Milk, Soap and the Surface Tension That Runs Away
A drop of washing-up liquid in a plate of milk sets off a burst that lasts about three seconds and draws a pattern you cannot repeat exactly. The driving force is a surface tension gradient, not the soap pushing the milk.
Materials
| Item | Amount | If you do not have it |
|---|---|---|
| Whole milk | 150 ml, room temperature | Semi-skimmed works; skimmed milk gives a much weaker burst |
| Washing-up liquid | 5 ml in a small dish | Liquid hand soap, diluted 1:1 |
| Shallow white plate | 1, at least 22 cm across | A rimmed baking tray lined with white paper underneath |
| Cotton bud | 1 | A pipette or a dropper |
| Food colouring | 4 drops in different colours, optional | None; the fat and soap still trace visible swirls |
| Phone camera | 1 | Nothing; the pattern is gone in seconds |
Method
Pour 150 ml of milk into the plate and let it settle for two minutes until the surface is perfectly still.
Watch for: Any movement in the milk will smear the pattern before you start.
If you are using colouring, touch four drops to the surface in a square, about 3 cm apart.
Watch for: The drops spread into discs roughly a centimetre wide and stay put.
Dip the cotton bud in washing-up liquid and touch it to the centre of the plate for one second, then withdraw it.
Watch for: The colours shoot outward in under a second, leaving a clean hole at the centre with a ring of milk pushing ahead of it.
Touch a fresh soapy bud to a different spot and watch the direction of travel change.
Watch for: The burst is weaker each time because the surface is already partly coated in surfactant.
Stir once gently with the tip of the bud to see what is left of the surface film.
Watch for: Stirring after the burst produces a dull, uniform mix with no further motion.
Why it works
Milk is an emulsion of butterfat droplets and proteins suspended in water, and its surface is covered by a protein film that has a high surface tension of about 50 millinewtons per metre. Washing-up liquid contains surfactants that drop the surface tension to about 25. When the bud touches the milk, surfactant spreads outward across the surface and drags the liquid underneath it. Water flows from the low-tension region toward the high-tension region, carrying colouring and fat with it. This is the Marangoni effect: flow driven by a surface tension gradient rather than by gravity or by stirring. Once the whole surface is coated, the gradient disappears and the motion stops.
Think of it like: Imagine a stretched rubber sheet anchored around its edge. Cut a small hole in the middle and the sheet pulls outward from the cut. The soap does not push the milk; it removes tension at one point and the rest of the surface does the pulling.
Clean-up and disposal
Pour the milk down the sink with running water. Rinse the plate thoroughly, because a film of milk fat left to dry becomes a smell. Both the milk and the soap are food-safe, so no special disposal is needed. Discard the cotton bud rather than reusing it, since it will carry surfactant into the next attempt.
Bench notes
Getting a still surface
This card fails more often from a moving surface than from any other cause. Pour the milk, wait, and resist touching the plate for a full two minutes before you start.
Observed / 实测Whole milk at 20 C gives a leading edge that travels about 9 cm in the first second. Skimmed milk travels about 4 cm.
What the fat is doing
The colouring does not dissolve in milk; it is carried on the surface and in the fat phase. That is why an uncoloured plate still shows faint white-grey swirls: you are watching the fat and the water moving at different speeds.
Observed / 实测If you leave the plate for two minutes after the burst, the fat droplets re-coat with protein and the surface tension climbs back to about 42 millinewtons per metre.
Everyday chemistry, still a bench rule
Variations worth running
- Compare whole, semi-skimmed and skimmed milk in three plates at once and rank the burst by how far the leading edge travels in one second.
- Try a drop of olive oil on a plate of water and then touch it with soap: the oil droplet splits and skates across the surface.
- Sprinkle a very thin layer of ground black pepper over water instead of using milk, touch it with soap, and watch the pepper retreat to the rim.
Questions that come up on the bench
- Is the soap pushing the milk outward?
- No. The soap lowers surface tension wherever it lands, so the surrounding high-tension surface contracts and pulls the liquid outward from that spot. The motion is the rest of the surface pulling, not the soap pushing.
- Why does whole milk work better than skimmed?
- The butterfat droplets give the moving surface something visible to carry, and they also interfere with the protein film. Skimmed milk still works but the swirls are fainter and fade faster.
- Why does it stop after a few seconds?
- Surfactant spreads until the entire surface is uniformly coated. With no difference in surface tension left anywhere, there is no gradient to drive flow, and the plate goes still.