Surface Tension
A liquid surface behaves like a stretched skin.
Lesson at a glanceShow the key points
- A molecule inside a liquid is pulled equally from all sides; one in the surface layer (about 10–15 molecular diameters thick) is not.
- Across any line of length l in the surface, the two sides pull each other with a force F ∝ l, tangential to the surface and perpendicular to the line: S = F/l (14.13), unit N/m.
- A thread loop on a soap film becomes a circle when the film inside is broken; a sliding wire on a U-frame is pulled in.
- Surfaces tend to shrink to the least area: drops are nearly spherical, wet brush bristles cling together.
- Typical values: water 0.075 N/m, mercury 0.465 N/m, soap solution 0.030 N/m.
A steel needle laid gently on water floats. Insects skate across ponds. Raindrops are round. The surface of a liquid behaves like a stretched elastic skin.
Why surfaces are special
A molecule deep inside water is surrounded by water molecules on every side, and their pulls balance. A molecule at the surface has water below but air above, so the forces on it are lopsided. The “surface” here means a layer about 10–15 molecular diameters thick: molecular forces become negligible beyond that distance, so a molecule deeper than this feels balanced forces.
Surface tension
Imagine a line AB drawn on the surface of a liquid. It divides the surface into two parts, and experiment shows they pull on each other with a force proportional to the length of the line. The forces are tangential to the surface and perpendicular to the line, just as two halves of a stretched rubber sheet pull on each other.
Seeing it happen
The thread loop
Dip a wire ring in soap solution to get a film, and lay a wet loop of thread on it. The loop sits in whatever shape it lands, because the film pulls it equally from inside and outside. Prick the film inside the loop: now only the outside film pulls, radially outward all round, and the loop snaps into a perfect circle.
The sliding wire
Make a soap film on a U-shaped wire frame closed by a light sliding wire. Held horizontal (and with little friction), the wire is pulled toward the closed end of the frame. Held vertical with the sliding wire at the bottom, a small weight hung from it can be balanced by the film’s pull.
A soap film (S = 0.030 N/m) forms on a frame closed by a 6.0 cm sliding wire. What is the force on the wire, and what mass could it support with the frame held vertical?
Surfaces try to shrink
Because every line in the surface is under tension, the surface shrinks whenever it can. Both demonstrations show this: the punctured film leaves the largest hole a loop of fixed length can enclose (a circle), so the remaining film is as small as possible; the sliding wire moves to reduce the film’s area. Paintbrush bristles splay freely under water but cling together once lifted out, cutting the area of the liquid surfaces between them. A small drop is nearly spherical, because a sphere has the least surface for a given volume (gravity flattens large drops a little).
| Liquid | S (N/m) | Liquid | S (N/m) |
|---|---|---|---|
| Mercury | 0.465 | Glycerine | 0.063 |
| Water | 0.075 | Carbon tetrachloride | 0.027 |
| Soap solution | 0.030 | Ethyl alcohol | 0.022 |
- 1Surface molecules feel unbalanced forces; the surface layer is about 10–15 molecular diameters thick.
- 2Surface tension : force per unit length across any line in the surface, tangential and perpendicular to the line.
- 3A soap film has two surfaces, so it pulls a wire of length l with 2Sl.
- 4Liquid surfaces shrink to the least area possible: drops are round, thread loops become circles.
Check yourself
Keep exploring
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