The surface tension of a liquid is easily measured with methods like pendant drop and Du Noüy ring. This measurement gives a surface tension value that can be compared to the other formulations or surface free energy of the surface. However, a single surface tension measurement is not able to tell anything about the nature of interactions between the molecules in the liquid. To gain more information, it is possible to determine the polar and dispersive parts of the surface tension. The method to calculate the component is defined in ISO 19403-5:2017 standard. Below the procedure, as it is done in OneAttension software is explained.
1. Measure the surface tension of the liquid studied
The first step is to measure the surface tension of the liquid with the pendant drop method. In a pendant drop measurement, a large drop size is used to achieve the elongated, pendant shape of a drop. The desired drop size is set in the measurement recipe together with the recording settings. Remember also to set the correct density of the liquid as that is used in calculations. Once the start button is pressed, the measurement is done automatically. After the measurement is completed, the results can be analyzed. The software will automatically define the surface tension value for the liquid.
2. Measure the contact angle against a known solid
The contact angle of the liquid against a known solid should be measured next. A completely dispersive solid with the known surface free energy should be chosen. Most typically some kind of Teflon surface is used. The surface must be clean before making the measurement. The information on the surface free energy of the solid is filled in the recipe. To measure the contact angle, a drop of liquid is placed on the sample surface. The volume of the drop is typically between 3 to 5 microliters. When the drop size and recording settings are set, the play button is pressed and the measurement is automatically done by the instrument. After measurement, the results can be analyzed and the contact angle value is automatically presented.
3. Combine into one surface tension component (STC) file
To calculate the surface free energy components, select both measurement files, right-click, and calculate STC. This will make a virtual measurement with both measurement files included and automatically calculate the surface tension components for the liquid with both OWRK and Wu methods.