Surface tension forces the surface of water to curve because molecules at the surface experience a net inward attraction toward other water molecules due to cohesive forces. This creates an effect where the surface behaves like a stretched elastic membrane, tending to minimize its surface area. When there is a pressure difference across the water surface, this tension causes the surface to curve to balance the forces. The curve allows surface tension forces to develop a normal component that counteracts the pressure difference, resulting in a curved surface like droplets or menisci. This relationship is quantitatively described by the Young-Laplace equation, which relates the pressure difference to the surface tension and the curvature radii of the surface. In essence:
- Water molecules at the surface are pulled inward by cohesion, creating a "skin" that resists external forces.
- A pressure difference across the surface causes it to curve so that the inward surface tension force can balance the pressure forcing outward.
- The curvature minimizes the surface energy by reducing the surface area relative to the volume.
Thus, surface tension forces water surfaces to curve in response to pressure imbalances, shaping droplets, bubbles, and menisci as forms that minimize surface energy.
