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.
Understanding Physics: Surface Tension and Capillarity
At the molecular scale, a molecule deep inside a liquid is pulled in many directions by nearby molecules. These pulls mostly balance. A molecule at the surface has no liquid molecules above it, so the inward pull is stronger.
Moving a molecule from the interior to the surface therefore needs energy. This is why a liquid naturally tries to reduce its exposed area.
A free drop becomes nearly spherical because a sphere encloses a given volume with the smallest possible surface area. Gravity can flatten a large drop, but very small drops are controlled mainly by surface effects.
The word tension can be misleading. The surface is not a solid sheet, and it cannot support every object placed on it. A carefully placed needle can remain afloat because its weight is small enough for forces around the contact line to help support it.
If the needle breaks through, the surface reshapes and it sinks. Detergent makes this easier because detergent molecules reduce the energy cost of making surface area.
This helps soap spread across a plate and helps washing water enter tiny gaps in fabric. It also explains why soap bubbles are easy to form but do not last forever.
Curved liquid surfaces create pressure differences. A smaller curved drop has a greater excess internal pressure than a larger one. This matters in sprays, fog, inkjet printing, and the tiny air spaces in wet soil.
Soap bubbles have two liquid surfaces, one facing inward and one facing outward. Both surfaces contribute to the effect, so a bubble behaves differently from a single liquid drop. When studying pressure in curved surfaces, first identify whether there is one interface or two.
Then pay attention to the radius. Mixing up radius with diameter is a common source of errors.
Capillarity appears when a liquid meets the wall of a narrow tube, a paper fiber, or a pore in a material. The liquid surface near the wall bends according to the competition between attraction to the wall and attraction within the liquid. In clean glass, water climbs at the edges and forms a concave meniscus.
The upward pull around the tube wall can raise the water column until its weight balances that pull. Narrower tubes give a larger rise because the wall contact remains important while the amount of lifted water becomes smaller. Plants use related effects in their fine channels, though evaporation from leaves provides much of the driving force for tall trees.
Paper towels, fountain pens, soil, and medical test strips all rely on liquid flow through small spaces. In calculations, use consistent units and remember that the contact angle decides whether a liquid rises or is depressed.
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.