Why does toothpaste harden?

Shutterstock.com/ Tasha Cherkasova

A simple question. Is toothpaste a liquid or a solid? What about cement? Of course, the answer depends on what stage of the material you are talking about. In the bottle toothpaste is, of course, a liquid and when it comes out and we use it, it is still a liquid. If you leave some toothpaste on your bathroom sink or the side of the tube it will soon turn to a solid. The process is similar to cement which starts as a liquid and over a number of hours turn to a very hard solid. The process that these pastes undergo is of no surprise to anyone. We are never surprised when cement turns hard and every time we are forced to clean the toothpaste off the top of the tube we don’t marvel in wonder at why it is now a solid, we just get mad at ourselves for allowing it to happen. While the result is no surprise the reason that it happens is a mystery to all of us. 

While there are many things that a mystery to the average person, science usually has an answer. I don’t understand how planes are so heavy an able to fly but science knows. Actually I don’t know how the internet works (at least not to a level that I could recreate if I was dropped in a world that had no internet) but science knows perfectly well. That is why it never worried me that I couldn’t explain why paste substances tend to harden over time. It didn’t seem to be a big deal and I was sure that science had an answer. Well, it turned out that science had no idea!

It turns out that the fact that pastes harden was a mystery in the science community as well. Scientists have known for years that pastes stiffen over time (no points for that one team) and they said that part of the reason was because of changes in the load that these materials undergo over time. However, they knew that wasn’t the entire explanation. Until recently they had no idea what was the main cause.

A new study was able to solve the question. It turns out that pastes have another process ongoing called contact-controlled aging. Basically, some of the particles in the paste come in contact with each other and this contact stabilizes their microstructure. These contacts then stiffen (the bit scientists knew about) making the entire paste now a solid. While it may sound like a small distinction, scientists say it is an important one.

The contact-controlled aging process has been largely ignored as people focused on the stiffening process to date. Now that this second process is understood these pastes can be created differently to use that process to their advantage. A wide number of materials use this process in their make up and knowing exactly how it works will fundamentally change the approach of manufacturers. 

Cement, clays, soils, inks, and paints are just a small number of materials that rely on this process. If you consider the use of cement, the fact that it becomes a solid is exactly why it is useful. We have already seen the introduction of quick-drying cement but now based on this discovery we could potentially see something like instant cement. Or we could see it used it in applications previously thought impossible.

As we come to understand the basic attributes of things and how they work we learn ways to augment and change them to our advantage. This small discovery of why paste hardens may sound like something insignificant but in reality, it could mean a huge change in the products we can produce and how they work.