HOW COME? Why your ice sports spikes
Several times I have noticed sharp spikes, usually at a 60- or 70-degree angle, sticking up from ice cubes in our freezer. Why do these formations sometimes appear on ice cubes? asks Marcus Givens.
Are your ice cubes mysteriously sporting Mohawks? If so, you've probably been making ice the old-fashioned way, in an ice cube tray. Cubes that tumble out of an ice maker tend to be uniformly shaped. But water left to freeze in open trays can undergo a startlingly spiky transformation.
One clue as to why spikes form: Have you ever noticed that, after you fill an ice cube tray, the frozen cubes bulge above the rim? Water is peculiar. When other substances transform from liquids to solids, molecules packing together, they contract. But when liquid H2O changes into solid ice, it actually expands -- by nearly 9 percent.
Why? At 32 degrees F, water molecules lock into a rigid, open lattice. The result is open spaces inside each cube, causing ice to swell out of the tray.
Scientists say that under the right conditions, thin ice sheets form on the surface first, near the edges of each tray compartment.
Meanwhile, ice also forms on the bottom of each compartment. As the freezing, glacier-like sheets advance in from the sides and up from the bottom, liquid water may find itself trapped between the jagged sheets.
As the freezing water expands, the still-liquid water may be squeezed through gaps in the icy sheets. Once at the cube's surface, the water may erupt in a tiny geyser, freezing in place. Presto: a hollow, pointy ice spike. A spike's angle varies, depending on the shape of the funnel the water squeezed through.
For spikes to form, freezing must be rapid, but not too rapid. Otherwise, water pushed through the cube's top will quickly freeze into a nondescript hill. According to studies, the ideal temperature for growing spikes is around 19 degrees F. It can take up to 10 minutes for a fledgling spike to grow to its full height. The tallest may reach 2 inches, convenient handles for pulling cubes from trays.
Want to make your own spiky cubes? According to researchers at Caltech, plastic trays work best, providing just the right amount of insulation on the sides and bottom. To grow the most spikes, fill the tray with distilled water. Dissolved minerals in tap water slow down freezing. Distilled water can "supercool," staying liquid as the temperature drops below 32 degrees, and then freezing rapidly.