A cornflour monster

11 March 2009

Ingredients

I waterproofed a 100W RMS speaker using a silicone spray, and then drove it using a signal generator via an amplifier. I then filled the cone about a third full with cornflour (cornstarch) mixture and turned on the power.

At about 30Hz (this depends on how much water you put in the cornflour) as I turned up the power it starts to change shape.

Small holes start to form which then grow until they meet one another and then it forms tendrils which grow and reform.

What is going on?

Cornflour is a shear thickening liquid which means that it gets stiffer as you apply forces to it.

Corflour in water Cornflour under pressure
Cornflour is made up of lots of particles which normally are lubricated by the water so it can flow. However if you apply a force the particles move slightly locking together so the mixture behaves as a solid.

This means that it will flow better when it is being pushed gently then when it is being pushed hard.

Normally if you have a liquid with a slight bump in it then gravity will cause the liquid to flow from higher areas to the lower. 

However if you vibrate the cornflour hard enough, at some parts of the cycle it will accelerate downwards at more than 10ms-1, this means that the cone is pulling the cornflour downwards, the opposite of the normal situation when it is pushing the cornflour up. This means that the cornflour is left behind and will flow in the opposite direction out of the hole.

When the cone accelerates upwards the forces due to the acceleration add to gravity creating a force large enough to stiffen up the cornflour so very little liquid flows into the hole. However when the cone is accelerating downwards the forces are much less so it becomes less stiff and the mixture can flow out of the hole easily. So overall the mixture flows out of the hole and it grows.

Cornflour moving upward Cornflour moving down
When the cone accelerates upwards the cornflour stiffens so very little flows into the hole. When it accelerates downwards the forces are less so the cornflour flows easily out of the hole.

You can see the holes growing as the cone accelerates downwards and smaller as it accelerates upwards in this very slow motion video (100x).

And the extreme case of this is when the cornflour forms tendrils, here slowed down by 20x

 

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