A Wilhelmy plate is a thin plate that is used to measure equilibrium surface or interfacial tension

Surface tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 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

Ludwig Wilhelmy

Ludwig Ferdinand Wilhelmy was a German scientist who is usually credited with publishing the first quantitative study in chemical kinetics.- Scientific work :...

, this method finds wide use in the preparation and monitoring of Langmuir–Blodgett films.

Detailed description

The Wilhelmy plate consists of a thin plate usually on the order of a few centimeters square. The plate is often made from glass or platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 which may be roughened to ensure complete wetting

Wetting

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

. 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

Tensiometer

A Tensiometer as it applies to physics is an instrument used to measure the surface tension of liquids.-Goniometer/Tensiometer:...

 or microbalance

Microbalance

A microbalance is an instrument capable of making precise measurements of weight of objects of relatively small mass: of the order of a million parts of a gram. In comparison, a standard analytical balance is 100 times less sensitive; i.e. it is limited in precision to 0.1 milligrams...

and used to calculate the surface tension () using the Wilhelmy equation:


where is the wetted perimeter (2 + 2) of the Wilhelmy plate and 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 () is assumed.

Advantages and Practice

Unlike a Du Nuoy 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.

Further reading

• Holmberg, K (ed.) Handbook of Applied Surface and Colloid Chemistry New York, Wiley and Sons: 2002. p. 219