Cornflour

Introduction
Public summary: 

Slimey, gooey and messy: cornflour is one of our favourite experiments! Come and stick your hands in, and figure out if it behaves like a liquid or a solid...

Explore the remarkable properties of cornflour mixed with water.
Useful information
Kit List: 
  1. Washing up bowls 3-4
  2. Cornflour 3kg min
  3. Water
  4. Laminated cornfour picture
Packing Away: 

Drain as much water off the cornflour mix as possible, and put dry-ish remains in to a bin bag. DO NOT pour the cornflour mix down drains, as it may block them.
Brush down the surface you were demonstrating on.

Frequency of use: 
4
Explanation
Explanation: 

In a nutshell
You and the children play with the gooey cornflour/water mix, exploring the concepts of solids, liquids and substances that have properties of both.
Cornflour is lots of irregular shaped particles that are separated by water normally so are lubricated and can move. If you squash them together it will push the water sideways a little bit and let them touch - now they lock together and behave as a solid.
The picture shows cornflour under a confocal microscope, which takes a 2-D slice through an image rather than looking at the surface.

How to set up the experiment
Slowly add water to cornflour until it works - ask a committee member for help if you're getting stuck. A ratio of 2.5 parts flour to 1 part water is suggested, but the ratio may vary.

What you need to know about the experiment
(1) Cornflour is shear thickening. This means the higher rate of shear, the higher the viscosity (i.e. the thicker it is). (Note: Shear can be explained by considering 'layers' of cornflour particles sliding past each other.)
(2) Try asking them whether it is a solid or a liquid. You may want to get them to come up with definitions of the terms solid and liquid - e.g. "what do we call hard things / things that flow...?" Cornflour is like a solid and a liquid - Acts as a solid under stress and a liquid otherwise
(3) It's like a room full of people and when you try and make it move quickly, everyone tries to move at once (while also moving closer together) and they all get in each other's way and so no one can move anywhere. And when you do stuff to it slowly, everyone has time to move out of the way and file out.

Microscope view of cornflour
(file here) (Bromley & Hopkinson reference here)

Want to know more?
Shear thickening is a problem in the oil industry, as when they are drilling they are getting rock fragments in the mud coming back up, if there are too many they behave similarly to the cornflour, with catastrophic results to pumps.
Some people are talking about making liquid body armour using this effect, to make the body armour more comfortable.
The opposite of shear thickening is shear thinning. Many substances are shear thinning because the higher rate of shear can break up interparticular interactions and reduce the viscosity - e.g. shampoo, toothpaste - when you shear them by squeezing them out of the tube, it flows, but when there's no shear, it sits quite happily on the toothbrush without flowing anywhere.

Explanation warnings
THICKENING SOUPS IS DIFFERENT: the cornflour grains open up when heated and release long starch molecules that tangle together forming a gel-like substance.
THIS IS NOT THIXOTROPY, which is concerned with time related effects. Thixotropy is a long word and shouldn't be used with children. Adults should be politely and gently explained the difference! The longer you shear a thixotropic fluid the lower the viscosity (the thinner it becomes) - e.g. paint - as you progressively break up interparticular interactions. Many fluids that are shear thinning are also thixotropic. Rheoplexy / Anti-thixotropy is the opposite - i.e. the longer you shear a fluid the higher the viscosity (the thicker it becomes). Xanthan gum might do this under certain conditions, but it's very rare for substances to do this.

Risk Assessment
Date risk assesment last checked: 
Mon, 01/01/2018
Risk assesment checked by: 
grh37
Date risk assesment double checked: 
Fri, 12/01/2018
Risk assesment double-checked by: 
Josh Garfinkel
Risk Assessment: 
DESCRIPTION Cornflour and water mix in a washing up bowl.
RISKS 1. Powder may trigger asthma attack.

2. Minor slip hazard.

3. Irritant to eyes.

4. After a while, the mixture accumulates some dirt, which is not recommended for consumption.
ACTION TO BE TAKEN TO MINIMISE RISKS 1. Clear up spilt powder. Where possible, do the experiment outside. Do not allow children to help to mix in new powder without first checking that they do not suffer from asthma.

2. Clear up spills promptly; if the floor is smooth, ensure that a mop is
available for this.

3. Avoid contact with eyes.

4. Encourage children to wash hands after use.
ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT

1. Move child out of area and sit them down. Call first aider.

2. Call first aider in event of injury.

3. Rinse with eyewash supplied if trained and confident to do so. Call first aider.

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Images
Publicity photo: 
Experiment photos: