I was asked a while back why the colours in striped toothpaste don’t mix inside the tube or even when you put it on your toothbrush. What an interesting question?! Certainly not something I have ever thought about before. Isn’t it interesting how we can use something daily and never notice the intricacies about it that makes it work?
It turns out that the colours stay separate due to a few different things, including the design of the tube to spread pressure evenly and the design of the hole where it squeezes out, but the most important factor that keeps the colours separate, is viscosity. Viscosity is the measure of a substance’s resistance against deformation. It is very closely related to the fluidity of a substance or liquid and is basically the “thickness” of the liquid. For example, syrup has a higher viscosity than water. It is not the same as density, which is the reason why oil floats on water, and I will explain the difference between density and viscosity in a later article. Each stripe in the toothpaste is thick enough to, at the right pressure and temperature, press against the other colours rather than to mix with it. It is basically just not “liquid” or fluid enough to mix.
Viscosity, the thickness of a substance, have many applications in our everyday lives that we are often unaware of. 1) In the formulation of medicine, viscosity affects the consistency of ointments and liquid medicines such as cough mixtures, making it easier or more difficult to swallow or spread. 2) In the food industry, viscosity is key to creating a wide range of products. It affects the texture of sauces, soups, and other products. Texture is the main contributor to the “mouthfeel” of different types of food. 3) It affects the flow of crude oil through pipelines and has a direct effect on the methods used for extraction of oil from reservoirs. 4) Viscosity is crucial in the paint and coatings industry, affecting how easily paint can be applied and how it adheres to surfaces. 5) Engine oils and lubricants need to have the right viscosity to ensure proper lubrication of engine components. 6) In the production of cosmetics and personal care products, such as lotions and creams, viscosity is crucial for product stability and user experience. Trying to wash your hair, for example, with shampoo that is either too thick to come out of the bottle, or too thin to stay in your hands, are almost impossible. 7) In inkjet printers, the viscosity of ink affects droplet formation and the quality of printed images. 8) Viscosity plays a role in the design of aircraft and spacecraft fuel systems. It is essential for ensuring the proper flow of fuel and lubricants in extreme temperature and pressure conditions. 9) In environmental monitoring, it is a crucial component in testing water quality. The viscosity of water gives an indication of the presence of oil, sedimentation and other pollutants in the water.
This is just a short summary to show you just how important the role of viscosity is in any of these applications. That is the wonderful thing about science. We can all continuously use different applications of scientific facts to make our lives easier and more comfortable, without having to know much about the science behind it. Let me know what questions you have regarding an everyday, or more intricate, phenomenon that may be rooted in science. I would love to do my part to find the answers that allow all of us to appreciate science a little bit more.
Written by Dr. Helen Prinsloo
