Probe Liquid
The surface energy is measured in units of Joules per area, which is equivalent in the case of liquids to surface tension, measured in newtons per meter. The overall surface tension/energy of a liquid can be acquired through various methods using a tensiometer or using the pendent drop technique and Maximum bubble pressure method.
The interface tension at the interface of the probe liquid and the solid surface can additionally be viewed as being the result of different types of intermolecular forces. As such, surface energies can be subdivided according to the various interactions that cause them, such as the surface energy due to dispersive (van der Waals) forces, hydrogen bonding, polar interactions, acid/base interactions, etc. It is often useful for the sessile drop technique to use liquids that are known to be incapable of some of those interactions (see table 1). For example, the surface tension of all straight alkanes is said to be entirely dispersive, and all of the other components are zero. This is algebraically useful, as it eliminates a variable in certain cases, and makes these liquids essential testing materials.
The overall surface energy, both for a solid and a liquid, is assumed traditionally to simply be the sum of the components considered. For example, the equation describing the subdivision of surface energy into the contributions of dispersive interactions and polar interactions would be:
S = SD + SP
L = LD + LP
Where S is the total surface energy of the solid, SD and SP are respectively the dispersive and polar components of the solid surface energy, L is the total surface tension/surface energy of the liquid, and LD and LP are respectively the dispersive and polar components of the surface tension.
In addition to the tensiometer and pendant drop techniques, the sessile drop technique can be used in some cases to separate the known total surface energy of a liquid into its components. This is done by reversing the above idea with the introduction of a reference solid surface that is assumed to be incapable of polar interactions, such as Polytetrafluoroethylene (PTFE).
Read more about this topic: Sessile Drop Technique
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