Heterogeneous Nucleation

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Heterogeneous Nucleation process is when ice forms at temperatures above minus 40 C or minus 40 F due to the presence of a foreign material in the water. This foreign material acts as the embryo and grows more rapidly than embryos of pure water. The location on which an ice embryo is formed is called an ice-nucleating site.
Heterogeneous Nucleation process is when ice forms at temperatures above minus 40 C or minus 40 F due to the presence of a foreign material in the water. This foreign material acts as the embryo and grows more rapidly than embryos of pure water. The location on which an ice embryo is formed is called an ice-nucleating site.

Heterogeneous Nucleation is when ice forms at temperatures above minus 40 C or minus 40 F due to the presence of a foreign material in the water. This foreign material acts as the embryo and grows more rapidly than embryos of pure water. The location on which an ice embryo is formed is called an ice-nucleating site. As with homogeneous nucleation, heterogeneous nucleation is governed by two major factors:

  • the free energy change involved in forming the embryo and the dynamic of fluctuating embryo growth.
  • In heterogeneous nucleation, the configuration and energy of interaction at the nucleating site become the dominating influence in the conversion of water to ice. Snowmaking involves the process of heterogeneous nucleation.

There are many materials and substances which act as nucleates, each promotes freezing at a specific temperature or nucleation temperature. They are generally categorized as high-temperature (i.e., sliver iodide, dry ice and ice nucleating proteins) or low-temperature (i.e., calcium, magnesium, dust and silt) nucleates. It is the low-temperature nucleators that are found in large numbers in untreated snowmaking water. The nucleation temperature of snowmaking water is between 15 F and 20F.

Why do you hear freezing warnings at temperatures around 32 F? The answer is that another factor is coming into play with the freezing process. That factor is called surface (i.e., roads, highways, trees). There is an energy interaction between the ice-nucleating site in the water with the surface. This causes the water droplets to freeze very near 32 F or 0 C.

In snowmaking it is the nucleator having the highest nucleation temperature that determines when a water droplet will freeze.

As a water droplet cools, heat energy is released into the atmosphere at a rate of one calorie per gram of water.

As it freezes into an ice crystal, the water droplet will release additional energy at a rate of 80 calories per gram of water. this quick release of energy raises the water droplet temperature to 32 F, where it will remain while freezing continues. This is one reason why we are accustomed to thinking that water freezes at 32 F. To be precise, the water will continue to freeze as long as it remains at or below 32 F, but only after it has first cooled to its nucleation temperature. Any excess energy will be dissipated into the atmosphere.


[edit] Also See

[edit] Reference;

  • 1 Wikipedia - Nucleation see [1]



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