thumb|Ice crystals in a frozen pond. When the water cools slowly, crystals are formed. Freezing quickly reduces crystal formation.

In physics and chemistry, flash freezing is a process by which an object is rapidly frozen by subjecting an object to cryogenic temperatures, or through direct contact with liquid nitrogen at .

This process is closely related to classical nucleation theory. When water freezes slowly, crystals grow from fewer nucleation sites, resulting in fewer and larger ice crystals. This damages cell walls and causes cell dehydration. When water freezes quickly, as in flash freezing, there are more nucleation sites, and more, smaller crystals. This results in much less damage to cell walls, proportional to the rate of freezing. This is why flash freezing is good for food and tissue preservation.

Flash freezing is commonly applied in the food industry and is studied in atmospheric science.

Impact of freezing

The surface environment does not play a decisive role in the formation of ice and snow. Density fluctuations within water droplets result in the possible freezing regions covering both the interior and the surface—that is, whether freezing from the surface or from within may be at random. 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 decay rate of the exponential gives the nucleation rate and is given by

:<math>R\ =\ N_S Zj\exp \left( \frac{-\Delta G^*}{k_BT} \right)</math>

where

  • <math>N_S</math> is the number of nucleation sites;
  • <math>Z</math> 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);
  • <math>j</math> is the rate at which molecules attach to the nucleus, causing it to grow;
  • <math>\Delta G^* </math> is the free energy cost of the nucleus at the top of the nucleation barrier;
  • <math>k_BT </math> is the thermal energy, where <math>T</math> is the absolute temperature and <math>k_B</math> is the Boltzmann constant.

thumb|Difference in energy barriers. Homogeneous nucleation (blue) has a higher nucleation barrier <math>\Delta G^* </math>at r<sub>c</sub> than heterogeneous nucleation (red).|255x255px

Classical nucleation theory is a widely used approximate theory for estimating these rates, and how they vary with variables such as temperature. It correctly predicts that the time needed for nucleation decreases extremely rapidly when supersaturated.

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 <math>\Delta G(r) </math> 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:

:<math>\Delta G={\frac{4}{3\pi r^{3}\Delta g+4\pi r^{2}\sigma </math>

The first term represents volume, and (assuming a spherical nucleus) this is the volume of a sphere of radius <math>r</math>. Here, <math>\Delta g</math> is the difference in free energy per unit volume between the thermodynamic phase in which nucleation is occurring, and the phase that is nucleating. The second term represents the surface area, again assuming a sphere, where <math>\sigma</math> is the surface tension.

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

:<math>r_c=-{\frac{2\sigma}{\Delta g</math>

The addition of new molecules to nuclei larger than this critical radius decreases the free energy, so these nuclei are more probable.

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 <math>\Delta G </math>, and thus lowers the nucleation barrier <math>\Delta G^* </math>. This lowered barrier is what makes heterogeneous nucleation much more common and faster than homogeneous nucleation.

Laplace pressure

The Laplace pressure is the pressure difference between the inside and the outside of a curved surface between a gas region and a liquid region. The Laplace pressure is determined from the Young–Laplace equation given as

:<math>\Delta P \equiv P_\text{inside} - P_\text{outside} = \gamma\left(\frac{1}{R_1}+\frac{1}{R_2}\right)</math>

where <math>R_1</math> and <math>R_2</math> are the principal radii of curvature and <math>\gamma</math> (also denoted as <math>\sigma</math>) is the surface tension.

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 <math>\gamma=\frac{W}{\Delta A}</math>. This work <math>W</math> is interpreted as the potential energy.

Applications and techniques

thumb|Flash freezing being used for [[cryopreservation]]

Flash freezing is used in the food industry to quickly freeze perishable food items (see frozen food). In this case, food items are subjected to temperatures well below the freezing point of water. Thus, smaller ice crystals are formed, causing less damage to cell membranes. American inventor Clarence Birdseye developed the "quick-freezing" process of food preservation in the 20th century using a cryogenic process. In practice, a mechanical freezing process is usually used instead due to cost. There has been continuous optimization of the freezing rate in mechanical freezing to minimize ice crystal size. This is done by submerging the sample in liquid nitrogen or a mixture of dry ice and ethanol.

Flash freezing is of great importance in atmospheric science, as its study is necessary for a proper climate model for the formation of ice clouds in the upper troposphere, which effectively scatter incoming solar radiation and prevent Earth from becoming overheated by the Sun. The results have important implications in climate control research. One of the current debates is whether the formation of ice occurs near the surface or within the micrometre-sized droplets suspended in clouds. If it is the former, effective engineering approaches may exist to tune the surface tension of water so that the ice crystallization rate can be controlled.