14.7 : Surface Tension of Fluid

Surface tension is a fundamental property of fluids, occurring at the boundary between a liquid and a gas or between two immiscible liquids. This phenomenon arises from the cohesive forces between molecules at the fluid's surface, creating an effect similar to a stretched elastic membrane. Inside each fluid, molecules are equally attracted in all directions by neighboring molecules, but surface molecules experience a net inward force, resulting in surface tension.

Surface tension varies with the type of fluid and its temperature, typically decreasing as temperature increases. Higher temperatures reduce molecular cohesion, thereby lowering surface tension. This property is crucial in explaining the formation of droplets, bubbles, and the behavior of liquid jets. For instance, water droplets form spherical shapes to minimize surface area, illustrating surface tension's role in reducing energy.

Capillary action, another effect of surface tension, is observed when a liquid moves up or down a thin tube compared to the surrounding liquid level. The tube's diameter influences the height the liquid reaches in the tube; narrower tubes result in higher rises.

Surface tension also allows lightweight objects to float on a liquid surface. For example, a steel needle can float on water if placed gently, as surface tension supports it. Similarly, insects like water striders exploit surface tension to walk on water without sinking.