What is Wilhelmy Plate Method

Wilhelmy Plate Method 

The apparatus consists of a thin mica, glass or platinum plate suspended vertically from a beam attached to a torsion balance . It provides a direct measurement of force exerted on the plate at the interface between two immiscible liquids or surface of a liquid. The force exerted is equal to the surface tension multiplied by the perimeter of the plate.

The liquid whose surface tension is to be measured is taken in a container and the plate is immersed into it. The container is then gradually lowered till the plate detaches from the surface of the liquid. The reading on the balance is noted just prior to the detachment.

If the liquid completely wets the plate, the detachment force is equal to the surface tension multiplied by the perimeter of the surface detached. Thus,

WL - W = 2 (L + T)γ
Or
γ =  WL - W/2 (L+T)

where WL is the reading on the balance prior to detachment, W is the weight of the plate in air, L and T are the length and thickness of the plate, respectively.

For the determination of interfacial tension, the plate is immersed into the lower liquid and the reading is noted just prior to detachment of the plate from the surface of the lower liquid.

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