Nucleation is the process by which water, a liquid, freezes to become ice or snow, both solids. There are two types of nucleation: Heterogeneous nucleaetion and Homogeneous nucleation. Ahh, Don't those two words just bring you back to the good ol' days of Biology? Nucleation is defined as "the intitial process that occurs in the formation of a crystal from a solution, a liquid, or a vapor, in which a small number of ions, atoms, or molecules become arranged in a pattern characteristic of a crystalline solid, forming a site upon which the additional particles are deposited as the crystal grows." (www.britannica.com) So basically, we've got this center embryo that forms as a crystal when water droplets freeze, and when more water droplets freeze, they latch on to the embryo--well, you'll figure out the rest.
Homogeneous Nucleation
In homogeneous nucleation there are no other substances affecting the freezing process-the liquid is of the same composition and contains no impurities. It's just the basics, pure water and air. In the homogeneous process, the temperature is the biggest influence on the change from water to ice or vice versa, but pressure changes also have an affect on the nucleation. As said in the atmospheric process page , the nucleation process can take place as low as -40 degrees C or -40 degrees F.
THE PROCESS
Nucleation gets the ball rolling when a few water molecules reach the solid state. They're hanging out in the atmosphere, staying cool (literally) and they are called the "embryo"- the center of the snowflake. The embryo is basically the center for any more water molecules that are converted to the solid state and it acts as the center until all of the water present has been converted. The rate at which the latent heat (the quantity of heat absorbed or released by a substance undergoing a change of state- such as water to ice) is released controls the timing of the process. So as more and more molecules attach to the embryo, energy releases. This released energy causes the temperature of the molecules attached to the embryo to be significantly lower than the temperature of any molecules unattached to the embryo. Water molecules continue to attach to the embryo. Once all molecules have attached, you have a solid state- ICE! Or, more aptly, SNOW!
Heterogeneous Nucleation
Heterogeneous nucleation occurs when a change of state from water to ice forms at temperatures above -40dC or -40dF. The change of state is enabled by a foreign material that is present in the water; basically an artificial embryo that assits the nucleation process. The foreign material, such as a dust particle, acts as the embryo and has a much faster growing rate than a natural embryo (pure water). The location of where the embryo is formed is called an ice-nucleating site. Heterogeneous nucleation depends on two factors as well, but they differ from those of Homogeneous nucleation. The two factors are the energy change used when the embryo is formed and the ways by which the embryo growth changes.
Heterogeneous nucleation is found most commonly in artificial snowmaking. Snowmaking employs two different types of nucleators: high-temperature and low-temperature.
High-temperature Nucleators:
-- soliver iodide (Dry ice)
-- Ice nucleating proteins (INP's)
Low-temperature Nucleators:
-- Calcium (Ca)
-- Magnesium (Mg)
-- Dust
-- Silt
The low-temperature nucleators are most commonly found in, for example, the reservoirs used by ski resorts for snowmaking.
"The Snow Additive"
Before, we talked briefly about heterogeneous nucleation and the role that nucleators play in snowmaking. But let's take a swan dive into the pool of York International, a company that makes the Snomax Snow Inducer, an ice-nucleating protein (INP).
Snomax uses Pseudomonas syringae, a bacterium that is found in nature in grass, trees, crops, even in the air. According to the company, there are about 40 organisms of P. syringae in one cubic meter of air, and about 10 billion organisms in one tomato leaf. P. syringae is an INP, or an ice-nucleating protein. INPs promote the growth of ice crystals by raising the nucleation temperature. Snomax sells actual bags of fermented, pelletized, freeze-dried and irradiated P. syringae to ski resorts, which they then use to make snow. The P. syringae acts as the embryo in heterogeneous nucleation (see above). Water molecules attach to the P. syringae at a much faster rate than they do to water in homogeneous nucleation. Also, the water freezes to the P. syringae at higher temperatures than it normally does.
The Snomax (P. syringae) is mixed into water to form a concentrate that is later injected into snowmaking water. Every water droplet shot from the snowgun is seeded with the P. syringae. The great benefit of Snomax is that it allows the ice to form normally at 27�F, and even up to 31�F in more humid areas. Obviously, Snomax is a high-temperature nucleator, which means that the great majority of the water droplets will freeze. York International states that "snowmaking water with Snowmax added will freeze faster, freeze more evenly, and make up to 40% more snow. This gives the ski operator the ability make snow over a wider range of temperatures / humidity conditions than making snow without Snomax or from using any other additive in the ski industry. In addition, snow made with Snomax will cure quicker, groom easier and last longer. Snomax is the best snow additive in the ski industry!"
www.interfab.at/ beschneiung_snowmax.html
