SPEED thrills. And for most of us, the mixture of heart-stopping velocities and spectacular mountain scenery is what gives downhill skiing a buzz that鈥檚 hard to beat. But going fast means minimising drag on the bottom of your skis. Unfortunately that usually requires plenty of wax, and the elbow grease needed to apply it.
What if there were a high-tech way to make those skis more slippery and take some of the hard graft out of the downhill dash? For a sport in which races are often decided by the tiniest fraction of a second, it could make all the difference. And what if the same device could also act as a brake, to give less experienced skiers a better chance of staying upright on the piste? Attach this device to the underside of your skis or snowboard and you won鈥檛 just reach the bottom of the run in record time, you might even be able to stop when you get there.
It鈥檚 the dream of Victor Petrenko, a Russian physicist who has immersed himself in the physics and chemistry of frozen water for more than 20 years. Now, it seems, his vision is becoming a reality. And the good news is that it won鈥檛 just be fans of winter sports who reap the benefit 鈥� we all could.
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Like many Russians, Petrenko has skied since he could walk, but he first became hooked on the science of ice and snow during a visit to England and the University of Birmingham鈥檚 ice physics lab in 1979. When he returned home to the Institute of Solid State Physics in Chernogolovka, north of Moscow, he decided to dedicate himself to the stuff. 鈥淢y boss鈥檚 reaction was to put ice to my head,鈥� Petrenko recalls. 鈥淗e thought I was crazy. But I was persistent and within two years I had built an ice laboratory.鈥�
What really captured Petrenko鈥檚 imagination was the fact that it is one of the few natural materials that act as a 鈥減rotonic鈥� semiconductor. In conventional semiconductors such as silicon, current flows as a stream of electrons, but in ice, current is carried by hydrogen ions 鈥� protons 鈥� which jump between water molecules in the ice lattice along the hydrogen bond 鈥渉ighways鈥� that hold the molecules together.
As well as transporting charge through the ice, the movement of protons can rotate the water molecules. This means that the flow of charge can alter ice鈥檚 mechanical properties. 鈥淚f you change the mechanical properties you should change the electrical properties, and vice versa,鈥� says Petrenko.
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. Rather than forming a solid layer, the surface is coated with an ultrathin 鈥渜uasi-liquid鈥� layer of jostling water molecules. At about 鈭�157 掳C this layer is only about one molecule thick, but as the temperature rises it reaches about one micrometre at 0 掳C.
Measurements have revealed something unexpected about this quasi-liquid layer: it 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. And after moving his lab to Dartmouth College in New Hampshire in 1990, he began to investigate how he could use the super-ionic state to make skis go faster.
Petrenko reasoned that if he could create a deep enough liquid layer beneath a ski, it should act like lubricating oil and reduce drag. One way to do this would be simply to heat the base of the ski so that it melts the ice beneath. And tests in Petrenko鈥檚 lab confirmed this, showing that at 鈭�10 掳C, heating the snow under the skis by 1 掳C reduced the friction by 16 per cent. Unfortunately Petrenko soon discovered that this is out of the question for professional skiers: 鈥淥lympic rules forbid the heating of the ski base,鈥� he says.
Luckily, Petrenko realised that the strange properties of the quasi-liquid layer offered an elegant way round the problem. Instead of heating the ski, he could use the layer itself as a heating element by sending a current through it. He set to work by modifying the underside of a ski, adding a pair of very thin comb-like metal electrodes arranged so that their 鈥渢eeth鈥� interlock (see Diagram). Then he hooked up one electrode to the positive terminal of a battery, and the other to the negative.
When he placed the ski on the ice, the voltage across the electrodes created a current in the quasi-liquid layer. Just as current in a wire generates heat, this flow of charge warmed the snow and created a liquid layer thick enough to lubricate the ski. And this trick doesn鈥檛 break any regulations. 鈥淣o one forbids us from heating the snow or ice,鈥� he says.
Ralph White from the English Ski Council thinks that Petrenko could be on to something. Professional skiers use different waxes to make the surface of their skis match the surface of the snow. 鈥淏eing able to do this electronically sounds interesting,鈥� he says.
One of Europe鈥檚 biggest ski manufacturers plans to do full-scale field tests of Petrenko鈥檚 device over the next year. He says that the results are so promising that both professional racers and recreational skiers will be carrying small battery packs to power their electro-skis down the slopes within the next few seasons.
But as well as acting as a ski accelerator, Petrenko has modified his device so that it can also function as a brake. The key, he discovered, is to pulse the electric field across the electrodes 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 (快猫短视频, 9 February, p 20). Petrenko believes that his ski brake could be hugely useful for beginners, and also for cross-country skiers who want better purchase when travelling across flat ground or climbing hills.
Commercialising his idea won鈥檛 be simple, however. First he鈥檒l have to toughen up the circuitry so that it can tolerate the rough and tumble of the ski slope. Skis regularly hit rocks, and the hair-thin metal electrodes need to withstand these impacts. For his brake, he got round this problem by cheating and building a heater into the bottom of the ski. Petrenko also needs to find batteries with the right mix of weight and capacity. He reckons a 3.6-volt lithium ion battery should do the trick, and last up to six hours or so on the slopes.
But some factors are beyond his control. For example, impurities in the snow such as salt can affect the efficiency of the device. Salt makes ice more conductive, so for the same electric current you generate less heat. And if temperature on the ski slope drops below 鈭�30 掳C, the conductivity of the ice becomes so low that you can鈥檛 create enough current to melt it.
These problems haven鈥檛 stopped Petrenko looking around for other ways to use his ideas. He hopes to build a version of the brake for the soles of shoes or boots, and possibly even one for car tyres. He has also applied the idea to stop the build-up of ice on power lines. That could prevent a repeat of the havoc caused in 1998 when ice storms in Canada and the American north-east encrusted power lines with so much ice that they collapsed, leaving millions without electricity. Rather than surrounding every power line with a heating element, Petrenko realised that it would be simpler to use the ice itself. 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.
Petrenko reckons this a really neat idea. The signal doesn鈥檛 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. Who would have guessed that a little ice could be the best way to beat the cold?