludge, and their structure allows them to incorporate other metals
such as zinc or aluminium, so the water left behind should be free of
metals.

Xu’s process still suffers from one major drawback—calcium molecules
disrupt the reaction, because they are too large to be incorporated smoothly
into the structure of the ferrites. This rules out the use of lime, calcium
hydroxide, for neutralising the acid water. “I have managed to extract ferrites
using sodium hydroxide, but this is more expensive than lime,” says Xu. “If
commercial uses for ferrites can be developed, this may not be a problem. But we
are working on a way of preventing calcium from inhibiting the reactions.”

The ants cut fresh leaf tissue and gather scraps of organic debris from the
forest floor. They use these to feed fungus gardens in their underground nests.
In return for the ants’ care, the fungi produce sterile “fruit”, on which ant
larvae feed.

Philip Howse, an entomologist at the University of Southampton, and his
colleagues upset this happy relationship by placing contaminated orange peel
containing the fungicide cycloheximide along ant trails in the Trinidad
rainforest. Orange peel is usually one of the ants’ favourite items, but within
two days, the ants refused to gather any orange peel that Howse put out for
them—contaminated or not (Experientia, vol 52, p 631).

The ants stopped gathering the fungicidal bait well before any visible
damage
occurred to the fungi. “The fungus must be producing a chemical distress
signal,” says Howse. The speed of the ants’ rejection ensures that the fungus
survives.

Though the ants stopped bringing citrus peel into the nest, they
continued to
collect other material. This suggests that they must have a method for
discovering that citrus peel was the culprit. Howse thinks they do this by
placing different food items in different parts of the garden, so they can
easily tell if any food type is causing problems for the fungus.

Communication within the nest is clearly not all it could be, however. “Some
of the nests are 20 metres across,” says Howse, “and contain many different
gardens.” Ants in different parts of the nest may have no more contact than
office workers on different floors of a tower. When Howse later laid down
contaminated orange peel on other trails emanating from the same nest, the
naive
ants gladly took it home with them.

The ants respond to the fungus’s food preferences so subserviently that
Howse
thinks the fungus is the controlling member of the partnership. “The
fungus gets
a good deal,” he says. “It gets a home, food, protection and a means of
reproduction—a queen carries spores with her during her nuptial flight to
establish a new colony. In return all the fungi do is feed the young
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