Editing Flash freezing

{{Short description|Quick freezing by exposure to cryogenic temperatures}}
[[File:Frozen Leaf (6886708197).jpg|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 [[cryogenics|cryogenic temperatures]], or through direct contact with [[liquid nitrogen]] at {{Convert|-196|C|}}.<ref>{{cite web |title=What is Flash Freezing? (with pictures) |date=27 February 2024 |url=http://www.wisegeek.org/what-is-flash-freezing.htm}}</ref>

This process is closely related to classical [[nucleation]] theory. When water freezes slowly, [[Crystal growth|crystals grow]] from fewer nucleation sites, resulting in fewer and larger [[ice crystal]]s. This damages [[cell wall]]s 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 (biology)|tissue]] preservation.<ref name="FAO">{{Cite web |title=Freezing of fruits and vegetables |url=http://www.fao.org/3/y5979e/y5979e03.htm |access-date=2020-04-06 |website=www.fao.org}}</ref>

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]].<ref name=":4">{{Cite web |title=Freezing water droplets form sharp ice peaks |url=https://www.sciencedaily.com/releases/2012/10/121005092911.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> Density fluctuations within water droplets result in the possible freezing regions covering both the interior and the surface<ref name=":1">{{Cite web |title=How water droplets freeze: The physics of ice and snow |url=https://www.sciencedaily.com/releases/2016/06/160621115439.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref>—that is, whether freezing from the surface or from within may be at random.<ref name=":1" />

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.<ref name=":3">{{Cite web |title=Superradiant matter: A new paradigm to explore dynamic phase transitions |url=https://www.sciencedaily.com/releases/2015/03/150318101355.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> Supercooled liquid water must become ice at {{Convert|-48|C|F}}, 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.<ref name=":0">{{Cite web |title=Supercool: Water doesn't have to freeze until -48 C (-55 F) |url=https://www.sciencedaily.com/releases/2011/11/111123133123.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> This suggests the structural change from liquid to "intermediate ice".<ref name=":0" /> The crystallization of ice from supercooled water is generally initiated by a process called [[nucleation]]. The speed and size of nucleation occurs within [[nanosecond]]s and [[Nanometre|nanometers]].<ref name=":2" />

As water freezes, tiny amounts of liquid water are theoretically still present, even as temperatures go below {{convert|-48|C|}} 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.<ref name=":0" /> 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.<ref name=":4" /> 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|electrical charges]].<ref name=":4" /> The water vapor collects on the tip and a tree of small ice crystals starts to grow.<ref name=":4" /> An opposite effect has been shown to preferentially extract water molecules from the sharp edge of potato wedges in the oven.<ref name=":4" />

If a [[Microscopic scale|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.<ref name=":0" /> The change in the structure of water controls the rate at which ice forms.<ref name=":0" /> Depending on its temperature and pressure, water ice has 16 different [[crystalline form]]s in which water molecules cling to each other with [[hydrogen bond]]s.<ref name=":0" />

== Concepts ==

=== Nucleation ===
{{Main article|Classical nucleation theory}}

Crystal growth or nucleation is the formation of a new [[Thermodynamics|thermodynamic]] 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 {{convert|0|C|}}, if the system is not evolving with time and nucleation occurs in one step, then the probability that nucleation has not occurred should undergo [[exponential decay]]. This can also be observed in the nucleation of ice in supercooled small water droplets.<ref>{{Cite book|title=Laboratory evidence for volume-dominated nucleation of ice in supercooled water microdroplets|last=Duft|first=D|publisher=Atmospheric Chemistry and Physics|year=2004}}</ref> 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]].
[[File:Hethomnucdifference.JPG|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.<ref>{{Cite book|title=Microphysics of Clouds and Precipitation|last=Pruppacher. Klett|first=H.R., J.D.|publisher=Kluwer|year=1997}}</ref><ref>{{Cite book|title=Nucleation: theory and applications to protein solutions and colloidal suspensions|last=Sear|first=R.P.|publisher=Physics Cond. Matt.|year=2007}}</ref>

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 [[Gibbs free energy|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.<ref>{{cite journal |last1=Liu |first1=X. Y. |date=31 May 2000 |title=Heterogeneous nucleation or homogeneous nucleation? |journal=The Journal of Chemical Physics |volume=112 |issue=22 |pages=9949–9955 |bibcode=2000JChPh.112.9949L |doi=10.1063/1.481644 |issn=0021-9606}}</ref>

=== Laplace pressure ===
{{Main article|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 [[Radius of curvature|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 ==
[[File:Cryopreservation.jpg|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{{clarify|What temperature? All freezers work well below freezing point but items put in it don't flash freeze|date=February 2017}} the [[freezing point|freezing point of water]]. Thus, smaller ice crystals are formed, causing less damage to [[cell membranes]].<ref>Da-Wen Sun (2001), Advances in food refrigeration, Yen-Con Hung, Cryogenic Refrigeration, p.318, Leatherhead Food Research Association Publishing, http://www.worldcat.org/title/advances-in-food-refrigeration/oclc/48154735</ref> American inventor [[Clarence Birdseye]] developed the "quick-freezing" process of [[food preservation]] in the 20th century using a cryogenic process.<ref>[https://books.google.com/books?id=UigDAAAAMBAJ&dq=1930+plane+%22Popular&pg=PA26 "Quick-Frozen Food Exactly Like Fresh."] ''Popular Science Monthly'', September 1930, pp. 26-27.</ref> 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.<ref name="FAO" />

Flash freezing techniques are also used to freeze biological samples quickly so that large ice crystals cannot form and damage the sample.<ref>{{cite web |title=Freezing Tissue |url=http://www.biotech.ufl.edu/EM/data/freeze.html |archive-url=https://web.archive.org/web/20120111233417/http://www.biotech.ufl.edu/EM/data/freeze.html |archive-date=11 January 2012 |access-date=2009-07-03 |publisher=Biotech.ufl.edu}}</ref> This is done by submerging the sample in [[liquid nitrogen]] or a mixture of [[dry ice]] and [[ethanol]].<ref>{{cite web |title=Preparing Competent E. coli with RF1/RF2 solutions |url=http://www.personal.psu.edu/dsg11/labmanual/DNA_manipulations/Comp_bact_by_RF1_RF2.htm |url-status=dead |archive-url=https://web.archive.org/web/20210923024451/http://www.personal.psu.edu/dsg11/labmanual/DNA_manipulations/Comp_bact_by_RF1_RF2.htm |archive-date=2021-09-23 |access-date=2009-07-03 |publisher=Personal.psu.edu}}</ref>

Flash freezing is of great importance in [[atmospheric science]], as its study is necessary for a proper [[climate model]] for the formation of [[ice cloud]]s in the upper [[troposphere]], which effectively scatter incoming [[solar radiation]] and prevent Earth from becoming overheated by the Sun.<ref name=":2">{{Cite web |title=Better understanding of water's freezing behavior at nanoscale |url=https://www.sciencedaily.com/releases/2013/05/130521152429.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> 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|surface tension of water]] so that the ice crystallization rate can be controlled.<ref name=":2" />

== See also ==

* {{anl|Frozen food}}

== References ==

{{refs}}

[[Category:Food preservation]]
[[Category:Preservation methods]]
[[Category:Phase transitions]]
[[Category:Cold]]