Icicles made from pure water give scientists brain freeze.
In nature, most icicles are made from water with a hint of salt. But lab-made icicles free from salt disobey a prominent theory of how icicles form, and it wasn’t clear why. Now, a study is helping to melt away the confusion.
Natural icicles tend to look like skinny cones with rippled surfaces — the result of a thin film of water that coats the ice, researchers think (SN: 11/24/13). As icicles grow, the film freezes. Any preexisting small bumps in the icicle get magnified into large ripples because the water layer is thinner above the bumps and can freeze more readily. But this theory fails to explain the salt-free variety, which have more irregular shapes reminiscent of drippy candles, says physicist Menno Demmenie of the University of Amsterdam.
So Demmenie and colleagues grew icicles in the lab, adding a blue dye that was visible only when the water was liquid. Salted icicles not only had ripples, but they also were covered in a thin, blue film. Icicles made from pure water had no such film. Only small droplets of blue appeared on those icicles, the team reports in the February Physical Review Applied.
In icicles with salt, the temperature at which the water on the surface freezes is lowered, allowing a liquid layer to coat the entire icicle. Without the salt, icicles must build up drop by drop.
2023-02-15 08:00:00
Article from www.sciencenews.org
With their twinkle of winter sunshine and the beauty of their constricted shape, Icicles prove to be one of the most stunning symbols of winter. However, the standard icicle is often defined by its linear shape and the lack of any kind of ripples or curves. This curious phenomenon stems from the physical properties of water itself and can prove to be one of the most fascinating aspects of the icy droplet.
When ice is created and suspended from a surface, such as a roof or window, it acts as a vessel for the flow of liquid dripping down. As the hot summer suns beats down on the ice, the water within it melts and creates a thin layer of liquid. That thin layer of liquid begins to slide off the side, but the shape of the icicle remains consistent and unchanged due to the tension of the water molecules.
The droplets of water that emerge from the icicle, or what scientists commonly refer to as ‘weeping’, are formed when the pressure of the moving liquid exceeds the tension of the molecules – this is also what creates ripples and curves on the icicle’s surface. However, as pure water does not contain any suspended particles, the number of ripples created is limited.
Ultimately, the lack of ripples and curves in pure icicles is the result of the spinning of water molecules as they move down and away from the icicle. When a liquid contains particles and impurities – such as sediment, silt or other suspended elements – the particles interact with each other, resulting in an increased number of curvy and undulating lines on the surface of the icicle.
By understanding the scientific principles behind the movement of water molecules and the impact of an impurity-free environment, one can appreciate the lack of ripples in icicles made from pure water. The draught of wavy and curved lines offers an interesting contrast to icicles full of impurities, making them an attractive sight all year round.