Wilhelmy Slide or Plate Method

The methods so far discussed have required more or less tabular solutions, or otherwise correction factors to the respective ideal equations. Further, if continuous measurements need to be made, then it is not easy to use some of these methods (such as the capillary rise or bubble method). 

The surface force will act along the perimeter of the plate (i.e., length [Lp] + width [Wp]). The plate is often very thin (less than 0.1 mm) and made of platinum, but even plates made of glass, quartz, mica, and filter paper can be used. The forces acting on the plate consist of the gravity and surface tension downward, and buoyancy due 

FIGURE2.13 Wilhelmy plate in a liquid (plate with dimensions length = Lp, width = Wp). 

The sensitivity of y using these procedures has been found to be very high ( 0.001 dyn/cm [mN/m]) (Birdi, 2008). The change in surface tension (surface pressure = II) is then determined by measuring the change in F for a stationary plate between a clean surface and the same surface with a monolayer present. If the plate is completely wetted by the liquid (i.e., cos(0) = cos(0) = 1), the surface pressure is then obtained from the following equation  

by using very thin plates with thickness 0.1 to 0.002 mm, one can measure surface tension with very high sensitivity. In practice, by using very thin platinum plates of well-known dimension (length = 1.00 or 2.00 cm), one can calibrate the apparatus with pure liquids, such as water and ethanol. The buoyancy correction is made very small (and negligible) by using a very thin plate and dipping the plate as little as possible. 

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