*** OVERVIEW ****

Twisting a rod in the polymer solution causes long molecules to wrap onto the rod and creep slowly upwards (like spaghetti on a fork).

Other things to do:
Try touching the surface lightly and lifting your fingertip (or lifting the rod out of the solution). The solution has a very high extensional viscosity (resistance to flow when pulling as opposed to pushing, which is how we usually think of viscosity). Imagine trying to pick up one strand of cooked spaghetti from a huge pile – it’s the same thing (except that you’re pulling out hundreds, if not thousands of strands of spaghetti).

The solution has some properties of a liquid, for example it finds its own level in a container, and some of the properties of a solid, for example it is elastic.

Tips for demonstrating:

Don't let them put their fingers in the slime. It's pretty much non-toxic, but it is very sticky and if it starts getting on peoples hands then we'll lose it all very quickly (and it's a pain to make).

*** BASIC PROCEDURE AND EXPLANATION ***

Get the kids to twist the rod in the slime. Wait for them to notice that it's doing something wierd (!) and comment on it.
Explain that everything's made out of molecules and how things behave is down to what kind of molecules it's made out of, so water's made of little round-ish molecules that pour past each other (like sugar, hundreds and thousands, insert example here...), but this stuff's made of long thin molecules more like spaghetti.
Let them have a play with the spaghetti-string and fork, and see that the string climbs up the fork.
Explain what's going on, that as the string wraps onto the fork out of the mixture, it pulls more string with it. As it pulls tight some of it gets forced up over the surface, pushed out of the way as more string wraps on. Only works with the prongs of the fork on the plate as otherwise the spaghetti is squashed out the bottom (which is preferred- gravity)
This is what's happening with the polyacrylamide, but on a much smaller scale so we can't see the individual "strings".

*** OTHER THINGS TO TALK ABOUT ***

Polymers in general

*** SCIENCE BACKGROUND FOR DEMONSTRATORS ***

The solution is polyacrylamide (between 0.5 wt% and 1 wt%) dissolved in glycerine (same thing as glycerol) with food colouring.

Glycerine is used as a cough mixture and is also added to icing to keep it soft.

Polyacrylamide is a polymer. This means that the molecules (the smallest parts that are still polyacrylamide) are very long and thin like cooked spaghetti, lengths of string or long hair. Polyacrylamide is used to thicken foods.

A bit more detail, adjust appropriately (for part 3 mathmos and similar, see end to shut them up):
Ok, so you want to know what happens when you eat spaghetti? Polymer chains and cooked spaghetti strands are the same in many ways!

The polymer chains get wrapped around the rod (trapping one end) and then as you keep turning, the free ends (not wrapped around the rod, but part of the bulk solution) are trapped in the tangled bulk (by other chains) and the end wrapped is being pulled by you, so the chain is under tension (a force on each end). To try and reduce the distance between the two ends (become straight, as a piece of string does when you pull both ends) it tries to move up or down the rod to a region where less chains are wrapped round and hence the effective diameter (the diameter of the rod plus wrapped around chains) is less and hence the distance is shorter. Because of the bottom of the container (Newton 3: action and reaction being equal and opposite), the only way is up (Yazz). For the ambitious, Newton 3 is also important in spinny chair, so you can link!

In case a concerned parent asks:

Yes, acrylamide (the monomer) is harmful, but there is none of this in the solution. The degree of polymerisation (the length of the chains) is so high that even though there is a distribution of lengths even the shortest is several thousands of monomers long – for how many take the molecular weight (on the tub) and calculate it!

For clever clogs (mainly passing professors and part 3 mathmos) it’s due to a non-zero first normal stress difference. The strain tensor of the motion of turning the rod produces a non-zero difference between the normal components of the resulting stress tensor – so there is a force up /down.

PLEASE, PLEASE DO NOT LET PEOPLE TOUCH THE SOLUTION. IT IS NOT HARMFUL, BUT STICKY AND IF PEOPLE TOUCH IT, IT STICKS TO THEM AND THEY TAKE IT AWAY WITH THEM (OR LEAVE IT IN A PAPER TOWEL) AND THEN THERE WON’T BE ANY FOR PEOPLE COMING ALONG LATER.