Figure 1: The pendant drop method makes it possible to determine the interfacial and surface tension of a drop of liquid. The results allow predictions of the wetting and mixing behavior of the liquid.
The pendant drop method is used to optically determine the interfacial and surface tension of liquids. For measurement, the image of a drop hanging from a needle is recorded. Using the Young-Laplace equation, the surface tension or interfacial tension can be calculated from this image.
In chemistry and materials science, the measurement of surface tension is important in order to understand the behavior of liquids on surfaces. This is relevant, for example, in the development of coatings and paints. The interfacial tension between two liquids is important, for instance, when two liquids are to be mixed together. An application from the food industry is the mixing of oils with water. The surface tension and interfacial tension can be determined optically using the so-called pendant drop method, which is presented below.
The pendant drop method is a measurement method that can be carried out using an optical contact angle measuring device, such as those in the OCA series from DataPhysics Instruments. The setup essentially consists of a camera connected to a PC, a light source and a drop of liquid suspended from a dosing needle (see Figure 2).
Figure 2: The schematic setup for the pendant drop method consists of a camera connected to a PC, a light source and a drop of liquid suspended from a dosing needle.
A software is used to analyze the recorded camera images. The contour of the hanging drop of liquid can be recognized via the contrast values of the image. The surface tension is calculated using a numerical method based on the Young-Laplace equation.
The relationship that can be derived from the Young-Laplace equation is even visible to the naked eye: the forces that determine the shape of the hanging drop are essentially surface tension and gravity. The surface tension aims to minimize the surface area and make the droplet as round as possible. Gravity, on the opposite, stretches the drop out of this spherical shape, resulting in the typical pear-like shape. If the surface tension is high, a drop is therefore more "spherical". If the surface tension is low, the drop is "stretched out".
The pendant drop method can also be used to determine the interfacial tension between two liquids. For this purpose, a similar setup is used as already described above. The difference is that the pendant drop is generated in a surrounding liquid.
Depending on the density ratio, the drop hanging on the needle can also rise in the surrounding liquid. In this case, a bent needle is used so that the drop can form its shape upwards without being disrupted. The interfacial tension is now calculated using the drop outline and approximation based on the Young-Laplace equation.
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