Measuring Contact Angles using the Sessile Drop Method

Figure 1: The sessile drop method is used to determine the contact angle formed between a drop of liquid and a solid surface.

The sessile drop method is used to optically determine the contact angle between a liquid and a solid surface. For the measurement, a drop is deposited on the solid surface. A contact angle meter records an image of the sessile drop. Based on this image, a software calculates the contact angle. Further analyses of the contact angle allow the surface energy or the wetting behavior to be determined.

In materials science, the determination of contact angles and the surface energy of solids are important to understand the interactions between solid surfaces and liquids. The contact angle describes the angle that a drop of liquid forms on a solid surface and provides information about the wetting behavior of this liquid-solid combination. The surface energy is a measure of how easy or difficult it is for a solid surface to be wetted by a liquid.

An optical measurement method for determining these two parameters is the so-called sessile drop method. The measurement setup, the calculation of the contact angle, and the resulting determination of the surface energy are explained below.

The measurement setup for the sessile drop method

The sessile drop method makes it possible to measure the contact angle of a liquid on a solid surface with high precision. The basic setup consists of a camera connected to a PC, a sample stage, a dosing unit (e.g., a syringe) and a light source (see Figure 2).

To achieve the desired measurement accuracy, the sessile drop method should be carried out using an optical contact angle meter, such as the systems of the OCA series from DataPhysics Instruments. For the sessile drop method, a dosing system is used to deposit a drop on a solid surface, placed on a sample table. The drop is illuminated by the light source and observed with a high-resolution camera. A connected PC then uses specialised software to record the images from the camera, which are subsequently analysed further.

Figure 2: Basic measurement setup of the sessile drop method

Calculation of the contact angle using the sessile drop methodAs said, the specialized software can analyse the recorded camera images. By evaluating the contrast values of the image, the so-called baseline, i.e. the contact line between the drop and the solid, and the drop outline can be recognized (see Figure 3). Ultimately, various mathematical models can be used to determine the contact angle.
The simplest, automatic calculation basis for determining the contact angle is to apply tangents to the drop outline at the two intersections with the detected baseline. In further automatic evaluations, the detected drop outline is approximated using different geometric forms. These forms include, for example, a circular shape, an ellipse, or a higher order polynomial. This allows the drop outline to be described and the contact angle to be calculated.
It is also possible to use the Young-Laplace equation to determine the contact angle from the drop contour. In contrast to the geometrical forms, as presented above, the Young-Laplace equation considers the physical properties of the droplet, too. As a rule, the Young-Laplace equation provides the most reliable results for larger contact angles and large droplets. It should be noted, however, that this calculation assumes a symmetrical droplet.

									
								
Figure 3: The sessile drop method in practice: here a software image with detected baseline, drop outline and contact angles
Extended contact angle evaluations: surface energy determination The optical determination of the contact angle using the sessile drop method also allows the determination of the surface energy of the solid with its disperse and polar components. To calculate it, the contact angles of at least two different test liquids, such as water and diiodomethane, are measured. The surface energy is important in many areas, namely whenever a solid surface is to be wetted with a liquid.

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