To what I know, surface tension is a force that appear at the interface between 2 bulk phases, and is parallel to that interface. Let consider a bulk of a fluid in the air. In the bulk of a fluid at rest, two sub-parts of a fluid exert a attractive force on one another due to surface tension. What I'm asking is that: Do two sub-parts of the air around exert a force on one another? (surface tension of the air) If they do, is the force equal the force in the fluid? If they don't, why? What if I replace the air by another suitable fluid?