Mixing together cornflour and water to make a non-newtonian liquid is great fun – it’s not only the kids who enjoy this one!
You can choose to simply demonstrate it as in Mark Miodownik’s step-by-step instructions on the BBC website*. I haven’t tried the trick of putting a raw egg inside a bag of the stuff as he did, and showing it doesn’t break, but I can imagine that is a really striking demonstration and would lead to an interesting discussion about what is going on.
There’s a simple but clear explanation of the principle of non-newtonian fluids on this website too. Perhaps you could ask one child to read through it, and get the others to act it out, representing the cornflour particles or the liquid to help them visualise the effect.
The alternative to just doing a demo is to ask the children to experiment by making their own, and to note down the ratio of cornflour and water needed to get the most effective mixture. This is what I did with my year 5 & 6 STEM club. We used small cups (and stuck to volume measurements not weight) so we didn’t use up too much of the ingredients in one go. This also emphasised that it was the ratio that mattered, not the actual quantities. Some of the children just wanted to ‘suck it and see’, but several were genuinely interested in honing in to the best ratio. And one group were even prepared to work out their final optimum ratio by adding a bit more of one ingredient or the other at a time and keeping a tally so as to calculate the final ratio.
I also encouraged them at the end to then make their work reproduceable by writing instructions to someone else to be able to make the optimum ooze, though I confess by this time, things were getting a bit messy.
I told them that for the TV show ‘The Big Bang’ my fellow presenter had walked on a bath of custard ooze. He was fine as long as he kept walking, but when he stood still he sank right in. We had a go with a small rectangular washing up bowl instead and used the best cornflour recipe from the previous experiments. It wasn’t as good as a bath of custard, but I still got squeals of excitement and everyone wanting a go - in bare feet of course.
Finally, I asked a few of them to try making custard ooze and see if that worked as well. And then to do some quick calculations to see how the cost compared. (They all agreed it was more smelly!)
This activity filled the full hour easily, and was probably the longest clearing up time too. I thoroughly recommend covering every surface with newspaper, and encourage the children to wear old clothes or aprons – and we weren’t even using food colouring.
*Photos are taken from the BBC website http://www.bbc.co.uk/science/0/22880407