I must confess: I have a crush on non-newtonian fluids. They are so strange and beautiful… They defeat intuition by respecting the laws of nature… and they are fun to play with!
Fluids can be characterized by a parameter known as viscosity. We all have direct experience of viscosity, as it measures the “ease of flow”: water flows easily when compared to honey. This behavior can be measured and quantified: higher viscosity for honey (a “thick” substance) and lower viscosity for water (a “thin” one). With more rigor, the term “viscosity” is a very generic term enclosing many different types of viscosities, depending on the experimental setup and the characteristics of the fluid. For the layperson, viscosity is a measure of thickness, what technically is called “dynamic viscosity”.
The pitch drop experiment at University of Queensland
Viscosity is not constant in a fluid. It is related to various parameters, the most typical is temperature: kitchen oil is rather thick when cool. Heat it up, and it becomes thin almost like water. Same for water, although it’s less evident. Boiling water is much thinner than cold water. For gases, it happens the opposite: the higher the temperature of the gas, the higher its viscosity is. Some substances can be so thick to behave like solids, for example glass and tar pitch: smash them with a hammer, they go into a thousand pieces. Heat them up, and they flow like a liquid. The fun part is that even when apparently solid, tar is actually showing its liquid properties in the so-called “pitch drop experiment”. Pitch is flowing through the funnel, but it’s very, very slow: since the experiment started, in 1927, only 8 drops of pitch fell, an average of one drop every ten years.
In classic, so-called “newtonian” fluids, viscosity is constant at a given temperature. Non-newtonian fluids are different: the applied stress itself, or the amount of time the stress is applied alter the viscosity. A non-newtonian fluid can be thin when left alone, but it becomes thick or almost solid when shaken or hit, or vice-versa.
A popular example of kitchen-chemistry non-newtonian fluid is corn starch: take starch, add a bit of water, and you obtain this
As you can see, the result is very thick when hit, but behaves more like a (relatively) thin liquid when left undisturbed. In other words, the mixture shows dilatant (or shear thickening) properties: the viscosity depends on the amount of applied stress, increasing thickness. Although the cornstarch and water mixture does not look particularly useful except as a curiosity, the same property can be exploited to engineer a material which is soft and pliable like rubber, but becomes hard enough to substitute polycarbonate when hit. It is the case with D3O
Pushing the dilatant property even further would effectively produce a “liquid body armor”: light and flexible in normal conditions, but able to stop a bullet when needed. Military and police forces all around the world have of course a great deal of interest in something like this.
Silly putty
Another very funny application of this kind of non-newtonian behavior is my preferred toy: Silly Putty. Basically the same concept: a rubber-like polymer flowing when left undisturbed, but hard when a strain is applied. It’s very soft, but it bounces hitting the ground. If you hammer it, it goes into pieces but it slowly flows and flattens if left undisturbed on a desk. Kind of interesting, its story. As frequently happens, one man’s discard is another man’s treasure.
There are other non-newtonian fluids that show shear thinning (or pseudoplastic) behavior. When stress is applied, their viscosity becomes lower, and from solid they become liquids. One example is quicksand, whose danger comes from its property of being relatively solid until pressure is applied (like when stepping on it). Applying pressure leads to a separation between the sand and the water from the mixture. This effectively produces a very thin liquid on top, and a thicker layer of sand deposit acting as an effective trap. To liquefy again this almost solid layer, the trapped person or animal should apply a lot of pressure above him to reintroduce water without exerting pressure below, something that would drag him deeper. This is not easy, and requires considerable strength. Quicksand is very dense, so drowning is not possible, and the human body would float half way like a cork. Nevertheless, being trapped and basically unable to walk away could become a considerable issue for other reasons, such as drowning for a raising tide, lack of food, or assault from wild animals.
Another example of pseudoplastic substance is Ketchup. It flows badly in normal condition, but if stress is applied (as when shaken, squeezed, or by hitting the bottom of the bottle) it liquefies, hence the saying “first a little, then a lottle, the Ketchup bottle“.
In some cases, shear thickening or thinning depends not on the amount of stress applied, but on the amount of time this stress is applied. The longer the application, the thicker (or thinner) the result. In these cases, we talk about rheopecty and thixotrophy respectively. Saint Januarius blood, a famous relic in Naples, Italy, contains a typical thixotrophic fluid, most likely a Iron(III) hydroxide compound. When the vial is left undisturbed, the gel behaves as a solid. Applying movement for a while is sufficient to show non-newtonian properties, leading to a liquid. An article on Nature has been published on this regard.
Shear stress is not the only factor influencing the viscosity of a fluid. New smart materials are now under development, where viscosity can be altered by exposition to light.