It may sound like a Zen koan, but it's a serious scientific question: How many molecules of water does it take to make the smallest possible ice crystal? Because crystals are defined by a repeated, three-dimensional arrangement of molecules, you can't necessarily take any small group of bonded-together molecules and call them a crystal. That's especially true for water: When it freezes, the weak hydrogen bonds that loosely bind the water molecules together pull the disordered clusters of molecules (left) into a more open—but also more rigid—cagelike arrangement (cross section of cluster at right). This roomy lattice is also why ice is less dense than water (and therefore floats). So to calculate the minimum number of molecules needed to make an ice lattice, a team of researchers shone infrared lasers on clusters of water molecules containing between 80 and 500 molecules. The team paid particular attention to how much energy the clusters absorbed from the lasers between the wavelengths of 2.63 micrometers and 3.57 micrometers—the range in which the oxygen-hydrogen bonds in water continually stretch and shrink. A particular peak of energy absorption occurred at a wavelength of about 3.125 micrometers—denoting the spectral characteristic of ice—and only appeared for clusters containing more than 275 water molecules, the researchers report online today in Science. That number of molecules yields a tiny ice cluster between 1 nanometer and 3 nanometers across—the ultimate in crushed ice.
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