TIRED of mini umbrellas and sick of olives on sticks? If the cocktails at your Christmas party are in danger of looking a little passé ths year, Ken Libbrecht has just the thing for you. Why not try adorning your guests’ drinks with “ice spikes”, gravity-defying icicles that sometimes grow out of ice-cube trays. “They’re bizarre things,” he says. “People are amazed by them.”
Libbrecht, a physicist at the California Institute of Technology in Pasadena, has recently become something of an expert in the art of growing ice spikes. Last summer he and his student Kevin Lui spent their days making ice cubes by the thousand. Their goal was to find out why ice spikes only occasionally rise out of freezing water.
They seem to have found the answer, or at least a big part of it, and along the way they have learned how to grow the perfect ice spike. The good news is that anyone with a freezer can do it.
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It all started earlier this year when, out of the blue, someone sent Libbrecht photographs of frozen needles over a centimetre long protruding from a tray of ice cubes. For the past six years, Libbrecht had been growing designer snowflakes in his laboratory in an effort to find out why they form such complex and delicate patterns. Though he had heard about people waking up on cold winter mornings to find the odd ice spike sticking out of their bird baths, he hadn’t given the phenomenon much thought. But when he received the photos of tiny ice towers made in a household freezer, he was intrigued, and tried growing them himself at home.
At first he had mixed success. Most of the time his ice cubes turned out cubic, as expected. But now and then, a tall spike emerged from his ice tray. After some experimenting, Libbrecht hit upon the secret for making as many as four spiky ice cubes per tray. The trick, he has found, is to use purified water rather than water straight from the tap. But leaving it at that wasn’t good enough for Libbrecht. “It bothered me why it worked with distilled water and not tap water,” he says. So he and Lui set out to find out exactly what affects the growth of ice spikes.
Pop an ice-cube tray filled with water in the freezer and, after about an hour-and-a-half, the surface begins to freeze. This freezing starts at the sides of each compartment of a plastic ice tray because they are covered with microscopic nicks and scratches. Water molecules wedge themselves in these tiny hollows, where they can form plenty of bonds with their neighbours as the temperature falls. And because ice crystals are less dense than water, they float to the surface. The freezing ice then creeps towards the middle until only a small hole remains unfrozen at the centre of the ice cap (see Graphic).
At the same time, more ice starts forming around the sides of the cube. And since ice expands as it freezes, the ice below the surface pushes water up through the hole. If the conditions are just right, the meniscus of water forced out of the hole freezes around its rim, forming the base of the spike. As this process continues, the ice spike grows taller until all the water has frozen or, more commonly, the tip of the tube freezes over. Using a video camera shut inside a lit freezer compartment, Libbrecht and Lui found that ice spikes grow to their full height surprisingly quickly, within 3 to 10 minutes.
Libbrecht has used experiments like these to work out why spikes won’t form readily in ordinary tap water. It is all down to impurities. As water freezes around the top of a growing ice tube, its saltiness increases because the dissolved minerals and metals do not fit snugly into the ice crystal lattice. Libbrecht reckons these impurities quickly build up to such high levels at the tip of the spike that the water there can no longer freeze. Any spikes that begin to form in tap water will just stall.
To test just how pure the water has to be for ice spikes to form, Lui compared ice cubes made from distilled water with those made from increasingly salty solutions of sodium chloride. With as little as 0.2 milligrams of salt added per litre of water, the chance of an ice cube producing a spike plummeted from 1 in 5 with distilled water to less than 1 in 20.
And since tap water typically contains 100 times this concentration of various salts, it is hardly surprising that ice spikes are so rare. So if you want to grow your own ice spikes, you’ll need distilled water like the kind sold in supermarkets for pouring into your steam iron or topping up your car battery.
What else do you need to watch out for? Though Lui managed to grow ice spikes in dozens of different kinds of freezers, from those found in labs to those found in student dormitories, he found that temperature does make a difference. At −7 °C, half the ice cubes turned spiky. If the temperature plunges, however, so does the likelihood of finding an ice spike. Only 1 in 10 cubes will grow a spike at −15 °C. Libbrecht isn’t certain why, but he suspects that in such cold conditions the tip of the embryonic spike freezes shut before it has had a chance to grow. So if your ice cream is really hard, your freezer is too cold to make many ice spikes.
Having a modern, frost-free freezer helps too. They hardly ever need defrosting because a fan circulates cold, dry air. The moving air chills the edges of the water droplet perched on top of the growing ice spike faster than in calm conditions. This faster freezing rate at the tip promotes the growth of longer ice spikes. “It also works better if the freezer is empty,” says Libbrecht. That’s because all those pizzas and bags of frozen peas stop the air from circulating freely.
But the good advice doesn’t stop there. If you want prizewinning spikes, use a plastic ice cube tray rather than an old-fashioned metal one. When Libbrecht spotted aluminium ice cube trays for sale on eBay, he knew he had to try growing ice spikes from them, but hardly any formed. Metal is such a good conductor of heat that the surface of the water freezes over quickly, completely sealing the hole from which the spike would normally grow. Plastic has just the right amount of insulation for ice spike formation.
Soon after they published their findings in October (), Libbrecht and Lui realised they had started a craze. Reports that their tallest ice spike was 5.6 centimetres long were like a challenge for people who had become hooked on ice spikes. Soon Libbrecht received an email from an enthusiast who distilled his water a second time to purify if further. “Using a cereal bowl filled with double-distilled water, he grew an ice spike measuring 10 centimetres long and 1 centimetre across,” Libbrecht says.
It may sound like a lot of trouble to go to for your Christmas party, but it’s time well spent. After all, spikes are the perfect ice-breaker.