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| In [[fluid mechanics]], '''dewetting''' is one of the processes that can occur at a solid–liquid or liquid–liquid [[Interface (chemistry)|interface]]. Generally, dewetting describes the rupture of a thin liquid film on the substrate (either a liquid itself, or a solid) and the formation of droplets. The opposite process—spreading of a liquid on a substrate—is called [[spreading]]. The factor determining the spontaneous spreading and dewetting for a drop of oil placed on a liquid substrate (water here) with ambient gas, is the so-called '''spreading coefficient''' <math>S</math>:<ref name="Rosen">{{cite book |first=Milton J. |last=Rosen |title=Surfactants and Interfacial Phenomena |publisher=Wiley-Interscience |location=[[Hoboken, New Jersey]] |edition=3rd |page=244 |year=2004 |isbn=978-0-471-47818-8 |oclc=475305499}}</ref>
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| :<math>S\ = \gamma_\text{gw} - \gamma_\text{go} - \gamma_\text{ow}</math> | |
| where <math>\gamma_\text{gw}</math> is the gas-water [[surface tension]], <math>\gamma_\text{go}</math> is the gas-oil surface tension and <math>\gamma_\text{ow}</math> is the oil water surface tension (measured on the fluids before they are brought in contact with each other)
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| When <math>S>0</math>, the spontaneous spreading occurs, and if <math>S<0</math>, dewetting occurs.
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| [[Spreading]] and dewetting are important processes for many applications, including [[adhesion]], [[lubrication]], painting, printing, and protective coating. For most applications, dewetting is an unwanted process, because it destroys the applied thin film.
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| In most dewetting studies a thin [[polymer]] film is spun onto a substrate. Even in the case of <math>S<0</math> the film does not dewet immediately if it is in a metastable state, e.g. if the temperature is below the [[glass transition temperature]] of the polymer. Annealing such a metastable film above its [[glass transition temperature]] increases the mobility of the polymer-chain molecules and dewetting takes place.<ref name="Leroux">{{cite journal |doi=10.1016/j.jcis.2008.09.062 |title=Polypropylene film chemical and physical modifications by dielectric barrier discharge plasma treatment at atmospheric pressure |year=2008 |last1=Leroux |first1=Frédéric |last2=Campagne |first2=Christine |last3=Perwuelz |first3=Anne |last4=Gengembre |first4=Léon |journal=Journal of Colloid and Interface Science |volume=328 |pages=412–20 |pmid=18930244 |issue=2}}</ref><ref name="karapanagiotis">{{cite journal |doi=10.1016/j.susc.2005.07.023 |title=Polymer film rupturing in comparison with leveling and dewetting |year=2005 |last1=Karapanagiotis |first1=Ioannis |last2=Gerberich |first2=William W. |journal=Surface Science |volume=594 |issue=1–3 |pages=192–202|bibcode = 2005SurSc.594..192K }}</ref>
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| When starting from a continuous film, an irregular pattern of droplets is formed. The droplet size and droplet spacing may vary over several orders of magnitude, since the dewetting starts from randomly formed holes in the film. There is no spatial correlation between the dry patches that develop. These dry patches grow and the material is accumulated in the rim surrounding the growing hole. In the case where the initially homogeneous film is thin (in the range of {{Convert|100|nm|abbr=on|sigfig=1}}), a [[polygon]] network of connected strings of material is formed, like a [[Voronoi]] pattern of polygons. These strings then can break up into droplets, a process which is known as the [[Plateau-Rayleigh instability]]. At other film thicknesses, other complicated patterns of droplets on the substrate can be observed, which stem from a fingering instability of the growing rim around the dry patch.
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| Surfactants can have a significant effect on the spreading coefficient. When a surfactant is added, it's amphiphilic properties cause it to be more energetically favorable to migrate to the surface, decreasing the interfacial tension and thus increasing the spreading coefficient (i.e. making S more positive). As more surfactant molecules are absorbed into the interface, the free energy of the system decreases in tandem to the surface tension decreasing, eventually causing the system to become completely [[wetting]].
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| ==References==
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| {{reflist}}
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| [[Category:Fluid mechanics]]
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I am Rodolfo and was born on 22 October 1988. My hobbies are Seaglass collecting and Geocaching.
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