Surface tension is the tendency of a liquid surface to act like a stretched elastic skin. It matters because it explains why water forms rounded droplets, why small insects can stand on water, and why a needle can float if placed carefully. The effect comes from cohesion, the attraction between molecules of the same substance.
In water, hydrogen bonding makes this cohesive pull especially strong.
Key Facts
- Surface tension is force per unit length: gamma = F/L.
- Surface energy form: gamma = W/A, where W is work and A is new surface area.
- For a liquid drop, the excess pressure is Delta P = 2 gamma/r.
- For a soap bubble with two surfaces, the excess pressure is Delta P = 4 gamma/r.
- Capillary rise is h = 2 gamma cos(theta)/(rho g r).
- Water wets clean glass because adhesion to glass is stronger than cohesion within water, producing a concave meniscus.
Vocabulary
- Surface tension
- Surface tension is the force per unit length along a liquid surface caused by cohesive molecular attractions.
- Cohesion
- Cohesion is the attraction between molecules of the same substance, such as water molecules attracting other water molecules.
- Adhesion
- Adhesion is the attraction between molecules of different substances, such as water molecules and glass.
- Capillary action
- Capillary action is the rise or fall of a liquid in a narrow tube due to surface tension and adhesive forces.
- Meniscus
- A meniscus is the curved surface of a liquid near a container wall caused by the balance between cohesion and adhesion.
Common Mistakes to Avoid
- Treating surface tension as a force only, rather than force per unit length. The correct unit is N/m, and the relevant length is the line along which the surface tension acts.
- Assuming molecules at the surface are pulled upward. In a water droplet, surface molecules have fewer neighbors above them, so the net cohesive pull is inward and slightly downward into the liquid.
- Using the capillary rise formula without the contact angle. The factor cos(theta) determines whether the liquid rises, falls, or shows no height change in a tube.
- Thinking all liquids make the same meniscus in glass. Water usually forms a concave meniscus, while mercury forms a convex meniscus because cohesion in mercury is stronger than adhesion to glass.
Practice Questions
- 1 A water surface has surface tension gamma = 0.072 N/m. What force acts along a 0.050 m line on the surface?
- 2 Water rises in a glass capillary tube of radius 0.50 mm. Using gamma = 0.072 N/m, theta = 0 degrees, rho = 1000 kg/m^3, and g = 9.8 m/s^2, calculate the capillary rise height h.
- 3 Explain why a water droplet on wax is more rounded than a water droplet on clean glass, using cohesion, adhesion, and contact angle.