Inphysics andchemistry,flash freezing is a process by which an object is rapidly frozen by subjecting an object tocryogenic temperatures, or through direct contact withliquid nitrogen at −196 °C (−320.8 °F).^[1]

This process is closely related to classicalnucleation theory. When water freezes slowly,crystals grow from fewer nucleation sites, resulting in fewer and largerice crystals. This damagescell walls and causes celldehydration. 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 andtissue preservation.^[2]

Flash freezing is commonly applied in thefood industry and is studied inatmospheric science.

The surface environment does not play a decisive role in the formation ofice andsnow.^[3] Density fluctuations within water droplets result in the possible freezing regions covering both the interior and the surface^[4]—that is, whether freezing from the surface or from within may be at random.^[4]

There are phenomena likesupercooling, in which the water is cooled below itsfreezing point but remains liquid if there are too few defects to seedcrystallization. One can therefore observe a delay until the water adjusts to the new, below-freezing temperature.^[5] Supercooled liquid water must become ice at −48 °C (−54 °F), not just because of the extreme cold, but because themolecular structure of water changes physically to formtetrahedron shapes, with each water molecule loosely bonded to four others.^[6] This suggests the structural change from liquid to "intermediate ice".^[6] The crystallization of ice from supercooled water is generally initiated by a process callednucleation. The speed and size of nucleation occurs withinnanoseconds andnanometers.^[7]

As water freezes, tiny amounts of liquid water are theoretically still present, even as temperatures go below −48 °C (−54 °F) 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.^[6] 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.^[3] Once the liquid is completely frozen, the sharp tip of the drop attractswater vapor in the air, much like a sharp metallightning rod attractselectrical charges.^[3] The water vapor collects on the tip and a tree of small ice crystals starts to grow.^[3] An opposite effect has been shown to preferentially extract water molecules from the sharp edge of potato wedges in the oven.^[3]

If amicroscopic droplet of water is cooled very fast, it forms aglass—a low-densityamorphous ice in which all the tetrahedral water molecules are not aligned but amorphous.^[6] The change in the structure of water controls the rate at which ice forms.^[6] Depending on its temperature and pressure, water ice has 16 differentcrystalline forms in which water molecules cling to each other withhydrogen bonds.^[6]

Crystal growth or nucleation is the formation of a newthermodynamic phase or a new structure via self-assembly. Nucleation is often found to be very sensitive to impurities in the system. For nucleation of a new thermodynamic phase, such as the formation of ice in water below 0 °C (32 °F), if the system is not evolving with time and nucleation occurs in one step, then the probability that nucleation has not occurred should undergoexponential decay. This can also be observed in the nucleation of ice in supercooled small water droplets.^[8] The decay rate of the exponential gives the nucleation rate and is given by

${\displaystyle R=N_{S}Zj\exp \left(\frac{-\mathrm{\Delta }{G}^{\ast }}{k_{B}T}\right)}$

where

• ${\displaystyle N_S}$ is the number of nucleation sites;
• ${\displaystyle Z}$ 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);
• ${\displaystyle j}$ is the rate at which molecules attach to the nucleus, causing it to grow;
• ${\displaystyle \mathrm{\Delta }{G}^{\ast }}$ is the free energy cost of the nucleus at the top of the nucleation barrier;
• ${\displaystyle k_{B}T}$ is thethermal energy, where${\displaystyle T}$ is the absolute temperature and${\displaystyle k_B}$ is theBoltzmann constant.

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.^[9][10]

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, thefree energy change of a droplet${\displaystyle \mathrm{\Delta }G(r)}$ 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:

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

The first term represents volume, and (assuming a spherical nucleus) this is the volume of a sphere of radius${\displaystyle r}$. Here,${\displaystyle \mathrm{\Delta }g}$ 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${\displaystyle \sigma }$ is thesurface tension.

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

${\displaystyle r_c=-\frac{2\sigma }{\mathrm{\Delta }g}}$

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${\displaystyle \mathrm{\Delta }G}$, and thus lowers the nucleation barrier${\displaystyle \mathrm{\Delta }{G}^{\ast }}$. This lowered barrier is what makes heterogeneous nucleation much more common and faster than homogeneous nucleation.^[11]

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

${\displaystyle \mathrm{\Delta }P\equiv P_{\text{inside}}-P_{\text{outside}}=\gamma \left(\frac{1}{R_1}+\frac{1}{R_2}\right)}$

where${\displaystyle R_1}$ and${\displaystyle R_2}$ are the principalradii of curvature and${\displaystyle \gamma }$ (also denoted as${\displaystyle \sigma }$) 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${\displaystyle \gamma =\frac{W}{\mathrm{\Delta }A}}$. This work${\displaystyle W}$ is interpreted as thepotential energy.

Flash freezing is used in thefood industry to quickly freezeperishable food items (seefrozen food). In this case, food items are subjected to temperatures well below^{[clarification needed]} thefreezing point of water. Thus, smaller ice crystals are formed, causing less damage tocell membranes.^[12] American inventorClarence Birdseye developed the "quick-freezing" process offood preservation in the 20th century using a cryogenic process.^[13] 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.^[2]

Flash freezing techniques are also used to freeze biological samples quickly so that large ice crystals cannot form and damage the sample.^[14] This is done by submerging the sample inliquid nitrogen or a mixture ofdry ice andethanol.^[15]

Flash freezing is of great importance inatmospheric science, as its study is necessary for a properclimate model for the formation ofice clouds in the uppertroposphere, which effectively scatter incomingsolar radiation and prevent Earth from becoming overheated by the Sun.^[7] The results have important implications inclimate control research. One of the current debates is whether the formation of ice occurs near the surface or within themicrometre-sized droplets suspended in clouds. If it is the former, effective engineering approaches may exist to tune thesurface tension of water so that the ice crystallization rate can be controlled.^[7]

• Frozen food – Food stored at temperatures below the freezing point of water, for extending its shelf life

• Food preservation
• Preservation methods
• Phase transitions
• Cold

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