Non-Newtonian Fluid
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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 sip of theory, we might ask ourselves whether this has any practical relevance. Newtonian fluids? Of course—we encounter them every day: we breathe one of them, and water is no stranger to us. So, there’s nothing particularly special about them. But what about those exotic non-Newtonian ones? Well, they’re not as exotic as you might think—you encounter them all the time. One example of a shear-thinning non-Newtonian fluid is ketchup in a bottle: it flows more easily when the bottle is shaken or tapped. Another example is whipped cream, which also shows lower viscosity when poured. However, in this experiment, we’re more interested in 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 water in small amounts. Initially, the cornstarch will absorb the water, but at some point, you will reach the stage of a non-Newtonian fluid. This is easy to recognize because the faster you try to stir, the harder it becomes. In extreme cases, the fluid can behave like a plastic solid (tearing and breaking into pieces, which then "melt" back into a uniform liquid when stirring stops). When stirred slowly, the fluid behaves like a relatively thin liquid. You can add a little more water if needed (but only a small amount, as adding too much water will destroy the non-Newtonian properties, causing the fluid to behave like a regular liquid). The mixture should be stirred frequently to prevent separation. Below is the prepared non-Newtonian fluid:
Playing with the freshly made goo is quite fascinating. When you hit the surface of the liquid quickly and forcefully with a spoon or another object, it resists like solid rubber. However, if you touch it gently, the spoon will sink in as if it were a regular liquid. Now imagine a pool filled with non-Newtonian fluid—by quickly and forcefully striking its surface with your feet, you can run across it! However, the moment you stop, the fluid around your feet becomes less viscous and behaves like a regular liquid, resulting in sinking.
Another interesting property is that you can mold this mixture into various shapes, just like clay. However, this can only happen under dynamic conditions or pressure—you need to act quickly. As soon as you stop moving or applying pressure, the fluid begins to flow, leaving a puddle behind.
Pour some of the fluid onto a flat, smooth countertop. It will look like a white puddle, similar to an ordinary liquid. However, try quickly scooping it up with a spoon or your hand. Surprisingly, the spilled fluid can be moved across the countertop (as long as you do it quickly) and can even be grabbed and lifted.
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 with power matching the amplifier, preferably round, with a plastic diaphragm (it will likely be damaged during the experiment, so avoid using new equipment—use something you were planning to discard)
- 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 (these are dynamic conditions—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—only 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