Cornstarch Oobleck: Mixing a Liquid That Punches Back
Two cups of cornstarch and one cup of water make a suspension that behaves like a solid when you hit it and like a liquid when you rest your hand in it. Getting the ratio right is the whole experiment; everything else is observation.
Materials
| Item | Amount | If you do not have it |
|---|---|---|
| Cornstarch (cornflour) | 250 g, about 2 cups | Potato starch behaves the same way |
| Cold tap water | 180 to 220 ml, about 1 cup | Room-temperature water; warm water mixes too fast |
| Wide mixing bowl | 1, at least 2 litres | A shallow roasting tin for a flatter puddle |
| Stiff spatula or wooden spoon | 1 | Your hand, if you do not mind the mess |
| Food colouring | 3 to 4 drops, optional | None; colour makes the flow lines harder to see |
| Kitchen scale | 1, accurate to 1 g | Measuring cups, but expect to adjust |
Method
Weigh 250 g of cornstarch into the bowl. Break up any lumps with the back of the spoon before adding water.
Watch for: Dry cornstarch squeaks faintly under pressure and puffs a fine dust if you stir it dry.
Add 180 ml of water and stir slowly for 30 seconds. Stop as soon as no dry powder remains.
Watch for: At this stage the paste is stiff and crumbly, and tears rather than pours.
Add water 10 ml at a time, stirring twice between additions, until the mixture flows off the spoon in a ribbon that breaks within a second.
Watch for: The correct mixture resists the spoon while you are moving it fast, then goes slack the moment you stop.
Punch the surface with a closed fist, then immediately press your open palm into it and hold still for five seconds.
Watch for: The punch produces a hard, almost drum-like thud. The open palm sinks to the bottom of the bowl within about three seconds.
Scoop a handful and squeeze it into a ball, then open your hand over the bowl.
Watch for: The ball holds its shape for about two seconds, then runs between your fingers as a thick liquid.
Why it works
Cornstarch grains are irregular, roughly one to twenty micrometres across, and they absorb a little water but never dissolve. In the right ratio there is just enough water to lubricate the grains past each other slowly, but not enough to keep them apart when you push. Fast shear forces the grains into contact and traps a thin film of water between them; that film plus the particle contacts creates a temporary jammed network that behaves like a solid. Remove the force and Brownian motion plus the water film lets the network relax back into a flowing suspension. This is shear thickening, and it needs the solid to be between roughly 40 and 50 percent of the mixture by volume.
Think of it like: It is the crowd at a busy doorway. Walking slowly, everyone slips through one at a time and the crowd behaves like a fluid. Shout 'run' and everyone presses together into a solid block at the door frame. Stop shouting and the block loosens again.
Clean-up and disposal
Never wash oobleck down a sink drain in bulk: it sets in the trap and can block the pipe. Scrape the bowl into the bin first, then rinse with plenty of hot water. Dried cornstarch on fabric brushes off once it is fully dry; washing it while wet just spreads it.
Bench notes
Getting the ratio right
Everything depends on the water fraction. Weigh both components the first three times; after that you can judge it by feel, because the correct mixture tears rather than drips off a spoon.
Observed / 实测With 250 g cornstarch, the working window is 195 to 215 ml of water. Below 195 ml it crumbles; above 215 ml it stops thickening on impact.
Testing like a rheologist
Instead of punching it, try to quantify the response. A slow stir and a fast stir of the same mixture tell you the direction of the change, and you can time how long a floating toy takes to sink after you stop stirring.
Observed / 实测The surface stays solid under a fast tap for roughly 0.1 seconds, which is why a finger flick bounces off.
Clean-up is part of the experiment
Variations worth running
- Run the same test with 200 g, 250 g and 300 g of cornstarch against a fixed 200 ml of water and find the point where it stops flowing at all.
- Drop a metal spoon in from 30 cm and compare the splash with the same drop into plain water.
- Float a small plastic toy on the surface and tap it sharply with a finger: it will not sink on a fast tap.
Questions that come up on the bench
- Why does warm water give a worse result?
- Warmer water lowers the viscosity of the thin film between grains and slightly increases starch swelling, so the grains pack differently. The effect is small but noticeable, and cold water simply repeats better from batch to batch.
- Can I keep it for tomorrow?
- It separates overnight into a hard layer with water on top. Stirring will bring it back, but the surface dries into a crust during the day. Cover the bowl with a plate if you want a second session.
- Is this the same as a non-Newtonian fluid in a physics lab?
- It is the same class of behaviour, but a lab-grade shear-thickening fluid is usually controlled for particle size and shape. Cornstarch is a mixture of sizes, so the transition point shifts each time you mix a new batch.