The sessile- and pendant-drop methods can be used to determine the static surface and interfacial tensions of liquids (Hunter 1986, Couper 1993). The shape of a liquid droplet depends on a balance between the gravitational and surface forces. Surface forces favor a spherical droplet because this shape minimizes the contact area between the liquid and its surroundings. On the other hand, gravitational forces tend to cause droplets to become flattened (if they are resting on a solid surface) or elongated (if they are hanging on a solid surface). A flattened drop is usually referred to as a sessile drop, whereas a hanging one is referred to as a pendent drop (Figure 5.15). The equilibrium shape that is adopted by a droplet is determined by its volume, density, and surface or interfacial tension.
The surface or interfacial tension is determined by measuring the shape of a drop using an optical microscope, often in conjunction with an image analysis program, and mathematical equations which describe the equilibrium shape (Couper 1993). This technique can be used to determine surface or interfacial tensions as low as 10-4 mN m-1 and can also be used to simultaneously determine the contact angle. The accuracy of the technique has been reported to be about 0.01% (Couper 1993).
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