92168.pdf

72 Liquid-gas and liquid-liquid interfaces

Wilhelmy plate methods

A thin mica plate or microscope slide is suspended from the arm of a

balance and dips into the liquid, as shown in Figure 4.5.

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Figure 4,5 Wilhelmy plate methods: (a) detachment; (b) static

When used as a detachment method (Figure 4.5a), the container

holding the liquid is gradually lowered and the pull on the balance at

the point of detachment is noted. For a plate of length x, breadth y

and weight W, assuming zero contact angle,

(4.10)

The plate method can also be used as a static method (Figure 4.5b)
for measuring changes in surface tension (see page 99). The change in
the force required to maintain the plate at constant immersion as the
surface tension alters is measured.

Ring method

In this method the force required to detach a ring from a surface or
interface is measured either by suspending the ring from the arm of a
balance or by using a torsion-wire arrangement (du Noiiy tensiometer).