This experiment demonstrates the ideas of time reversability in fluid flow.

It consists of two concentric cylinders that can be rotated relative to one another, with the space between the cylinders filled with syrup. Using icing sugar, draw a picture or write an initial on the surface of the syrup. Turn the inner cylinder about one rotation, and see the picture get stretched and maybe even vanish. Now rotate the inner cylinder back again (slowly!) and watch your picture come back. Simple!

What to do

Put three cans of golden syrup in the white bucket. Sprinkle some of the icing sugar(or similar) in a straight radial line on the surface of the syrup. Whilst holding the bottom of the bucket firmly, get the children to SLOWLY turn the handle to rotate the inner cylinder (say 90°). Then get them to rotate it back slowly. The powder should be in the same pattern as initially.

How to explain it

Imagine the syrup as little blobs of syrup. When you turn the inner cylinder they experience a force and so move. When you turn the cylinder back, they experience the same magnitude of force but pushing them in the opposite direction. So in the end it's as if they never moved at all.

Related stuff

A fish which swims in water by wiggling its tail from side to side wouldn't get anywhere if you put it in syrup. Moving the tail one way and then the other in syrup just puts the fluid back where it started, with no forward push. Sperm get round this by having a spiral tail which rotates, so their swimming motion is never 'reversed' and they can travel forwards.

What are we seeing?

'The syrup is sticky so all the bits next to each other stay together' - close...

If the syrup is deep enough and the turning was not too fast, the flow has very little turbulence. This is not just because the syrup is "sticky" - if you turn too fast you will lose the picture no matter how sticky the syrup is! How a flow behaves depends not only on the stickiness of the fluid, but also on speed of flow (and density of flow and lengthscales). In the case of very low Reynolds number and nice boundary conditions, reversing the boundary conditions reverses the flow almost exactly - which is why the picture comes back almost exactly and why flapping fish can't swim in golden syrup.

Science background

Golden syrup has a very high viscosity, so the flow should be laminar. It is the little bit of turbulence that is unavoidable that will mean that you will have to occasionally clean it out and put fresh syrup in! The slowly is to minimise turbulence. This causes turbulent mixing which stops the demonstration working.

Other things to talk about

Dimensionless numbers in fluid dynamics (e.g. Reynolds number, etc.) and how these can be used to describe flow in systems of completely different size, but same dimensionless number.

Some background information from 'Chemistry & Industry' 02/01/06

Mixtures are Reversible by Lisa Richards
"When you were making your Christmas pudding last month, did you consider that if you chose to stir the mixture anti-clockwise, not only are you breaking a tradition and giving yourself bad luck, you may also have caused the mixture to separate rather than combine?

As crazy as this may sound, it has been discovered that two liquids seemingly irreversibly mixed can be returned to their original components.

David Pine and his team at the University of California describe this phenomenon as the 'equivalent to reversing time' (Nature 2005,438,997).

Using two concentric cylinders and tiny beads suspended in solution, Pine and his colleagues showed that, when stirred in reverse, the beads retrace their movements and return to their starting positions.

Troy Shinbrot, of Rutgers Univsersity, New Jersey, believes that this has great implications for pharmaceuticals. Shinbrot explained that viscous fluids such as honey have been known to have this property for some time, but it was generally assumed that suspensions behave irreversibly. Many pharmaceutical preparations are sold as suspensions, for example children's antibiotics such as Amoxicillin. Understanding mixing behaviour helps in the understanding of which suspensions will mix, and the way in which this occurs.

Applications such as tissue engineering performed on carrier particles, or cell cultures grown in viscous, slowly agitated sugar solutions, may also benefit from this new knowledge."