Wilhelmy plate

The term Wilhelmy plate method refers to a method and apparatus which measures the force exerted on a thin plate (Wilhelmy plate) oriented perpendicular to an air-liquid or liquid-liquid interface to measure equilibrium surface or interfacial tension. The Wilhelmy plate method, based on the work of Ludwig Wilhelmy, finds wide use in the preparation and monitoring of Langmuir-Blodgett films.

Description of Method

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 which may be roughened to ensure complete wetting. 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 (σ) using the Wilhelmy equation.

:$sigma\; =\; frac\{F\}\{l\; cdot\; cos\; heta\}$

where l is the wetted length of the Wilhelmy plate and θ is the contact angle between the liquid phase and the plate. In practice the contact angle is rarely measured, using either literature values or assuming complete wetting (e.g. θ=0).

Bibliography

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