Embedded Files
Non-Newtonian Fluids are fluids that don't have constant viscosity, under different levels of stress.
On the contrary, the viscosity of Newtonian fluids or "normal fluids" only change when the pressure of the surroundings or the temperature of the fluid are changed.
Definitions:
Viscosity - A fluid's resistance to flow
Stress - The force per unit area applied to a body (the result of this is strain in the body of the material)
There are two general types of Non-Newtonian Fluid:
Shear Thickening
Shear Thinning
However, shear thickening fluids are much less common and much more fascinating so we will concentrate on them for now.
>Shear Thickening:
These are fluids that become more viscous the more stress that is applied to them.
Examples: Custard, Oobleck (mixture of cornflour and water), silly putty
Shear thickening fluids are often solid particles in suspension in a liquid (custard and oobleck). It is the combination of solid particles and liquid that give shear thickening fluids the combined properties of solids and fluids both.
Custard
It is quite shocking to some that custard (now tasty AND fascinating) is a shear thickening fluid. Custard is simply cornflour suspended in milk, with some added yummy flavourings. If you were to place your hand slowly in custard, you would be in no doubt that the gooey mess that is now your hand just touched a liquid. In fact you merely need to look at it and see that custard is a liquid.
However, when custard is impacted, or in other words, when it is subject to high stress, it can be said that the custard behaves like a solid. There is indeed deformation on the surface of the custard, a liquid like rippling, however if the stress induced is large enough, the custard provides a normal reaction to the stress that is almost equal in magnitude! This is very unlike our standard Newtonian fluids such as water, which provide only very minute normal reaction forces. Very weird indeed.
It is even possible to walk, or more run, on custard. All you need to do to stay dry is provide enough stress to the custard with your footfalls, creating a large enough reaction force on your body. Sounds easy doesn't it? Oh, i forgot to mention! You can't stop moving otherwise you sink hastily, helplessly and hilariously. Nearly forgot to tell you that. Oops.
Are you still gawping at the screen in disbelief? Good, then gawp at this:
Also here is a video of people talking about and walking on a similar Non-Newtonian Fluid:
Now we have addressed the non-believers, we can move on to the more fun (or less fun depending on your inclination) part of the page.
The Physics
So why are we able to run on custard (or other shear thickening fluids)?
As was mentioned briefly before, custard is hard particles (cornflour) suspended in liquid (milk) and it is this fact that lets people run on custard. It was, until very recently, thought that the shear thickening properties of custard were solely responsible for this but there are now new theories as to why we are able to do this. Let's not forget, we know for a fact that custard gets more viscous with increased shear force and therefore is a shear thickening fluid but it can't be this property that allows us to run on some of these liquids. This is because the impacts of our feet on the surface would not activate the shear thickening properties of the custard due to the orientation of the force in relation to the plane of suspension.
More detail on this:
A particle/liquid mixture being sheared
A particle/liquid mixture being impacted
As you can see from the images, shear thickening wouldn't be activated by impacts on the surface. This is because a shear force applied to a suspension would be parallel to the plane of suspension whereas the impact force applied by footfalls on the surface would be perpendicular to the plane. This means that scientist's previous explanations as to why we can walk on custard must be incorrect. New theories try to explain the phenomenon using other properties of these liquids.
So why can we walk on custard?
Scott R. Waitukaitis and Heinrich M. Jaeger, 11 July 2012, researched this phenomenon (link here to their research article), using an aluminium rod and an extremely concentrated mixture of cornflour and water, because they believed that the behaviour of particle/liquid mixtures under an impact was not caused by the shear thickening but by a different mechanism. We will explore this mechanism in detail:
Waitukaitis and Jaeger theorised that rather than the particles being moved over each other (shearing theory) causing the solidifying of the liquid, it is more the particles in the solution being compressed together and forming a jammed dense region bellow the point of impact. In other words, the high forces created by the impact pushes the particles in the mixture into each other below the point of impact.
To further illustrate and help you understand the jamming of particles under an impact, here is a basic image:
As you can see, the impact force has compacted the particles under it into a jammed region where the particles are so close together that the mixture underneath the impact point will behave like a solid.
This wave of compacting flows through the liquid and instantaneously reaches the bottom of the container of the mixture, due to the almost inelastic nature of the collisions between the particles. Thus there is now an almost solid "pillar" of jammed particles below the impact point all the way down to the bottom of the container. An image to help understanding:
This jammed pillar pushes on the bottom of the container which provides a normal reaction force on the pillar. This force is transferred almost instantaneously to the impactor through the compact pillar and pushes against the force of the impact. The more concentrated the mixture is, the faster the force is transmitted through the pillar and the larger the force from the container is. This is because the particles in the mixture will be closer together and more numerous so have to travel less distance to collide with another particle.
Here is a video from Waitukaitis and Jaeger's experiment (975KB download) that shows the initial impact of the aluminium rod.
There is no splash upon impact as there are with your standard Newtonian fluids such as water. You can also see a depression/deformation that spreads away from the impact point and across the surface of the mixture. This is because the compacted particles pull on the particles next to them, creating the depression around the impact point and jammed region. This shows the solid properties that are adopted by the mixture under impact.
So what happens when we stop moving?
After the impact occurs there is no impact force keeping the particles compacted together in a jammed region. This means the particles are allowed to enter back into suspension, letting the particles slide passed each other easier, in other words, the viscosity of the fluid decreases and the solid characteristics stop existing. Thus there is now little normal reaction supplied by the liquid.
This is when things start to sink. Frequent firm impacts must be made on the surface of the fluid to continue to make jammed regions under the impacts which, in turn, keep supplying a normal reaction force. This is why we can run on custard.
References:
Brown, E., Forman, N. A., Orellana, C. S., Zhang, X., Maynor, B. W., Betts, D. E., DeSimone, J. M. and Jaeger, H. M., 2010. Generality of shear thickening in dense suspensions. Nature Materials [Online], 9. Available from: http://www.nature.com/nmat/journal/v9/n3/full/nmat2627.html [Accessed 18 October].
Waitukaitis, S. R. and Jaeger, H. M., 2012. Impact-activated solidification of dense suspensions via dynamic jamming fronts. Nature [Online], 487(7406). Available from: http://www.nature.com/nature/journal/v487/n7406/full/nature11187.html [Accessed 18 October 2013].
Wagner, N. J. and Brady, J. F., 2009. Shear thickening in colloidal dispersions. Physicstoday [Online], 62(10). Available from: http://scitation.aip.org/content/aip/magazine/physicstoday/article/62/10/10.1063/1.3248476 [Accessed 18 October 2013].
Videos used:
Other images used were drawn using Microsoft Paint.
Page updated
Google Sites
Report abuse