that it concerns a pure liquid) and are different manifestations of the same
phenomenon: work has to be done to enlarge the surface area. For surface
or interfacial tension, the symbolgis used. In Figure 10.1, the force by
which the film pulls at the horizontal bar of the frame isgtimesLtimes two,
since the film has two surfaces.
The phenomenon just mentioned allowsmeasurementof the surface
tension, as illustrated in Figure 10.2a. One measures the net downward force
FIGURE10.2 Surface tension and surface pressure. (a) Measurement of surface or
interfacial tension by means of a Wilhelmy plate (widthL, thicknessd). The plate is
attached to a sensitive balance. (b) Illustration of surface pressure (P) caused by
surfactant molecules (depicted by vertical dashes). Between the barriers the surface
tension is lowered and a net two-dimensional pressure acts on the barriers.