LOOK closely at any icicle and you’ll see that it carries a series of circular ridges, just 8 millimetres apart. It’s the same no matter how big or cold the icicle, or how much water trickles down. But why?
Naohisa Ogawa became hooked on the puzzle after seeing it presented on TV by Yoshinori Furukawa, an ice researcher at Hokkaido University. Now together they’ve worked out the explanation.
When ice crystals grow, heat escapes fastest from the tips of bumps on their surface, so water there freezes fastest too, accentuating the bumps. The closer the bumps are, the steeper the curves and the faster it freezes. All shapes in nature also tend to smoothe themselves out, and competition between these effects helps explain why snowflakes take on their complex shapes. But for icicles, the physics can’t be the same because the bumps are so much bigger than they are on snowflakes.
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Now Furukawa and Ogawa have worked out that the key lies in the thin film of water that trickles down the ice. Heat transfer is much more rapid in the water stream, so the effects occur on a larger scale. By crunching through the maths, the team showed that the bumpy ridges should be 5 to 13 millimetres apart – exactly what they are. They report their findings this week at a conference at Memorial University, Newfoundland.
