(Inside Science) -- With the wintry holiday season now upon us, icicles will soon join luminous and festive decorative lights along roofs and rafters. Natural icicles are more than convenient decorations, however, for University of Toronto physicists Antony Szu-Han Chen and Stephen Morris. They are an icy enigma waiting to be solved.
One riddle, in particular, is the origin of the ripple patterns that form around the circumference of icicles. By growing both smooth and rippled icicles in their laboratory, the pair discovered one ingredient that is vital to the formation of icicle ripples: salt.
Adding sodium chloride -- plain table salt -- to water introduces what are called ionic impurities. These form due to the presence of positively and negatively charged atoms. Although others have studied icicle formation, no previous models have considered that ionic impurities could be the primary source for ripples.
Icicles grown from salt water exhibit ripples while icicles grown from pure water are smooth, Chen and Morris reported in the New Journal of Physics this October. The experimental results challenge leading theories, which stipulate that ripples will form on icicles regardless of the water's purity.
"It was a complete surprise that the salt made a difference," said Morris.
In 2010, Chen and Morris built a device that grows icicles under controlled wind and temperature conditions. They found that icicles grown under windless conditions -- in still air -- developed multiple, branch-like pointed tips instead of the familiar single tip often exhibited in nature.
They also discovered that icicles grown from tap water were less uniform in shape, bulging and twisting more than icicles grown from distilled water. Using the same machine three years later, the team grew 67 icicles from solutions of distilled water mixed with different amounts of sodium chloride. This time, instead of analyzing the shape they studied the formation of ripples on the icicle surface.
The icicles grow in a refrigerated box that includes a camera, a nozzle that drips water and a support from which the base of the icicle eventually forms. Like meat on a skewer, the icicle attached to the support rotates at a leisurely speed of one revolution every four minutes as the nozzle continues to drip.
The researchers used six different solutions in their experiment, each with a different amount of dissolved salt. With saltier solutions, the team measured more pronounced ripples that protruded further away from center of the icicle.
Chen and Morris also tested solutions with other types of impurities, such as those formed by including dissolved gases in the water, but found they made no difference to the formation of ripples. Therefore, they concluded that the ionic impurities of a salty solution were key to the formation of ripples. In the future, they plan to test other ionic substances.
Their experimental results are in line with what scientists have observed and known for more than twenty years. In 1990, a pair of scientists at the University of Alberta in Edmonton developed a model based from their observations of "marine" icicles made from salty solutions that "developed horizontal ribs or ridges." In that study, the researchers did not consider the ionic properties of salt dissolved in water.
In fact, all current theories of ripple formation focus on other factors, such as surface tension, said Chen, a physics graduate student. Chen and Morris are still grappling with the theory that will match their experiment.
One person who is especially interested in a working theory that could readily explain icicle ripples is Christopher Batty, a computer graphics researcher at the University of Waterloo, in Ontario.
"With computer graphics, we're getting more interested in detail and realism," Batty said. "With simulations we can explore the theoretical understanding behind effects like icicle ripples and even more obscure phenomena like tip splitting effects."
Batty has developed methods for simulating the flow of honey and animating splashes and droplets of water. While Batty's work combines computer graphics and computational physics for academic purposes, today's commercialized digital age desperately depends on people like Batty who can model fluid dynamics.
For example, in order for Disney animators to create a realistic winter wonderland through which the characters in its latest film Frozen could tromp, it called upon the skills of a few UCLA computer scientists. Although the characters in the film reflect the classic Disney cartoon style, the film's snow is as realistic as ever. To achieve that level of realism, the Disney-UCLA team developed a novel snow simulation method that could model both the clumping and packing behavior of real snow.
"Ideally, it would be great to do something comparable to what UCLA did with Frozen for icicle formation by drawing on what Stephen Morris' experiments reveal," Batty said.
Jessica Orwig is a contributing writer to Inside Science News Service.