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Rod Climbing

Introduction
Public summary: 

Discover a strange goo with same strange properties. Watch as it is able to climb up a rod in front of your eyes!

A polyacrylamide solution, which "climbs up" a rod when it is rotated in the solution
Useful information
Kit List: 

Pot of pre-made polyacrylamide-in-glycerine. (x2)
Stirring rod (plastic)
Box of string "spaghetti"
Fork
Hand Drill with rod attachment

Frequency of use: 
1
Explanation
Explanation: 

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

How to set up the experiment
The premade solution is polyacrylamide (between 0.5 wt% and 1 wt%) dissolved in glycerine (same thing as glycerol) with food colouring, which goes with the rod. The fork and spaghetti can be used to demonstrate how a polymer behaves when it moves.

What you need to know during the event
1) Get the kids to twist the rod in the slime. Wait for them to notice that it's doing something weird...
Explain that all materials, like the slime, are made of molecules. The slime molecules are really long and thin, like spaghetti...,The slime contains a polymer called polyacrylamide. 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.

2) Let them have a play with the spaghetti-string and fork, and see that the string climbs up the fork.
Encourage them to see how the length of the molecule makes the spaghetti get tangled, creating a viscous solution. Explain what's going on - 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. This is what's happening with the polyacrylamide, but on a much smaller scale so we can't see the individual "strings". *Demo only works with the prongs of the fork on the plate as otherwise the spaghetti is squashed out the bottom (which is preferred because of gravity)*.
This occurs due to the circular flow of the liquid around the rod becoming sheared, which causes the forces perpendicular to the surface (upwards) to become much larger than those effects causing the circular motion, leading to the liquid rising up onto the rod. This effect (called the Weissenburg effect) can occur in any liquid capable of shear (think batter or similar things in baking partially rising up the whisk when mixed), but as the rising is due to shearing effects, this will become magnified for more viscous liquids, such as the polyacrylamide. Also, note that this effect just requires some rotational motion, and if this can be achieved without the rod, a bump will still form around the vortex formed by the stirrer. As seen in this youtube video: https://www.youtube.com/watch?v=tK2ajzCfNBU

What kind of molecule do they think might pour much more easily? (e.g. water, sand, sugar which are small and round molecules instead of chains).

Sometimes they will ask why it forms a 'blob', and doesn't just rise up, this is because of this effect falling off as the width of the 'blob' decreases (less shearing can occur), and so gravity pulls the layers down until a blob is formed

3) Try lifting the rod out of the slime or touching the surface of the slime lightly with the rod. 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).

Tips for demonstrating
Don't let them put their fingers in the slime
1) Acrylamide (the monomer) is harmful, but there is none of this in the solution. The slime (POLYacrylamide) that we're using is non-toxic. The degree of polymerisation (the length of the chains) is so high that even the shortest is several thousands of monomers long.
2) It is also very sticky and if it starts getting on people's hands, then we'll lose it all very quickly.

Want to know more?
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.

Polymers behave elastically due to the different states they can adopt - extremes being the coiled and extended states. In the coiled states, there are lots of different conformations which come about from molecule rotating in different parts along its length, but only one that makes it straight. That means that the coiled state is much more likely (this is entropy!), and so the polymer tries to go back to it if stretched.

The glycerine that the polyacrylamide is dissolved is used as a cough mixture and is also added to icing to keep it soft.

Risk Assessment
Date risk assesment last checked: 
Wed, 17/01/2018
Risk assesment checked by: 
grh37
Date risk assesment double checked: 
Sun, 04/02/2018
Risk assesment double-checked by: 
Giedre
Risk Assessment: 
DESCRIPTION Weissenberg effect. If rod is rotating fast in a polymer solution the polymer will climb the rod.
RISKS 1. Eating polymer (note: polyacrylimide is non-toxic).

2. Slip hazard.
ACTION TO BE TAKEN TO MINIMISE RISKS

1. Stop kids from eating polymer. If you are using polyacrylimide, use a very high molecular weight so there are no monomers. Children must not handle the slime during the activity!

2. Clear up any spills.

ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT 1. Polyacrylimide is non-toxic. Advise parents what has been ingested and to see their GP should illness develop

2. Call first aider in event of injury
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