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 or large plastic trays
  2. Cornflour 3kg min
  3. Water
  4. Laminated cornflour picture
Packing Away: 

Drain as much water off the cornflour mix as possible (this is easier if you can leave it for a while for the cornflour to settle first), 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.

Putting down a tarpaulin or taping bin bags to the floor first may make cleaning up easier. If possible set up near a sink or have an extra bowl of water nearby for hand washing.

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.

PLUS Explanation

The above explanation works well even with sixth-formers (they often just like playing with the cornflour). Perhaps focus more on real applications (e.g. the bullet proof vests, oil etc), and then add in some discussion of the points below.

*CHaOS Plus Further Ideas*
The starch granules themselves are composed of a mixture of long-chain polysaccharides - essentially lots of “sugar molecules” (glucose) stuck together.

A simple model for describing a non-Newtonian fluid is

η =k (d(gamma))/dt)**n-1

Here η is the viscosity (how “thick” the fluid is) and gamma is the shear (how much the material is deformed). The differential with respect to t tells us we’re interested in the strain rate (how fast you’re shearing). k is a constant.

If n=1, we get classical Newtonian behaviour (no dependence on shear rate). If n<1, then the viscosity decreases with increasing shear rate (shear thinning) and if n>1, then the viscosity increases with increasing shear rate (shear thickening).

Below is a schematic shear rate vs stress graph for various materials (labelled). The gradient gives the inverse of viscosity:

The “proper” definition of viscosity is the ratio of the shear stress to the velocity gradient in a fluid from a stationary boundary:

η= tau/(Δu/Δy)

Higher viscosity means a larger stress is needed for a given velocity gradient to be achieved (basically need to push harder to move a viscous fluid).

Another graph of shear stress vs velocity gradient (this time the gradient is the viscosity):

A Bingham plastic is one which is solid up until some yield stress, and then subsequently deforms with increasing stress. Mayonnaise and toothpaste are examples.

There is no “one” theory for why shear thickening happens - there are a couple of mechanisms… Talk about intermolecular forces (e.g. van-der-Waals forces), which hold molecules in suspension.

For large shear rates, intermolecular repulsion can be overcome, and the molecules are pushed out of their equilibrium positions and “mosh” together. This essentially makes the suspension “less ordered”, and hence increases the viscosity (less easy for molecules to move past each other).

Another mechanism involves the molecules joining to form small groups (“hydroclusters”), which may be thought of as long rods of particles that cause a “traffic jam” and increasing viscosity.

Risk Assessment
Date risk assesment last checked: 
Thu, 09/01/2020
Risk assesment checked by: 
Holly Smith
Date risk assesment double checked: 
Fri, 10/01/2020
Risk assesment double-checked by: 
Esmae
Risk Assessment: 

Cornflour and water mix in a washing up bowl.

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Cornflour Powder may trigger asthma attack. All 2 4 8 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.
In the event of an adverse reaction, move child out of area and sit them down. Call first aider.
1 3 3
Water/water-cornflour mixture Minor slip hazard. All 3 3 9 Clear up spills promptly; if the floor is smooth, ensure that a mop is available for this. Put wet floor sign down on cleaned floor.
Set up near a sink or have a bowl of water for hand washing nearby so children don't drip cornflour on their way to a sink.
Call first aider in event of injury.
2 2 4
Cornflour Irritant to eyes. All 4 3 12 Avoid contact with eyes and tell children to as well.
Call first aider in event of injury, who may perform an eyewash if trained and happy to do so.
2 3 6
Old cornflour mixture After a while, the mixture accumulates some dirt, which is not recommended for consumption. Public 3 2 6 Encourage children to wash hands after use. Do not allow children to ingest the mixture. 1 2 2
0
0
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Images
Publicity photo: 
Experiment photos: