Consider a liquid under the atmospheric pressure. Nonzero pressure inside the liquid means repulsion between molecules. On the other hand to explain the surface tension it is usually said that molecules attract each other!
Which one is correct: attraction or repulsion?
Best Answer
Liquid molecules which are too close to one another will repel one another and liquid molecules which are too far apart will attract one another.
The comparison has been made to the equilibrium separation of the liquid molecules when the net force on the liquid molecule is zero.
A liquid molecule in the body of a liquid has to support the weight of the liquid molecules above it.
To provide an upward force on that liquid molecule it gets closer than the equilibrium separation so that there is a net repulsive force on the liquid molecule due to its nearest neighbours.
The density of a liquid increases with depth but by very little.
At the surface the liquid molecules do not have nearest neighbours above them and so there is a net downward force due to the neighbours below the surface molecules.
To counteract that downward force the surface molecules have a greater than equilibrium separation which means that there is a net force of attraction between the surface molecules - this is the origin of the surface tensional force.
The sketch graph below illustrates the transition from repulsive to attractive as one goes through the equilibrium separation $r_0$.
So the answer is that both are correct and the actual force acting, attractive or repulsive, depends on the separation of the liquid molecules.