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Why can cornstarch goo feel solid and liquid?

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What else makes you wonder?

What would one extra cup of water do to the next fast punch?

Keep the number of grains fixed. Imagine how their gaps and possible contact paths change when the mixture has more room.

Could someone keep running across a whole pool of this goo?

Each foot needs a fresh fast load. What happens if one foot stays planted for too long?

Could a grain-filled fluid help make flexible body armor?

Think about the two jobs: bending comfortably during normal motion and resisting a sudden impact.

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After you watchWhy can cornstarch goo feel solid and liquid?

The short answer

Cornstarch goo is a crowded mixture of solid grains and water. During a slow push, water helps the grains slide around one another, so the mixture flows. During a fast push, enough neighboring grains can touch and form a temporary force-carrying network, so the same mixture resists like a solid.

Try this next

  • What might change if the same grains had more room between them? Keep the starch scoop and push speed fixed, then predict whether a wall-to-wall contact path would become easier, harder, or unchanged.
  • What happens the instant a fast load stops? Watch the normal batch after a quick hit. Look for the moment the contact links disappear and water paths open again.
The whole story

How it works

The goo is a dense suspension: many tiny starch grains packed into water. A gentle, slow load leaves time for grains and water to rearrange. A quick load can force nearby grains into frictional contact. When those contacts connect across the pushed region, they share the force and the goo suddenly feels firm. Remove the load and the temporary network relaxes, so the mixture flows again.

What people get wrong

The mixture does not permanently switch between a true solid and a true liquid, and temperature is not changing it. Its resistance depends on how its grains are loaded. The same bowl can flow under one kind of motion and resist under another.

The catch

Shear-thickening materials can stay flexible during ordinary motion yet resist a sudden impact. That response depends on the recipe, particle crowding, grain shape, and loading speed, so engineers must tune a suspension for the range of forces it will meet.

Questions kids ask

Is oobleck really a solid or a liquid?

It is a suspension whose response changes with loading. It flows during gentle motion but can resist a quick impact when its grains form a temporary contact network.

Why can people run across a pool of it?

Repeated fast footfalls can create brief load-bearing contacts. If the person stops supplying those quick impacts, the grain network relaxes and the mixture flows around them.

Does a fast punch freeze the water?

No. The water stays liquid. The sudden resistance comes from the solid grains pressing into a connected frictional network.

Do all non-Newtonian fluids get thicker when pushed?

No. Some become less viscous under shear, and some have a yield stress before they move. Non-Newtonian simply means the relationship between stress and flow is not the simple constant-viscosity rule.

Talk about it

  • What has to connect before the mixture can carry a push across the bowl?
  • What changed between the fast punch and slow press if the ingredients stayed the same?
  • Where else would a material that is flexible normally but firm on impact be useful?

For grown-ups

This is a dense non-Newtonian suspension. Under rapid shear or compression, lubrication films between particles can give way to frictional contacts. A percolating contact network raises the effective viscosity and can support load transiently. When stress drops, the network relaxes and the suspension flows again.