A few days ago, I sat down to write what I thought would be a simple explanation of surface tension and how it is measured in the laboratory (a blog which will be published shortly if I can figure all of this out). In doing the normal background research, however, I started to see contradictions that did not align with what I thought I knew about surface tension. The culprit was wettability. Soon I was in a circular argument with myself regarding the two and how to differentiate them. Contact angle, for example, is a measure of wettability but we also use it (in an inverse way) to measure surface tension. In fact, the concepts of surface tension and wettability are so intertwined that it is difficult, for me at least, to view them independently.
In an effort to get a grip on this, let’s start by looking at the definitions of the terms wettability and surface tension.
Wettability – Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. (Wikipedia)
The classical model is a drop of liquid on a smooth solid surface. The contact angle is established by the balance of the adhesive force (the liquid wanting to maintain contact with the solid) and the cohesive force within the liquid (both the internal cohesive force and the force of surface tension). An increase in adhesive force between the liquid and the solid or a decrease in the cohesive force (surface tension) within the liquid will result in greater wettability and a smaller contact angle. It is important to note that a change in the adhesive force can be the result of a change in either the solid or the liquid while a change in the cohesive force can only be the result of a change in the cohesive force or surface tension of the liquid.
Surface Tension – Surface Tension is the attractive force exerted upon the surface molecules of a liquid by the molecules beneath that tends to draw the surface molecules into the bulk of the liquid and makes the liquid assume the shape having the least surface area. (Mirriam-Webster)
It works to think of surface tension like the rubber of a balloon attempting to form the contents into the most compact shape (although not exactly since in the case of the balloon, the initial shape of the balloon may have an influence on the final shape). It is surface tension that is the “cohesive force” described above. Without gravity and the influence of a solid in contact with the liquid, a water droplet will assume a perfectly spherical shape (as I’m sure we’ve all seen in those science broadcasts from space). Things change as soon as the droplet comes into contact with something.
The above mentioned quandary may be fueled by the fact that it is common for one to say that they have, for example, reduced the surface tension of water by adding a surfactant. In fact, most surfactants change the surface properties of the liquid and may not affect the chemical nature of the liquid at all. There is, however, a change in the apparent surface tension.
To Be Continued
– FJF –