The plate tensiometry, first described in detail by L. Wilhelmy [[i]], is the classical method to determine the surface tension of liquids. This method is a real static technique, i.e. the liquid surface is completely at rest during the measurement.
Theoretical basis
The Wilhelmy plate method involves dipping a thin plate into a liquid and directly measuring the force acting on the plate normal to the interface. The increase in force F measured by the balance is related to the surface tension as follows
[i]. L. Wilhelmy, Ann. Phys. Chem., 29(1863)177
Here w and d are the width and length of the plate, respectively. The contact angle q is usually difficult to measure. The plate is therefore made of glass or platinum, which will be fully wetted (q = 0) if thoroughly cleaned. In that case Eq. (V.3) reduces to simply g = F/P, where P is the circumference of the plate. Surface roughness is often added to the plate to assure complete wetting. Sometimes, mainly in monolayer studies, the plate material is filter paper. Practically, though, contact angle is never measured but assured to be zero. With the use of a modern electro-balance, very precise surface tension measurements can be obtained without the use of any theoretical corrections. This fact, along with the fact that it is a static technique, makes the plate method a popular choice for precise equilibrium surface tension measurements. Note also that commercial versions of this method are readily available. The common disadvantage of this method and the ring method is that a relatively large amount of liquid is needed to perform accurate measurements.
Experimental procedure
Two different approaches can be used, the plate can be left suspended, bridging the two phases, or the plate can be pulled through the interface. The experiments can run for hours. In practice, a wire attached to a balance usually suspends the plate. The solution of interest is raised so that its surface just touches the bottom of the plate. The principle of the plate method is so simple that it can be set up easily in any laboratory equipped with a balance (schematic in Fig. V.3).
