What do quicksand, ketchup, and oobleck all have in common? The answer: they are all non-Newtonian fluids. While this may sound like a foreign material that should have no place in everyday life, non-newtonian fluids are all around us, and understanding them can help you understand the world (and get ketchup out of a glass bottle without really trying).

In order to learn about non-Newtonian fluids, it makes sense to start by understanding Newtonian fluids. Newtonian fluids are best exemplified by two of the most essential substances in the world: water and air.

A Newtonian fluid is a fluid in which viscosity is not affected by shear force i.e. moving through the fluid more quickly leaves the fluid with the same viscosity, but the resulting opposing force increases linearly.

This is why when you move slowly through water, it feels like there is very little resistance, but when you try to swing your arms wildly through it, the resistance feels significantly higher. The reason Newtonian fluids behave as they do has a lot to do with how the individual particles flow past one another.

Because the particles are free to move, an object may move through a fluid. However, in order to properly move out of the way of the object, the particles must also flow past each other. When you move slowly, the particles have plenty of time to move, and so they provide little resistance to your motion.

However, as soon as you try to go tearing through the water, the particles begin having trouble moving past each other, causing them to push back on your hand and generate resistance. These are the key characteristics of Newtonian fluids.

A non-Newtonian fluid is anything that is, well, non-Newtonian.

There are two general strands of non-Newtonian fluids, shear thinning and shear thickening. These two groups, as described by their names, react to an applied shear force in one of two ways, by either thickening or by thinning. To begin, we will examine the simpler of the two groups, shear thickening substances.

Shear thickening substances include quicksand and oobleck, among a multitude of other fluids. When these substances are subjected to a shear force, their viscosity increases dramatically. The most notable example of this is in oobleck, which, when moved through slowly, feels like a very thick paste, but, when moved through (or on) quickly, becomes hard enough to run on or to stop a punch.

The reason for this behavior is similar to the cause of Newtonian behavior, except with the addition of a compound that disperses itself in an ordered manner throughout the fluid. In the case of oobleck, the corn starch distributes itself evenly throughout the water, and for the most part it remains fairly distributed.

Because of this distribution, objects may continue to move freely through the fluid. However, when a shear force is applied, it begins to knock groups of corn starch particles together, eventually forming small clumps of solids that result in the whole fluid acting as a solid, resulting in a sudden increase in viscosity. The result is that, with sufficient speed, you can run across oobleck.

Another interesting point is that this is also the format by which quicksand operates, so, if you ever pull an Indiana Jones and find yourself stuck in a quicksand pit, don’t panic, just move slowly, and you will be able to swim right through the world’s most dangerous non-Newtonian fluid.

Having just examined fluids that become nearly rock hard when shear force is applied to them, it only makes sense to look at exactly the opposite group, fluids that flow more and more easily the more force is applied to them.

These fluids, called shear thinning fluids, are rarer and harder to understand than their thickening brethren, but are just as ubiquitous, namely, in the form one of the most common condiments in the world, ketchup. Shear thinning is thus far less thoroughly explained when compared to shear thickening.

The current theory is that as the shear forces are applied to the fluids, the force generates sufficient heat within the fluid itself to overcome the usual random motion of the particles and lubricate the fluid itself. This means that as the force is applied, the particles actually move more easily, reducing the viscosity of the fluid.

Now, you are probably wondering “well how can this help me with my ketchup-based struggles?”

Well, if you consider that ketchup will become thinner as force is applied to it, and shaking only applies force for a short period of time, then one must consider a different method of movement, namely spinning. If you spin the bottle between your hands, the duration of the force increases, allowing the ketchup to move from the bottle to your plate more freely.

As you can see, non-Newtonian fluids are both plentiful and strange. Having learned all of this material, it is my hope that you may avoid any ketchup or quicksand related troubles in the future.