The traditional view is solids can support stress — you can walk on a solid, whereas liquids cant. Why is this? It’s because atoms in solids act as springs. When you apply a certain force on a spring, it recoils back with the same force. Think about jumping on a mattress or a trampoline — the harder you jump down, the equivalently large force that pushes you upward.
Scientifically, you are applying a stress (force/area) on the solid, which results in the atoms in a solid displacing like springs (strain). This causes the atomic springs in the solid pushing back just like a spring and supporting the applied force. Turns out these atomic springs are essential for walking. When you walk on a pavement, you are compressing concrete atoms ever so slightly and that provides the equivalent supporting force.
For liquids however, any stress you apply leads to the atoms moving or flowing at a certain strain rate. Thus, more force you apply to the liquid, more the flow leading to an opposite effect as a solid. If you dive into a pool, you fall in deeper as compared to if you just waded in. However, most everyday materials break this traditional view of solids and liquids. One classic example is a common household thickening agent, cornstarch.
Cornstarch powder when mixed with water gives a thick, soupy solution. As the ratio of cornstarch powder relative to water increases, the solution becomes thicker. However, this solution is not simply more viscous. Cornstarch in water is also known as Oobleck. When you have a large container with a mixture of equal parts water and cornstarch, try to slowly push a spoon through it. You will find that it moves like spoon through honey or any other viscous liquid. However when you try to push the spoon at a faster rate, the Oobleck resists flow and at an even higher rates, it breaks up like a solid. The viscosity of Oobleck seems to depend on how you handle it!
But why is this happening? For that we need to first understand how to quantify a liquid’s viscosity. Viscosity is the liquid’s response to spreading. Mathematically, it is defined as the stress (Force per area) required to deform a liquid at a certain rate (strain rate), related to the speed at which the liquid spreads.
Turns out Oobleck has different viscosity based on the force you apply on it. The reason behind this is cornstarch powder is composed of microscopic grains. When you push slowly on a dense mixture of cornstarch in water, the grains of cornstarch flow past each other without much disturbance. But if you push hard, these grains suddenly bump into each other and get stuck. Because of this, when more force is applied, cornstarch becomes more viscous.
The property of increasing viscosity with applied stress is called shear thickening.
If you thought that shear thickening pretty much explains why you can run on Oobleck; think again! Shear thickening explains why cornstarch-water mixtures get more viscous with higher applied force, which means that cornstarch gets harder to spread. However it still doesn’t explain why cornstarch acts like a solid and supports the stress of a person jumping or running on it. According to a 2012 article in Nature, physicists Scott Waitukaitis and Heinrich Jaeger find the reason this is possible is because once you jump in a pool of cornstarch-water, you quickly create a solid by compacting nearby grains together. This is similar to inducing a traffic jam at a bottleneck.
Let’s pause to think about this a bit. As vehicles enter a roadway with a bottleneck such as a few lanes blocked due to an accident, they need to cooperate with each other to navigate smoothly. More often than not, some people try to swerve in at the last minute, and others are too polite. All this sudden confusion at the bottleneck leads to inefficient navigation and pile ups close to the origin of the disturbance. It’s the same idea for jumping on cornstarch. the grains next to the point of the jump can’t efficiently rearrange and cooperate in the immediate aftermath of the jump. Instead, they just get compacted, providing enough force to support the weight of the person.
But once the jump is finished, the compacted grains slowly return back to their original configuration as if nothing ever happened. This is similar to a traffic bottleneck caused due to an accident that dissipates some time after the lanes are cleared.
The same underlying physics of congestion is seen both in household cornstarch as well as traffic jams ; even though grains of cornstarch and human drivers are quite different. Isn’t that fascinating!
The 2012 study also found this effect is stronger in shallower pools, where an immediate solid core forms that supports your stress, all the way down to the bottom of the pool. If you had a deep pool, running on it gets much harder — keep that in mind for your next cornstarch pool party!
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