Flash freezing

 ( Food Preservation ) 

There are phenomena like supercooling, in which the water is cooled below its freezing point but remains liquid if there are too few defects to seed crystallization. One can therefore observe a delay until the water adjusts to the new, below-freezing temperature. Supercooled liquid water must become ice at , not just because of the extreme cold, but because the molecular structure of water changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others. This suggests the structural change from liquid to "intermediate ice". The crystallization of ice from supercooled water is generally initiated by a process called nucleation. The speed and size of nucleation occurs within and Nanometre.

As water freezes, tiny amounts of liquid water are theoretically still present, even as temperatures go below and almost all the water has turned solid, either into crystalline ice or amorphous water. However, this remaining liquid water crystallizes too fast for its properties to be detected or measured. The freezing speed directly influences the nucleation process and ice crystal size. A supercooled liquid will stay in a liquid state below the normal freezing point when it has little opportunity for nucleation—that is, if it is pure enough and is in a smooth-enough container. Once agitated it will rapidly become a solid.

During the final stage of freezing, an ice drop develops a pointy tip, which is not observed for most other liquids, and arises because water expands as it freezes. Once the liquid is completely frozen, the sharp tip of the drop attracts water vapor in the air, much like a sharp metal lightning rod attracts Electric charge. The water vapor collects on the tip and a tree of small ice crystals starts to grow. An opposite effect has been shown to preferentially extract water molecules from the sharp edge of potato wedges in the oven.

If a microscopic droplet of water is cooled very fast, it forms a glass—a low-density amorphous ice in which all the tetrahedral water molecules are not aligned but amorphous. The change in the structure of water controls the rate at which ice forms. Depending on its temperature and pressure, water ice has 16 different in which water molecules cling to each other with .

$R\backslash \; =\backslash \; N\_S\; Zj\backslash exp\; \backslash left(\; \backslash frac\{-\backslash Delta\; G^*\}\{k\_BT\}\; \backslash right)$

• $N\_S$ is the number of nucleation sites;
• $Z$ is the probability that a nucleus at the top of the barrier will go on to form the new phase, not dissolve (called the Zeldovich factor);
• $j$ is the rate at which molecules attach to the nucleus, causing it to grow;
• $\backslash Delta\; G^*$ is the free energy cost of the nucleus at the top of the nucleation barrier;
• $k\_BT$ is the thermal energy, where $T$ is the absolute temperature and $k\_B$ is the Boltzmann constant.

Nucleation can be divided into homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation is the rarer, but simpler, case. In homogeneous nucleation, classical nucleation theory assumes that for a microscopic, spherical nucleus of a new phase, the free energy change of a droplet $\backslash Delta\; G(r)$ is a function of the size of the nucleus, and can be written as the sum of terms proportional to the nucleus' volume and surface area:

$\backslash Delta\; G=\{\backslash frac\{4\}\{3\}\}\backslash pi\; r^\{3\}\backslash Delta\; g+4\backslash pi\; r^\{2\}\backslash sigma$

At some intermediate value of $r$, the free energy $\backslash Delta\; G$ goes through a maximum, and so the probability of formation of a nucleus goes through a minimum. This occurs when $\backslash frac\{dG\}\{dr\}=0$. This point, $\backslash Delta\; G^*$, is called the critical nucleus and represents the nucleation barrier; it occurs at the critical radius

$r\_c=-\{\backslash frac\{2\backslash sigma\}\{\backslash Delta\; g\}\}$

Heterogeneous nucleation occurs at a surface or impurity. In this case, part of the nucleus boundary is accommodated by the surface or impurity onto which it is nucleating. This reduces the surface area term in $\backslash Delta\; G$, and thus lowers the nucleation barrier $\backslash Delta\; G^*$. This lowered barrier is what makes heterogeneous nucleation much more common and faster than homogeneous nucleation.

$\backslash Delta\; P\; \backslash equiv\; P\_\backslash text\{inside\}\; -\; P\_\backslash text\{outside\}\; =\; \backslash gamma\backslash left(\backslash frac\{1\}\{R\_1\}+\backslash frac\{1\}\{R\_2\}\backslash right)$

The surface tension can be defined in terms of force or energy. The surface tension of a liquid is the ratio of the change in the liquid's energy and the change in the liquid's surface area (which led to the change in energy). It can be defined as $\backslash gamma=\backslash frac\{W\}\{\backslash Delta\; A\}$. This work $W$ is interpreted as the potential energy.