Finally. Lunchtime. Your hamburger is waiting patiently on the plate, the french fries sit next to it, glistening. Perfect, except for one thing. Ketchup. You grab a new bottle, twist off the cap and turn the bottle upside down. Nothing happens. The ketchup just sits there in a big lump, defying gravity. You shake it. Nothing. You shake it again more violently. Nothing. You shake it really hard this time, like a very enthusiastic polaroid photographer and then, disaster. The ketchup starts flowing into your plate and before you have time to stop the flow, what was once supposed to be fries with ketchup on the side has become ketchup with a side of fries. You look up to heavens and sigh. Why?
No, really. Why?
Ketchup behaves this way because it is a non-newtonian fluid. A newtonian fluid, like water, has constant viscosity (resistance to flow, the fluid’s ‘thickness’). No matter how much force is applied to it, the viscosity will not change. Non-newtonian fluids, however, don’t have constant viscosity. They exhibit properties of fluids AND solids.
The ‘thickness’, or viscosity, of a non-newtonian fluid depends on the force applied to it.
Ketchup is a ‘shear thinning’ fluid. At rest, it is very viscous but the more (shear) force you apply to it, the less viscous it becomes. That’s why it’s hard to get it flowing out of the bottle, and why it’s hard to control the flow once it does.
Ketchup is not the only shear thinning liquid. Blood, whipped cream, nail polish and syrup are all examples of such fluids.
Walking on custard
The opposite of shear thinning fluids are, quite obviously, shear thickening fluids, which flow quite freely under little stress and become almost solid under high stress. A good example is custard. You can slowly sink your hand into a bowl of custard, but if you punch it, it will become solid.
A shear thickening fluid commonly used for demonstation purposes is Oobleck, a simple mixture of water and corn starch, which you can easily make at home (more cheaply than custard, though definitely less delicious). In the video at the bottom of this post, you can watch a bowling ball rolling on top of Oobleck as if it was solid floor and some other really cool super slow motion shots including a couple of guys walking and dancing on ‘water’!
Quite recently, researchers at the university of Chicago discovered exactly why Oobleck behaves this way. The water leaves the impact point more quickly that the corn starch. The starch left behind gets jammed up, forming a solid.
Quicksand is another example of a shear thickening fluid. You can walk across it, but stop for too long and you start sinking. Once you’re inside, quick sudden movements are completely useless as it will resist the motion. The way out of a quicksand pit is to pull yourself out (if you have a rope or vine handy) really slowly, thus keeping viscosity low, or by slowly wiggling your legs, then spreading arms and legs apart, allowing you to float, as the surface area of your body increases. Quicksand is far too dense for you to sink deeply. Drowning in quicksand is, in fact, a myth. However, getting stuck in it will obviously put you more at risk from dehydration or getting eaten by a curious predator.
Shear thickening fluids are also used for armour. They’re nice and flexible, but get very hard when hit by a bullet.
Rheology is the study of such flows. As you can imagine, an understanding of these liquids is fundamental in lots of industries. Making soap, transporting oil, making ice cream, making paints, shampoo, toothpaste involves non-newtonian fluids. In the references at the bottom of this post you’ll find a link to a PhD thesis called “Rheology of Caramel”. Two hundred and twenty pages all about how caramel behaves in different conditions. It might sound pointless to you, but if you’re making caramel products on an industrial scale, knowing how the caramel will behave under different conditions and how it will flow is crucial.
Try it out
If you do want to try making a non-newtonian fluid at home (and you should, it’s fun) just mix 1.5-2 parts corn starch to 1 part water.
See what happens, punch it, hit it with a hammer, let your non-newtonian flag fly. If you have kids, it’s a great experiment to try together (if your definition of experiment is slapping bowls of custard).
More Physics from Pie Cubed
- http://etheses.nottingham.ac.uk/1837/1/Phd_thesis_Giuseppina_Barra.pdf -Giuseppina Barra – Rheology of Caramel – PhD Thesis
- Miracle mix looks like liquid but shatters like glass – New Scientist
- Zapped crude oil flows faster through pipes – New Scientist
- Robert W. Fox, Alan T. McDonald, Philip J. Pritchard – Introduction to Fluid Mechanics – ISBN 978-0471202318
- “Quicksand Science: Why It Traps, How to Escape”. National Geographic News.