Alan Stahler: ‘Nevada’ is Spanish for ‘snow’
Water is sticky — it sticks to our bodies, which is why we need towels.
Water sticks to itself, to make cloud droplets, raindrops and snowflakes.
Water sticks to itself for pretty much the same reason a balloon, rubbed on your shirt, sticks to the wall. As it’s rubbed, the balloon pulls electrons — sub-atomic particles, pieces of atoms — off your shirt. Electrons carry electric charge. Accumulating electrons, the whole balloon becomes electrically-charged.
Bring that charged-up balloon near the wall, and it pushes some of the wall’s electrons away, deeper into the wall. With a deficit of electrons, the part of the wall near the balloon also become electrically-charged, but with an opposite charge from the balloon.
Opposites attract; the balloon sticks to the wall.
Water is composed of molecules, small clumps of atoms, glued together. Each water molecule is composed of just three atoms: two hydrogens, glued onto an oxygen — H2O. The three atoms make a V, one hydrogen atom at each end of the V, the oxygen at the vertex (the point where the arms of the V meet).
Oxygen and hydrogen atoms do not share electrons fairly — oxygen hogs the electrons, which gives the oxygen atom an electric charge, just like that on the balloon. The hydrogens, with a deficit of electrons, acquire an opposite charge, just like the wall.
Again, opposites attract. The electron-rich oxygen atom of one molecule attracts an electron-poor hydrogen of another, and the water molecules stick together.
In liquid water — the water in your glass, in your body — water molecules stick catch-as-catch-can. A hydrogen of one molecule might slide between the hydrogen arms of another, to get close to the oxygen between. Or a hydrogen on one molecule might approach an oxygen atom from behind, outside of either hydrogen arm.
In a solid crystal, however, such sloppiness is not allowed. Each V-shaped water molecule has just one way to attach to the others. The result? A six-sided snow crystal.
The enforced pattern of the solid takes up a bit more space, a bit more volume, than the liquid, so water expands when it freezes, busting that bottle of beer you’d put in the freezer for “just a few minutes.” It’s also why ice floats.
But why the intricate pattern of the snowflake? And why the symmetry? How does each arm of the snowflake “know” how the others are growing? I visited physicist Ken Libbrecht in his lab at Caltech to learn more about snowflakes. Libbrecht has built an apparatus that allows him to grow snowflakes, one at a time, and watch what happens as he changes the temperature and moisture of the air.
Molecules in air (including the water molecules) fly fast — better than a thousand miles an hour at room temperature. The warmer the air, the faster they fly; the colder the air, the slower. A thermometer is a speedometer.
Libbrecht is investigating, but has yet to figure out why, the patterns water molecules make when they link-up are exquisitely sensitive to how fast the molecules fly, how fast they collide with the crystal. Warm the air by just a few degrees, and the pattern changes. But — even more perplexing — warm the air just a bit more, and the pattern changes back.
How does one arm know how the other arm is growing?
As a snowflake wafts about in a cloud, as it falls downward through the cloud, it encounters pockets of air that are warmer or colder, wetter or drier. Each pocket dictates a different growth pattern. Since all six arms are engulfed in the same pocket of air, all six respond by growing the same sort of pattern at the same time.
Libbrecht’s latest experiments, his latest attempts to understand snowflakes, involve growing ice needles on the ends of high-voltage wires, then growing snowflakes on the ends of those needles. I’ll be speaking with him again soon, and will report back what he’s found.
Al Stahler enjoys sharing nature with friends and neighbors. His science stories can be heard on KVMR-FM (89.5 MHz), and he may be reached at email@example.com.
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