Snowflakes: water film as a key to the change in shape

Temperature, humidity and ultra-thin water layer determine the crystal form

Snow Crystal © SXC
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Snowflakes are not only beautiful and diverse, they are still a mystery. One of them has now been aired by an American chemist. He discovered that the thickness of an extremely thin water layer on the crystal surface plays a crucial role in the shape and shape of the flakes. This knowledge is also important for the interactions of snow with its environment.

One who not only studies snowflakes in winter, but also breeds and examines them all year round, is the analytical chemist Travis Knepp. In a special laboratory chamber, where temperature and humidity can be adjusted, the researcher grows snowflakes along a thread and observes how their shape changes under different conditions. He came across a relatively abrupt transition between two typical crystal forms:

Change of shape at certain temperatures

At temperatures just below freezing, between -2.8 ° C and 0 ° C, the sides of the snow crystals grow faster than the top and bottom. As a result, these flakes take on a compact, plate-like structure. In contrast, crystals grown between C. and C. resemble large prisms or ice needles. The higher the humidity in the formation of crystals, the more they branch out. During their lifetime, the snowflakes may recrystallize several times, depending on the conditions they are exposed to.

Water layer determines Umspringpunkt

But why is there such a sharp limit to shaping at certain temperatures? And what factors determine the location of this limit? The scientist also found out in his experiments. "There is a very thin layer of water on the surface of each ice, " says Knepp. "Even far below freezing, water exists in a quasi-liquid form in this very thin layer. That's why ice is so smooth. Whenever you slip, you do not slip on the ice, but on this thin layer of water. "

And this layer is responsible for the different forms of snow crystal, as the researcher now proved. "The thickness and presence of this thin layer of water determines the general shape that the snow crystal assumes, " says Knepp. "If we changed the thickness of this layer, we also changed the temperature at which the crystal changed its shape. So far, it has not been known that this quasi-liquid layer plays such a significant role in shaping. display

Importance for environmental chemistry

However, this knowledge not only explains one of the riddles of snowflakes, it also has an impact on the interaction of the snow with its surroundings. "There's a lot of chemistry happening on the surface of ice, " the researcher explains. By understanding the physical structures of the snow crystal how it grows and why it takes a particular shape, we can learn more about what happens on its surface.

An example of this is the complex reactions of ozone depletion in the Arctic and Antarctic. Some of these chemical processes take place on the surface of ice crystals and snowflakes. Because the shape of the crystals determines the size of the surface, it also influences the intensity of the ozone-depleting reactions. All the more important is the exact knowledge of the so convertible snowflakes.

(Purdue University, 30.12.2009 - NPO)