a force that acts on the particles of a liquid at the surface is called

The force that acts on the particles of a liquid at the surface is called surface tension. Surface tension is caused by the cohesive forces between the liquid molecules at the surface. Molecules at the surface experience a net inward force due to the lack of similar molecules above them, creating a "film" that makes the surface behave like a stretched elastic membrane. This force minimizes the surface area of the liquid and causes phenomena such as droplets forming spherical shapes.