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| [[Image:Wilhelmy plate.PNG|frame|right|Illustration of Wilhelmy plate method; <math>l</math> is, unlike shown, NOT the height of the plate; the magnitude of force on the plate is instead directly proportional to the perimeter of the plate.]] | |
| A '''Wilhelmy plate''' is a thin plate that is used to measure equilibrium [[surface tension|surface or interfacial tension]] at an air–liquid or liquid–liquid interface. In this method, the plate is oriented perpendicular to the interface, and the force exerted on it is measured. Based on the work of [[Ludwig Wilhelmy]], this method finds wide use in the preparation and monitoring of [[Langmuir–Blodgett film]]s.
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| ==Detailed description==
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| The Wilhelmy plate consists of a thin plate usually on the order of a few square centimeters in area. The plate is often made from filter paper, glass or [[platinum]] which may be roughened to ensure complete [[wetting]]. In fact, the results of the experiment are irrelevant of the material used, as long as the material is wetted by the liquid.<ref>{{cite book|last=Butt|first=Hans-Jürgen|title=Physics and chemistry of interfaces|year=2006|publisher=Wiley-VCH-Verl.|location=Weinheim|isbn=9783527406296|page=16|edition=2., rev. and enl. ed.|coauthors=Graf,, Karlheinz; Kappl, Michael}}</ref> The plate is cleaned thoroughly and attached to a scale or balance via a thin metal wire. The force on the plate due to wetting is measured via a [[tensiometer]] or [[microbalance]] and used to calculate the surface tension (<math>\gamma</math>) using the '''Wilhelmy equation''':
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| :<math>\gamma = \frac{F}{l \cdot \cos \theta}</math>
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| where <math>l</math> is the wetted perimeter (2<math>w</math> + 2<math>d</math>) of the Wilhelmy plate and <math>\theta</math> is the contact angle between the liquid phase and the plate. In practice the contact angle is rarely measured, instead either literature values are used, or complete wetting (<math>\theta=0</math>) is assumed.
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| == Advantages and Practice ==
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| Unlike a [[Du Noüy ring method|Du Noüy ring]], no correction factors are required when calculating surface tensions when using the Wilhelmy plate, assuming a zero contact angle. In addition, because the plate is not moved during measurements, the Wilhelmy plate allows accurate determination of surface kinetics on a wide range of timescales and it displays low operator variance. In a typical plate experiment, the plate is lowered to the surface being analyzed until a meniscus is formed, and then raised so that the bottom edge of the plate lies on the plane of the undisturbed surface. If measuring a buried interface, the second (less dense) phase is then added on top of the undisturbed primary (denser) phase in such a way as to not disturb the meniscus. The force at equilibrium can then be used to determine the absolute surface or interfacial tension.
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| == See also == | |
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| * [[Tensiometer]]
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| * [[du Noüy ring method]]
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| * [[Sessile drop technique]]
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| == Further reading ==
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| *Holmberg, K (ed.) ''Handbook of Applied Surface and Colloid Chemistry'' New York, Wiley and Sons: 2002. Vol. 2, p. 219
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| ==References==
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| {{Reflist}}
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| [[Category:Laboratory equipment]]
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| [[Category:Materials science]]
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