Wilhelmy plate method force balance

The Wilhelmy plate method, as shown in Figure 4.10, is similar to du Nouy s ring method, but it uses a thin mica plate or microscope slide. The plate is suspended from a balance and dips into the liquid. The force, F, required to detach the liquid meniscus surrounding the plate depends on the surface tension or interfacial tension by ... 

There are two modifications to the Wilhelmy plate method. In the first modification, the cup carrying the liquid is mobile and is lowered until the previously immersed plate becomes detached from the liquid surface, and the maximum vertical pull, / max on the balance is noted, similarly to the ring method. Then the capillary force, for the zero contact angle, can be given as... 

The essential difference between the two studies is that Elliott and Riddiford, Schwartz et al. (and many others) used optical methods to determine 0 whereas Johnson et al. used the Wilhelmy plate method. The Wilhelmy balance derives the contact angle from a force measurement. 

A stand-alone static method is the popular Wilhelmy plate method (Figure 1.18). In this method, a completely wetted platinum plate is brought into contact with a liquid surface, and a pull force is applied to the plate. Equilibrium is achieved when that force, corrected by the buoyancy force acting on the immersed part of the plate, is balanced by the surface tension, that is, F + dbHpg = 2(d + b)a. The force F is measured with a sensitive dynamometer, which typically forms the core of modem surface tension meters. 

The Wilhelmy plate method (Ludwig Ferdinand Wilhelmy [1812-1864]) can be used to measure surface or interfacial tension at an air-liquid or liquid-liquid interface. In this method, a thin plate is oriented perpendicular to the interface, and the force exerted on it is measured. 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 (y) using the Wilhelmy equation ... 

Devise a method to accurately use two Wilhelmy plates (see Fig. 6) to measure the surface tension of a Langmuir film. Do not forget about the gravitational and buoyancy forces. Such a device is called a Wilhelmy balance. 

The Wilhelmy slide method is somewhat similar to the ring-pull method. A very thin plate, such as a microscope cover glass or a sheet of mica, is hung from one arm of a balance and allowed to dip in the solution (Fig. 18.3). If p is the perimeter of the slide, the downward pull on the slide due to surface tension is yp. If F and F are the forces acting downward when the slide is touching the surface and when it is suspended freely in air respectively, then... 

There have been various methods for measuring the surface tension [2]. The Wilhelmy plate is commonly employed in the usual LB apparatuses to monitor the change in the surface pressure. In the Wilhelmy method [8], the force balanced with the surface tension is determined by a plate vertically suspended and partially immersed in the water phase. Figure 3 illustrates the experimental configuration. 

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