surface tension
Summary
Surface tension is the force per unit length (or energy per unit area) that makes a liquid surface act like a stretched sheet, because cohesive forces pull surface molecules inward. It generally falls with higher temperature and with surfactants such as soap.
Detail
Meaning: Surface tension is the tendency of a liquid surface to act like a stretched sheet and shrink to the least area. A molecule in the bulk is pulled equally in all directions by cohesive forces, but a surface molecule has neighbors only beside and below it, so it feels a net inward pull. Enlarging the surface takes work, so free drops pull into spheres. Surface tension generally falls as temperature rises and when surfactants such as soap are added.
Equation: γ = F/L = W/ΔA, where γ is surface tension (N/m, same as J/m²), F is the force along the surface (N), L is the length of edge it acts across (m), W is the work to enlarge the surface (J), and ΔA is the added area (m²). A soap film has two surfaces, so F = 2γL.
On the MCAT: Expect a water strider or floating needle, a drop's shape, or capillary rise in a narrow tube, where adhesion to the wall versus cohesion sets the meniscus. Predict that detergent lowers γ. In biology passages, lung surfactant lowers alveolar surface tension so alveoli collapse less.
Common confusion: A needle denser than water floats by surface tension, not buoyancy. Also, γ depends on the liquid and its surroundings, not on how large the surface is.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.