Re: Surface tension force is downwards or parallel to surfae of liquid?

Thanks for the question.

Surface tension is a force, which is a vector and must have a direction and magnitude associated with the definition of the quantity. The purpose of the surface tension is to reduce the total energy of the liquid, which results in a minimized surface area. This is the reason that drops tend to be spherical unless there are other forces such as gravity acting on the drop to change its shape.

The direction of the surface tension force is perpendicular to the surface. This is a result of the attractive forces between the liquid molecules.

Imagine a molecule "deep" in the bulk of the liquid with other liquid molecules all around exerting attractive forces in every direction with each force being balanced by an equal force on the other side. The result is zero net force on the bulk molecule.

Now, picture a molecule at the surface. The attractive forces acting on a surface molecule from the molecules in the bulk of the liquid act to pull the surface molecule toward the liquid because there are no attractive forces to balance the force on the opposite side of the molecule.

The attached image gives a pretty good visual description of the differences. If you add all the forces for the surface molecule, you get an inward pointing force while the forces in the bulk neutralize each other.

Here is the source of the image: