I really want to ask: Why don’t the colors squeezed out of the colored strip toothpaste mix together?

Speaking of color striped toothpaste, I believe many people have it at home. What is puzzling is that no matter how you squeeze this color striped toothpaste, the color of the paste squeezed out is still clear and will not mix together.

Why don't these pigments mix together in the toothpaste packaging? Some people say that the reason why the colors can be automatically distinguished is because toothpaste is a non-Newtonian fluid, while others say that it is because toothpaste is a Bingham fluid. So, who can make the toothpaste color strips automatically distinguished? What is the relationship between Bingham fluid and non-Newtonian fluid? Let's talk about it below.

Color strip toothpaste, Image source: Screenshot of e-commerce platform

Newtonian and non-Newtonian fluids

First, let’s talk about Newtonian fluids and non-Newtonian fluids.

In daily life, we see various kinds of fluids, such as water, milk, juice, air, salad dressing, ketchup, etc. These fluids are actually not the same.

Common fluids such as water, milk, and juice can be regarded as Newtonian fluids. The characteristic of this type of fluid is that the shear stress is proportional to the shear strain rate. In simple terms, under the same temperature and pressure, the viscosity of the liquid has nothing to do with the external force it is subjected to.

For example, if we pour water directly from a bottle or squeeze the bottle hard to squeeze the water out, the viscosity of the water will hardly change.

With the analogy of Newtonian fluid, non-Newtonian fluid is easier to understand. It refers to the fluid whose viscosity is related to the external force it is subjected to.

There are three types of non-Newtonian fluids: shear-thickening fluids (dilatant fluids), shear-thinning fluids (pseudoplastic fluids), and Bingham fluids.

1

Shear thickening fluid

As the name suggests, this type of fluid is characterized by increasing viscosity when the external force increases. In daily life, a mixture of corn starch and water is more likely to be seen.

If you are interested, you can try it. Mix water and corn starch in a ratio of 1:1.25 to 1:1.3. The resulting starch paste is a shear-thickening non-Newtonian fluid.

When the force is relatively small, its viscosity is not high, and you can easily stir it with your fingers. But if you suddenly apply a relatively large force to it, such as punching it hard, it will suddenly become "hard" due to the external force and its viscosity will increase.

But toothpaste obviously does not belong to this category of non-Newtonian fluids, otherwise the harder you squeeze the toothpaste, the harder it would be to squeeze it out.

2

Shear thinning fluid

In contrast to shear-thickening fluids, the greater the external force on this non-Newtonian fluid, the less viscous it becomes, that is, it becomes thinner. In fact, it is also very common in daily life. Human blood, ketchup, peanut butter, etc. are all this type of non-Newtonian fluid.

If you have bottled ketchup at home, you can give it a try. If you pour it directly from the bottle, the ketchup will be very sticky and difficult to pour out. But if you shake the bottle while pouring it out, the ketchup will become thinner and will flow out by itself (be careful not to splash it on others).

Toothpaste can be squeezed out after being subjected to external force, which looks a bit like this type of non-Newtonian fluid, but it is not actually a non-Newtonian fluid. Toothpaste belongs to the third type of non-Newtonian fluid - Bingham fluid.

3

Bingham fluid

The mathematical model of this fluid was proposed by Eugene C. Bingham, so it is called Bingham fluid.

This fluid is different from the shear-thinning fluid mentioned above. When the external force on the shear-thinning fluid increases, the viscosity will slowly decrease (that is, the fluidity will slowly increase).

When there is no external force or the external force is relatively small, Bingham fluid shows the characteristics of a solid. Before reaching the critical value, it will not show fluidity. When the external force reaches the critical value, it will suddenly show fluidity. Toothpaste belongs to this type of fluid.

During normal storage and transportation, the toothpaste is subject to very little external force and can almost be regarded as a solid. Only when it is squeezed by external force will it be squeezed out like a viscous liquid. And once the external force stops, the toothpaste inside will not continue to flow out, so the toothpaste will not be spread everywhere.

Of course, if the squeezed toothpaste accidentally falls into the sink, it will not flow away by itself, but will stay there like a solid until you clean it up.

Bingham fluid allows color strips to be “automatically isolated”

Knowing the characteristics of Bingham fluid, we can understand why the color stripes can be "self-isolated". When there is no external force or insufficient external force, the color stripes of different colors can be regarded as solids, and they will certainly not mix with the colors next to them.

After being squeezed, the toothpaste can flow, but it is still relatively viscous, so it can appear on your toothbrush in the form of separate strips.

Of course, there are some color-striped toothpastes whose colored and white parts are distributed front to back, and their caps have special structures inside.

A specially designed toothpaste cap, Image source: Reference 4

When squeezing, the white toothpaste at the back is squeezed out from the thick hole in the middle, while the colored toothpaste at the front is squeezed out from the smaller holes next to the thick hole. In this way, the squeezed toothpaste can also appear in the shape of colored strips.

Special color striped toothpaste cap, Image source: Play with me - Toys for Kids

The color of the small square at the end of the toothpaste.

Can you tell whether toothpaste ingredients are natural or chemical?

When searching for content related to "colored strips" in toothpaste, I also saw a rumor related to colored strips, which I would like to explain to you here.

I don't know if you have noticed that there is often a small "color stripe" square at the end of the toothpaste package. There was once a widely circulated rumor about it: the color of the small square at the end of the toothpaste with pure natural ingredients is green; the color of the small square at the end of the toothpaste with natural ingredients and medicine is blue; the color of the small square is black, which means that the toothpaste is completely chemical.

Image source: Science Popularization China

This correspondence between the tail label and the color is completely unfounded. The colored label at the tail of the toothpaste is only to facilitate the machine to locate the tail of the toothpaste and cut and package it based on this position.

The different colors are only for the convenience of distinguishing from the packaging color and facilitating machine recognition, and have nothing to do with the ingredients of the toothpaste. If you look closely at the ingredients of the toothpaste, this rumor will be self-defeating. In the ingredients of the toothpaste with a green label, there are also a lot of so-called "chemical substances" written prominently, so there is no basis for judging the ingredients based on the color of the label, and there is no need to panic about "chemical substances" itself.

References

[1]https://nvlpubs.nist.gov/nistpubs/bulletin/13/nbsbulletinv13n2p309_A2b.pdf

[2] https://www.colgate.com.hk/oral-health/selecting-dental-products/what-is-a-toothpaste-color-code

[3] https://www.sciencefocus.com/science/striped-toothpaste

[4] https://patents.google.com/patent/US5324505A/en

[5] http://www.cocia.org/detail/985/

Planning and production

Author: Science Scraps Popular Science Creator

Review丨Sun Mingxuan, Professor of Shanghai University of Engineering Science

Planning丨Zhong Yanping

Editor: Zhong Yanping

Proofread by Xu Lailinlin

Note: The cover image is a copyrighted image. Reprinting it may cause copyright disputes.