杏吧原创

IT IS not often helps save a life 鈥� he is a physicist, after all, not a physician. But when an architect telephoned him in his office at the University of Toronto, Canada, last year, with a potentially lethal problem, his advice was to the point.

The architect鈥檚 problem was icicles. He had designed a building whose windowsills accumulated snow in bad weather. Worried about a passer-by being engulfed by a sudden avalanche and suing, he had installed heaters on the windowsills. Consequently meltwater was dripping off the sill and forming enormous icicles that loomed dagger-like overhead. The architect was still worried, and with good reason. Falling icicles reportedly .

Fortunately, Morris understands icicles. In fact, he is the world鈥檚 leading expert on their formation. Perhaps that鈥檚 a matter of location. 鈥淚 live in a cold country,鈥� he says. Although Toronto鈥檚 location on Lake Ontario gives it one of Canada鈥檚 balmier climates, the surrounding area can seem like one big ice slab in winter.

There is more at stake with icicles than just the risk of an impaling. In January 1998, an ice storm destroyed much of the power infrastructure in the Canadian province of Quebec. Pylons buckled under the weight of the ice and some power lines snapped, leaving millions of homes and businesses without electricity for days. 鈥淭he cost was estimated at more than 20 billion Canadian dollars,鈥� says Masoud Farzaneh, head of the based at the University of Quebec in Chicoutimi.

Morris鈥檚 contributions to limiting such costs emerge principally from a large, styrofoam-insulated box standing in the middle of his lab. 鈥淲e call it the rotisserie,鈥� he says. Inside the box a downward-pointing wooden spike attached to a turntable rotates at the stately rate of 12 revolutions per hour. A nozzle drips water onto the wide base of the spike, while an air fan sits in each of the box鈥檚 corners, blowing cold air at it. The whole contraption is cooled by antifreeze that circulates through the box鈥檚 walls.

This is enough for Morris and his graduate student Antony Chen to strip away layers of assumption about icicles that have accumulated over the years. Take the established picture that they grow with a 鈥渟elf-similar鈥� shape: that the ratio of an icicle鈥檚 length to its circumference is always the same. According to this idea, an icicle鈥檚 precise shape depends on local factors such as the difference between the icicle鈥檚 surface temperature (0掳C), the ambient air temperature (significantly colder), and the rate at which this falls as you move away from the icicle鈥檚 surface.

Not so, say Morris and Chen. Their experiments show that self-similarity appears in only a small fraction of icicles, when conditions 鈥� air movement and water purity, for instance 鈥� are just so. 鈥淭hings that you wouldn鈥檛 think matter turn out to matter a great deal,鈥� says Morris. Only when the fans in the rotisserie gave the air a gentle stir, for example, did a classic icicle with a single tip form; with still air, the tip tended to split in two ().

It is unlikely that engineers will be able to do much about air movement around power lines, of course. But knowing the conditions under which ice will build up fastest could be a first step towards designing equipment and systems that are more robust. 鈥淚cicles are the simplest thing you can think of in this class of problem,鈥� Morris says.

In another of his experiments, Morris tackled an idea about icicles floated by Farzaneh and his colleague , now at Kyushu University in Japan. Many natural icicles have a rippled surface, as if composed of a narrowing succession of rings. Theories of icicle growth had traditionally had nothing to say about such features, but Farzaneh and Ueno suggested surface tension between the freezing water and the surrounding air was the culprit: the higher the surface tension, the fewer ripples on the icicles ().

That seemed reasonable. Natural icicles form from dirty water that sits on roofs and in guttering, which has a low surface tension and so should form ripples. Sure enough, when Morris performed his rotisserie experiments with distilled water, no ripples formed. But there was a sting in the tip: when he used tap water, also a pretty pure sort of water, ripples suddenly appeared.

, a theoretical geophysicist at the University of Cambridge, in the less frozen, but damp east of England, confesses himself baffled by this. 鈥淭he level of impurity even for tap water should be so low, it鈥檚 hard to conceive of how it should affect the experiment 鈥� but it does seem to,鈥� he says. It certainly suggests surface tension is not the primary factor.

Neufeld is particularly interested because he performs Morris鈥檚 experiments in reverse: he studies how icicles melt. Working with Cambridge colleagues Raymond Goldstein and Grae Worster, he left a cylindrical block of ice in a warm room and watched how its shape changed over time, exploring how things such as the rate of heat transfer to and from the block and its own material consistency changed matters ().

That might seem akin to watching paint dry, but it could prove important. Understanding how layers of air insulate the surface of glaciers, for example, is vital to making accurate estimates of how fast they will melt 鈥� and sea levels will rise 鈥� as the Earth warms under its blanket of greenhouse gases.

There is another, more unexpected area in which icicle research is followed with interest: Hollywood. You might think that ice is only a problem there when bar staff can鈥檛 find any. But in an age where many films owe more to CGI wizardry than sharp camera work, requests for realistic simulations of melting, freezing, crystal growth and so on come up 鈥渕ore often than you might at first expect鈥�, says , who researches computer graphics at Columbia University in New York City. For instance, Batty was part of a team asked to create a Kryptonian crystal spaceship for the film Superman Returns. 鈥淪ome of the internal decor and controls of the ship were actually supposed to appear to grow organically,鈥� he says.

鈥淭here is another, more unexpected area in which icicle research is followed with interest: Hollywood鈥�

The scene was ultimately deleted 鈥� which was disappointing, but the work goes on. 鈥淚f we could grow icicles on the computer, in a physical way, then we can create virtual models of chasms, caves, ice storms, shut-down airports and so on in days instead of months,鈥� says Batty鈥檚 colleague .

That could now be on the cards: Batty, Grinspun and Morris have just teamed up in a new collaboration to master ice-making for the big and small screen once and for all. It鈥檚 the next problem for Morris to solve, after he gave the architect the benefit of his accumulated wisdom. 鈥淚 told him to stop heating the windowsills; you have to just leave the snow,鈥� Morris says. 鈥淚t didn鈥檛 turn out to be a very profound solution.鈥�