Non-Newtonian Fluid
| Polish version is here |
A Very Unusual Liquid - A Bit of Theory
According to the definition, a fluid is any substance that can flow, meaning it can change its shape depending on the container it is in and can also move freely (flow). It is important to emphasize that the concept of a fluid is broader than that of a liquid, as it also includes all gases and mixtures composed of different physical phases, such as foam, emulsions, or suspensions.
The primary measurable property of fluids is their viscosity (not to be confused with density, as often happens in everyday language). Viscosity, also known as internal friction, is a property of fluids that describes their internal resistance to flow. It is not the resistance that occurs at the boundary between the fluid and the container walls, but rather the resistance between the fluid particles themselves.
Based on the relationship between shear stress and shear rate, there are two main types of fluids:
- Newtonian
- Non-Newtonian (with many subcategories, including plastic-viscous, pseudoplastic, thixotropic, and others)
In Newtonian fluids (perfectly viscous), the shear stress τ is directly proportional to the shear rate γ, as shown in the graph below. This is known as the flow curve:
This type of model accurately describes fluids such as water and gases.
The second category of fluids does not follow this linear relationship, meaning they do not obey Newton's law! The viscosity of non-Newtonian fluids is not constant under steady pressure conditions and changes over time. According to this definition, the flow curve of such a fluid is non-linear. These fluids can be divided into:
- Shear-thickening (the more force you apply to make them flow, the more viscous they become)
- Shear-thinning (opposite to the previous case; they exhibit lower viscosity under dynamic conditions)
Enough Theory - Let's Get Practical!
After this brief dose of theory, we might wonder whether it has any real practical relevance. What about Newtonian fluids? We encounter them every day. Air is one example, and water is another familiar case. There is nothing particularly unusual about them. But what about the so-called exotic non-Newtonian fluids? In reality, they are not as rare as they might seem, and we come across them quite often. A good example of a shear-thinning non-Newtonian fluid is ketchup, which flows more easily when the bottle is shaken or tapped. Whipped cream behaves in a similar way, showing lower viscosity when it is poured. In this experiment, however, we will focus on a shear-thickening fluid, specifically a simple mixture of cornstarch and water.
To proceed with the experiment, you will need a small container to mix the ingredients, along with a portion of cornstarch and cold water (at or below room temperature).
Pour a certain amount of cornstarch into the container, about a quarter of a cup to start with. Then, while stirring continuously, slowly add small amounts of water. At first, the cornstarch will simply absorb the water, but after a while you will notice a change as the mixture turns into a non-Newtonian fluid. You can easily recognize this moment because the faster you stir, the harder it becomes to move the spoon. In extreme cases, the mixture behaves almost like a plastic solid, breaking into pieces that melt back into a smooth liquid when you stop stirring. When mixed slowly, it behaves like a fairly thin liquid. If needed, you can add a bit more water, but only in small amounts, because too much will make it behave like a regular liquid and lose its special properties. Stir the mixture often to keep it uniform. The photo below shows the finished non-Newtonian fluid.
Playing with the freshly made mixture is surprisingly fascinating. When you strike the surface of the liquid quickly and with force using a spoon or another object, it resists like solid rubber. If you touch it gently, however, the spoon sinks in as if it were an ordinary liquid. Now imagine a pool filled with non-Newtonian fluid. By striking the surface quickly with your feet, you can actually run across it. The moment you stop, though, the fluid around your feet becomes less viscous and starts behaving like a regular liquid, causing you to sink.
Another interesting property is that you can shape this mixture into different forms, much like clay. This only happens while the fluid is under pressure or in motion, so you need to work quickly. Once you stop moving or applying force, the mixture begins to flow again and spreads out into a puddle.
Pour some of the fluid onto a flat, smooth countertop. At first, it will spread out like a white puddle, resembling an ordinary liquid. If you try to scoop it up quickly with a spoon or your hand, you will find that it can be moved across the surface and even picked up, as long as you act fast enough.
The Fun Part - Bringing the Goo to Life
Now we come to what I consider the most fascinating and visually impressive phenomenon you can achieve with this type of fluid. However, this requires a bit more preparation than the previous experiments. You will need:
- Freshly prepared non-Newtonian fluid (slightly thicker)
- An audio amplifier
- A speaker, preferably round and with a plastic diaphragm. It will likely be damaged during the experiment, so avoid using a new one and choose one you were planning to discard instead.
- An audio frequency generator (preferably producing sine waves, with a range starting from 1 Hz; computer-based software generators work well)
The setup is very simple: connect the speaker to the amplifier as usual, then connect the amplifier to the frequency generator (e.g., the sound card output). When the amplifier and generator are turned on, the speaker diaphragm will vibrate at the selected frequency. Next, pour a small amount of non-Newtonian fluid onto the diaphragm (if you want to protect the speaker, you can place a thin sheet of plastic foil over the diaphragm before pouring the fluid). The setup should look something like this:
After pouring a small amount of fluid onto the diaphragm, turn everything on. By adjusting the frequency, you can create a standing wave in the fluid (at different points in the fluid, the vibration forces will differ, so the viscosity of the fluid may vary significantly within short distances. This is the basis of the observed effects). Sometimes, the phenomenon needs a trigger, such as blowing air onto the fluid’s surface through a thin tube or gently stirring it. Other times, it occurs spontaneously. The fluid’s surface becomes uneven; some areas begin to collapse, while others rise, forming finger-like protrusions that can extend quite high and move around. It looks truly fascinating. The whole scene resembles some kind of living creature, like an amoeba. Occasionally, a portion of the fluid separates from the rest and astonishingly begins to climb up the sloped wall of the diaphragm. Notice that these upward movements defy gravity, occurring at the expense of the speaker diaphragm's vibration energy. When the vibrations stop, all effects cease, and the fluid settles at the bottom of the diaphragm to "come alive" again when vibrations are resumed. Below is a video showcasing the results I achieved:
Enjoy!
Further readings:
- Rao M.A., Rheology of Fluid and Semisolid Foods: Principles and Applications (2nd ed.), Springer, 2007, pp. 8
- Schramm L.L., Emulsions, Foams, and Suspensions: Fundamentals and Applications, Wiley VCH, 2005, pp. 173
- Tropea C., Yarin A.L., Foss J.F., Springer handbook of experimental fluid mechanics, Springer, 2007, pp. 661-676
Marek Ples