Sessile drop method — Optical determination of the contact angle
Contact angles can be determined with an optical contact angle measuring and contour analysis system of the OCA series. The setup drafted in figure 1 is used to capture an image of a liquid droplet that sits on a solid (sessile drop) and to subsequently analyze it with a software. The evaluation of the grey scale values of the recorded image allows for the detection of the so-called base line (contact between droplet and solid) and of the drop outline (see figure 2). The easiest method to determine the right and left contact angle is to apply tangents at the intersections of the drop outline and the baseline. In addition to this tangent method the SCA 20 software of DataPhysics Instruments offers four more automatic as well as one manual method to evaluate the contact angle.
For the automatic methods a contour line is fitted to the drop outline based on a mathematical equation. The latter describes a circle when using the height/width method, an ellipse when using the ellipse method and a higher-order polynomial when using the polynomial method.
In contrast to these purely mathematically motivated fits, the fourth automatic method, the so-called Young-Laplace method, takes the droplet’s physical properties into account (compare explanations for the pendant drop method). The image scale, the local gravity acceleration and the densities of the liquid and the surrounding medium have to be known, respectively.
In general the Young-Laplace method yields the most reliable results, especially for large contact angles and big drops. It has to be kept in mind, however, that with the Young-Laplace method one always obtains a common contact angle for the left and right intersection with the base line because the calculation assumes a symmetric drop.
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