Tiny dirty water droplets, often mistaken for air bubbles, tell the story of the growth of rippling icicles.
Icicles made with purified water are smooth.But salt or other impurities can form icicles ripple as they hang from tree branches, bridges and power lines (Serial Number: 11/24/13). Impurities can also be responsible for the fuzzy appearance of icicles, which is often attributed to air bubbles.Those bubbles are actually tiny polluted water blobsthe researchers reported in November Physical Review E.
When examining a 3 mm thick icicle cross-section grown in the labUniversity of Toronto physicists Stephen Morris and John Ladan discovered some impure liquid water surrounded by relatively pure ice (Serial Number: 8/13/10). “It turns out that there are very few air bubbles in the icicles,” Morris said. He called the pockets of water “inclusions” to distinguish them from air bubbles.
What’s more, “the inclusions record the history of the growth of the ripples. It’s like growth rings on a tree,” Morris said. Inclusions form in layers close to the surface of the icicle, with older layers being covered by newer layers as the icicle grows. “You can infer something about the growth history by looking at the patterns of the inclusions.”
To track icicle formation, the researchers mixed fluorescent dyes into water to replace other types of impurities and used the water to grow icicles in the lab. The dye ends up concentrating in higher concentrations in the liquid content of the icicle, just like any other pollutant. It also glows brightly in ultraviolet light, which makes inclusion layers easier to see.
By varying the concentration of dye in water, researchers demonstrate one way impurities can affect dye patterns icicle ripples (Serial Number: 10/10/02). “All it takes is contamination comparable to tap water before their shape changes from smooth to corrugated,” Morris said.
The underlying cause of the ripples caused by pollutants is unknown. Morris said that, being an experimentalist, he would leave that conundrum to theorists.
