DIY · Card 08 · Physics · Materials
Two ingredients in a bowl. Stir slowly and it pours like cream. Punch it and your fist bounces off as if you’d hit a brick.
01The brief
Make a liquid that turns solid when you hit it, and back into a liquid the moment you stop.
40 %
of the slime’s volume is cornflour grains, packed just short of locking up
0.01 mm
roughly the size of a cornflour grain, small enough to stay suspended in water
100×
the stress it pushed back with when struck, against stirring, in a 2012 Chicago experiment
Cornflour and water should make nothing more interesting than gravy. Instead, in about the right proportions, they make a substance that seems to change its mind. Let it run off your fingers and it drips like honey. Slap it and it is as hard as a table. Squeeze a handful into a ball and it holds its shape only for as long as you keep squeezing.
It is usually called oobleck, after a gooey green substance that falls from the sky in a 1949 Dr. Seuss story. Physicists call it a dense suspension and a shear-thickening fluid, and for a substance you can make with two kitchen ingredients, it kept them arguing for a surprisingly long time.
02Kit
Cornflour and water. The only skill is adding the water slowly enough to stop at the right moment.
Tick off what you have
0 of 4 ready
03Method
Mix it, find the sweet spot, then spend as long as you like hitting, sinking into and squeezing it.
Step 1 of 8 · Minute 0
Tip about 60 g of cornflour, roughly eight heaped tablespoons, into a mixing bowl.
It crumbles like wet sand.
Why
Too little water: the grains are already jammed even when you’re gentle.
Fix
Add water a teaspoon at a time and stir slowly until it flows when you tilt the bowl.
It just splashes when you hit it.
Why
Too much water: the grains are too far apart to jam.
Fix
Stir in more cornflour, a spoonful at a time, until a fast stab of the spoon meets resistance.
It went hard at the bottom of the bowl.
Why
Left standing, the heavy grains settle out into a packed layer.
Fix
Stir it slowly back together. It always comes back.
04Physics
Look at oobleck under a microscope and you’d see grains of starch, each a hundredth of a millimetre or so across, packed so tightly that they nearly touch. Between every pair of grains is a thin film of water. When you push the slime gently, the grains slide past each other on those films, the way wet tiles slide. Slowly, it flows.
Push it fast and the grains are shoved together harder than the water films can hold them apart. The films squeeze out, the grains touch, and now they are rubbing against each other with real friction, like dry sand. Locked by friction, each grain props up its neighbours, and a solid network shoots through the slime, spreading ahead of whatever hit it. In 2012, physicists in Chicago filmed this jammed region growing beneath a falling rod, like snow piling up in front of a shovel.
The trick only works in a narrow window. Too much water and the grains never get close enough to touch. Too little and they are jammed already, a crumbly paste. Have a poke:
Poke the slime
Swipe down onto it fast, or press and hold gently. Or use the buttons.
05The big idea
Solid or liquid?
Depends how fast.
We sort the world into solids and liquids as if they were fixed kinds of stuff. Oobleck says that is too simple. Whether something behaves like a solid or a liquid can depend on how quickly you ask it to move.
Oobleck is the dramatic case, but it is not alone. Glaciers are solid ice and still flow downhill. Silly Putty bounces and then oozes. Even rock in the Earth’s mantle, solid as anything, flows over millions of years. Time is part of what a material is.
06The math
1 · Newton’s rule
τ = η · γ̇
The resisting stress τ in a sheared fluid is its viscosity η times the shear rate. For a Newtonian fluid, η is a fixed number.
2 · Shear rate
γ̇ = v / h
How fast the fluid is being sheared: the speed difference v across a gap of thickness h. Units: per second.
3 · How packed it is
φ = Vgrains / Vtotal
The share of the slime’s volume taken up by grains. Shear thickening appears when φ gets close to the point where grains can no longer move freely.
Isaac Newton assumed in 1687 that a fluid’s resistance is simply proportional to how fast it is sheared. For water, honey and oil, that is very nearly true, and fluids that obey it are named after him. Oobleck doesn’t.
Put numbers on it. Tilting the bowl shears the slime at maybe 0.1 per second. Stirring with a spoon at 20 cm/s past a 2 cm gap gives γ̇ = 0.2 / 0.02 = 10 per second. A punch at 3 m/s into a centimetre of slime is about 300 per second. The slime’s jamming kicks in somewhere in the middle of that range, which is exactly why stirring is hard and tilting is easy.
And the recipe sets φ. Cornflour grains are about 1.5 times as dense as water, so 60 g of them take up about 40 ml. Stirred into 60 ml of water, they fill about 40 / 100 = 40 % of the mixture. Just a few percent more and the grains lock up with no push at all; a few percent less and they never lock. Try the recipe on the curves:
The slime flows
Below about 10.0 per second the grains slide past each other on films of water. Push faster and watch the green line leap.
07Real life
Some of these thicken when pushed, like oobleck. More of them thin, which turns out to be just as useful.
Running on custard
Custard powder is mostly cornflour, and television shows have filled swimming pools with custard and sent people sprinting across. Each foot lands hard enough to jam the custard underneath. The ones who stop to wave sink in up to their knees.
Ketchup, the opposite
Ketchup sits stubbornly in the bottle until you shake or tap it, then pours far too fast. It is shear-thinning: the harder you work it, the runnier it gets. Most everyday non-Newtonian fluids are like this; oobleck is the unusual one.
Paint that doesn’t drip
Good paint is thick in the tin, thins out under the brush so it spreads easily, then thickens again on the wall before it can run. The same trick keeps toothpaste on the brush and lets blood flow through the narrowest vessels.
Impact pads
Some sports pads and phone cases are made from materials that stay soft and bendy when you flex them slowly, then stiffen in an instant under a sharp blow and spread the force out, the oobleck idea worked into a solid.
Quicksand, the other opposite
Quicksand is fine sand, clay and salt water that sits firm until it is disturbed, then suddenly liquefies. It is about twice as dense as a person, so you can’t sink all the way: you stop at about your waist. Wriggle your legs slowly to let water back in, rather than yanking.
Silly Putty
Invented in the 1940s by engineers trying to make synthetic rubber, it bounces like a ball if you throw it, flows into a puddle if you leave it overnight, and shatters if you hit it with a hammer. How it behaves depends entirely on how fast you ask.
1885
Walk on wet sand at the beach and the sand around each footprint suddenly looks dry and pale. In 1885 the engineer Osborne Reynolds explained why: packed grains can only be pushed out of shape by spreading apart, and as they spread, the gaps between them grow and suck water in from the surface. He called it dilatancy.
For a century, dilatancy was the favourite explanation for oobleck too. It is part of the story, but it took computer simulations and high-speed experiments in the 2010s to show that friction between grains, switched on when they are forced into contact, is what really makes the slime lock. A kitchen experiment, still being worked out, much like the tea leaves Einstein puzzled over.
08Is and isn’t
When you hit it, the water squeezes out and leaves dry starch behind.
The surface can look dry for a moment, but that’s not the cause. A hard push forces the grains into direct contact, breaking the thin films of water that kept them slippery, and friction between them locks them into a jammed network.
Oobleck turns into a solid.
It acts solid only while you are pushing on it hard, and only where you push. Take the push away and, in a fraction of a second, it is a liquid again. It never stops being a thick liquid full of grains.
Non-Newtonian fluids get thicker when you push them.
Most get thinner: ketchup, paint, yoghurt, blood. Fluids that thicken under force, like oobleck, are the rare ones.
Quicksand sucks you under.
Quicksand is roughly twice as dense as the human body. You sink to about your waist and stop. The danger is getting stuck, not drowning in it.
09Go further
A
The dancing slime
Pour a thin layer onto cling film stretched over a speaker and play a loud, low note. Fingers of slime rise up and squirm about.
B
Drop a weight
Drop a heavy marble onto a deep bowl of slime, then lay one gently on the surface. One bounces, the other slowly vanishes.
C
Drip it from a height
Hold a lump high above the bowl and let it fall as a stream. It flows smoothly down, then breaks into blobs when it lands.
D
Find the recipe
Make three small batches with different amounts of water. Weigh both ingredients and find the ratio where the hit-it test starts to work.
E
Try other powders
Potato starch works too. Flour and sugar don’t. Compare them and work out what the starch grains have that the others lack.
Compare it with the density tower, where honey, oil and water each kept a fixed thickness however you poured them. Oobleck is the first liquid in this deck that changes its thickness depending on how you treat it.
10Questions
Put about 60 g of cornflour (cornstarch) in a bowl and stir in water a tablespoon at a time, until it flows when you tilt the bowl but resists a fast stab of the spoon. You will need a little under the same weight of water as cornflour.
Oobleck is cornflour grains packed close together in water. Pushed slowly, the grains slide past each other on thin films of water. Hit hard, they are forced into contact faster than the water can flow away, friction between them locks them into a jammed solid, and it pushes back.
A fluid whose thickness, or viscosity, changes depending on how hard or fast you push it. Water and honey are Newtonian: their viscosity stays the same however you stir them. Ketchup and paint get runnier when pushed; oobleck gets much thicker.
Yes, as long as you keep running. Each footstep hits it hard enough to jam it solid underneath you for a moment. Stand still and you slowly sink in.
Cornflour and water are safe to touch and wash off easily. Don’t pour it down the sink, though: it can settle and clog the pipes. Let it dry out and throw it in the bin.
Safety · It’s just cornflour and water, safe to touch, but keep it off carpets. Never pour it down a sink or toilet: it settles and blocks the pipes. Let it dry, then bin it.
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