Cornflour - solid or liquid?

I admit to being a little less than keen on this demo when I used to do it with Cambridge Hands-On Science (CHaOS).  Not because it wasn't fun or interesting, but because of the scale of the mess that could be created by smaller-than-average visitors who tended to gravitate towards it!  Now a large portion of my life centres around cleaning up mess after little people, I thought it wouldn't make much difference if I added cornflour to the mix!  Whilst I mention them, if CHaOS happen to be in your area for their summer roadshow (Sheffield/Leeds/Nottingham still to go this year), I really recommend a visit, particularly if you've got slightly older children.

The curious thing about this demo is that cornflour is made of lots of little bits (particles) of different shapes.  If they are surrounded by water, they move past each other, and a cornflour/water mix behaves like a liquid.  If you squeeze the water out, the particles can't move past each other easily and the mix behaves like a solid.  So if you move your hands through it slowly, it's a liquid and if you move quickly then it becomes like a solid.  It's quite fun to play with, even for adults.  This is called "shear thickening", although I don't suggest trying to tell a toddler that.

When you make the mix, add the water a little at a time and stir it slowly until you get a gloop that pours slowly but you can't push your finger into quickly.  I used about half a pack of (out of date) cornflour which filled a normal sized yogurt pot.  I decided to give some scope for experimenting with a couple of clean containers from our plastic recycling bin and a lolly stick, but put them in a washing up bowl to try and constrain the inevitable mess.

The first attempt was not a success.  He stuck his fingers in, wiggled them around, shouted "urgh, get it off me, Mummy wash my hands!" and that was that. He doesn't like finger paints or similar, so I guess this shouldn't have been a surprise.  However, an hour or so later, he asked to go back to it, so we did.  This time I encouraged him to use the lolly stick rather than his hands, and he spent a good 15 minutes pouring it, poking it and making a mess.  We talked about how the cornflour was lots of little bits and if you moved them too quickly there was a traffic jam and they got stuck.  He liked jabbing the stick into a pot of cornflour quickly to see it wouldn't go, and then slowly to see it go all the way to the bottom.  The mess was largely constrained to the washing up bowl, but I wouldn't say that none ended up on the floor...

Cornflour
Playing with cornflour
I scooped as much of it up as I could and put it in the pot in the fridge so we can have another go tomorrow.

Comments

  1. Chortle. Yeah. Cornflour does really want a 1:1 ratio of adults to small people... and/or a small field.

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  2. Maybe next time (probably tomorrow) we'll do it in the garden instead...

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