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Cornflour monster

Make cornflour dance on a speaker
Useful information
Kit List: 

Speaker (NB: This experiment is seriously damaging to speakers, a new solution to protecting it is needed if we're to do this again)


Description taken from Dave's write-up on the Naked Scientists website:

A cornflour monster

This is a really strange effect only discovered about 6 years ago. Basically it involves shaking cornflour mixture very hard and it starts to grow tendrils, in a very 1950's B-movie way.

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.

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.

Risk Assessment
Date risk assesment last checked: 
Tue, 08/01/2013
Risk assesment checked by: 
Date risk assesment double checked: 
Sun, 20/01/2013
Risk assesment double-checked by: 
Richard H
Risk Assessment: 
DESCRIPTION Cornflour and water mix in a washing up bowl.
RISKS 1. Powder may trigger asthma attack.

2. Minor slip hazard.

3. Irritant to eyes.

4. After a while, the mixture accumulates some dirt, which is not recommended for consumption.
ACTION TO BE TAKEN TO MINIMISE RISKS 1. Clear up spilt powder. Where possible, do the experiment outside. Do not allow children to help to mix in new powder without first checking that they do not suffer from asthma.

2. Clear up spills promptly; if the floor is smooth, ensure that a mop is
available for this.

3. Avoid contact with eyes. Ensure that eye-wash is available near the experiment.

4. Encourage children to wash hands after use.

1. Move child out of area and sit them down. Call first aider.

2. Call first aider in event of injury.

3. Call first aider. Allow parent to rinse with eyewash supplied if desired; a demonstrator must not do this themselves unless trained to do so.

This experiment contains mains electrical parts, see separate risk assessment.