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Freezing and Supercooling of Liquids

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Freezing and Supercooling of Liquids Christoph Stähle, Mat.Nr. 1211028 11.06.2014 Freezing and supercooling of liquids Doing research about low temperature procedures in liquids, this paper gives information about freezing and supercooling of liquids. Liquids need structured molecules, so called clusters, for being able to freeze. The amount of clusters needed for the act of freezing depends on the temperature of the fluid. If the pressure is right and the fluid has a high clarity, it's possible to cool it down below 273,15 K without crystallization. Analyzing the state of a supercooled water reveals that further cooling leads to an increasing viscosity until it reaches a condition comparable to glass. So supercooling is a metastable state. A supercooled liquid is going to freeze, if some sort of impulse is transmitted or the fluid is contaminated by some foreign material. How freezing and supercooling appears in nature is explained in the example of the arctic ground squirrel. Another way of using principle of supercooling is the latent-heat storage system, better known as hand warmer. However, it' even possible to use fluids as energy storage. As we know there are three different states of matter, solid, liquid and the gaseous state. These are normally shown in a state of matter diagram. But taking a closer look at the state of matter diagram of water reveals that it can stay liquid underneath the freezing temperature of 273,15 K at the right circumstances. As many people might not know a lot about low temperature procedures in liquids such as nucleation as a requirement for freezing, the author did research about what happens in fluids if temperature is lowered below the freezing temperature. So this brief article gives some information especially about the act of freezing and the metastable state of supercooling. Fig. 1.: state of matter diagram for a normal substance In general all states of aggregation for a normal substance are shown in a state of matter diagram (Fig.1). Furthermore there are two special points, the so called triple point and the critical point. At the triple point all three states can appear together. Above the critical point there is no more significant difference between liquid and gaseous state. For water as an abnormal liquid the state of matter diagram looks slightly different. As it can be seen in the diagram (Fig.2), water can remain liquid although it is cooled beyond its freezing temperature without changing into the solid state, which is actually called supercooling. The opposite, heating water above its boiling temperature without changing from liquid to the gaseous state, is possible too and called boiling Fig. 2.: state of matter diagram for water retardation or super-heating. Christoph Stähle, Mat.Nr. 1211028 11.06.2014 If a liquid is cooled down at its freezing temperature it is crystallizing. Therefor 1 W = ∗σ∗Ο (1) nucleation is needed for the ice crystals to be 3 α able to attach [4]. On the one hand such a nucleus can consist of, for example dust particles or any other contamination in the fluid. On the In that equation W is the work itself, σ is the other hand so called clusters take in that position surface tension and O α is the surface area of the [1]. In general clusters are accumulations of nucleus [5]. atoms or molecules that can appear in different geometric shapes, so called platonic body's [2]. Now let's take a closer look at supercooling: These are three dimensional objects with Preventing water from freezing needs a certain polygon surface areas. The most important one amount of pressure, the lower the temperature is is the icosahedron [9]. the higher the pressure has to be. As we said before dust and other contamination support the development of ice crystals. So preventing distilled water from freezing is easier than normal water. Supercooling is a metastable state. That means, the system remains the same during small changes but it is unstable during higher modifications. So a supercooled liquid is going to freeze if some sort of impulse is transmitted or the fluid is contaminated by some foreign material. For example: Hitting a table with a bottle keeping a supercooled drink might be enough for spontaneous freezing, but it has to be opened before. Otherwise the pressure inside keeps your drink liquid. Fig. 3.: structure of a icosahedron Supercooled liquids can even be seen as some sort of energy storage that releases it's saved energy through freezing. In the context of ice formation clusters are connections of water molecules. For the Cooling down a liquid leads to an increase of formation of an ice crystal the nucleus needs a viscosity. The connection between viscosity and certain size, the so called critical size. If this is temperature is given by following equation : achieved by the meeting of lots of particles it is called homogeneous nucleation. Because the E clusters break up as fast as they are built together ( A ) (R∗T ) a lot of them are needed. The exact amount of η=η0∗e (2) particles needed, depends on the temperature of the liquid. The lower the temperature is, the less particles are needed. At a temperature of 268,15 In this formula η0 is a material constant, E A is the K about 5*10 4 structured water molecules are activation energy, R is the gas constant and T is needed. If the temperature is lowered at about the temperature [7]. 253,15 K still a few hundred molecules need to get together. As you keep cooling down already supercooled The work that needs to be done for creating a water it's viscosity is increasing until it's texture nucleus of critical size can be calculated by is similar to glass, which is known as amorphous following equation: ice. Christoph Stähle, Mat.Nr. 1211028 11.06.2014 The difference between normal and amorphous Summing up, freezing of a liquid needs a certain ice is, that the second one has no frequent amount of structured molecules, so called structure like the crystal of normal ice. clusters. Freezing can be avoided by keeping the There are three known types of amorphous ice: clusters at a minimum by the use of clear liquids such as distilled water and high pressure at low • low density amorphous ice (LDA) temperatures. Supercooling however gives the • high density amorphous ice (HDA) opportunity to save energy in form of heat for a • very high density amorphous ice later use such as hand warmers do. (VHDA) As we said before, supercooling is a way of [1]http://de.wikipedia.org/wiki/Cluster_(Physik); saving energy in form of heat. For example this 11.06.2014 is used for latent-heat storage systems (hand warmers) [8]. Charging such a system works by [2]http://de.wikipedia.org/wiki/Platonischer_K melting special crystals, which leads to saving a %C3%B6rper; 11.06.2014 lot of thermal energy. Unloading the storage is connected with crystallization and the release of [3]http://de.wikipedia.org/wiki/Eis; 11.06.2014 the thermal heat that has been saved before. [4]http://de.wikipedia.org/wiki/Nukleation; Even animals use supercooling for surviving. 11.06.2014 The arctic ground squirrels (Fig.4) for instance, are able to cool their body temperature down to [5]http://web.sg.ethz.ch/users/fschweitzer/until20 270,15 K [6]. So the blood of the little animals 05/download/Schmelzer,%20Schweitzer%20- stays liquid and circulating during their winter %201985%20-%20Thermodynamik%20und sleep. %20Keimbildung,%20I.%20Isotherme However, they have the same problem as the %20Keimbildung%20in%20finiten bottle hit against a table, we mentioned before. If %20Systemen.pdf; 11.06.2014 there is a tremor, like an earthquake, they spontaneously freeze and die. [6]http://www.newscientist.com/article/dn23107- zoologger-supercool-squirrels-go-into-the-deep- freeze.html#.U5jx42zmTIU; 11.06.2014 [7]http://de.wikipedia.org/wiki/Viskosit %C3%A4t#Temperaturabh.C3.A4ngigkeit; 11.06.2014 [8]http://de.wikipedia.org/wiki/W %C3%A4rmekissen; 11.06.2014 [9]http://de.wikipedia.org/wiki/Ikosaeder; 11.062014 [10]http://de.wikipedia.org/wiki/Unterk Fig. 4.: arctic ground squirrel %C3%BChlung_(Thermodynamik); 11.06.2014.
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