Measuring Surface Tension and Contact Angle Round Two

8 MEASURING SURFACE TENSION AND CONTACT ANGLE ROUND TWO 

We have now established that both 7 and 6 have thermodynamic significance and have seen that their values as well as their temperature coefficients are of interest. In addition, we have seen that the measurement of contact angles presents some complications of its own. All this adds up to a need for more reliable and more accurate methods for the measurement of these parameters than those presented in Section 6.2. One of the most powerful strategies for this involves a second measurement made with the Wilhelmy plate. 

We saw in Section 6.2 that the Wilhelmy plate offers an accurate method for measuring 7 cos 6. We thus have one experiment with two unknowns. The Wilhelmy balance measures the weight of the meniscus in this section we examine the height to which the meniscus climbs on the same surface. We shall see that this distance — which may be accurately measured with a traveling microscope or cathetometer — also depends on 7 and 6. The functional relationship between these parameters and the experimental variables is different from the case of the meniscus weight. Therefore we have two experiments with two unknowns that can be solved for 7 and 6. 

In Section 6.4 we discussed the pressure difference that exists across a curved surface. The Laplace equation, in the form provided by Equation (35), gives a general description of this pressure difference Ap. Our objective is to apply this relationship to the meniscus formed by a liquid surface at a flat solid wall. The first thing to notice about this is that one of the R s in Equation (35) becomes infinite since the support is planar hence this R l term disappears from the Laplace equation. 

13 Definitions of variables used to describe a meniscus formed by liquid A displacing liquid B in terms of (a) x and z and (b) j . 

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