Here Is Why Toothpaste and Cement Harden over Time

An international collaboration has unveiled exactly what happens to certain pastes over time.
Fabienne Lang

You know that toothpaste hardens over time as you see it in the cap of your old tube, and that cement solidifies from liquid to solid after a few hours. 

What wasn't fully understood until now, however, was exactly what was happening in these pastes' structures that brought this change on.

An international collaboration between a University of Delaware professor and a team from the Ecole des Ponts and Université Paris-Est in France has now got to the bottom of exactly why this happens. 

Their findings were published in Nature Materials


Dense colloidal suspensions

Another name for paste materials is "dense colloidal suspensions" and it's been well-known that these stiffen as they age. This partly happens because of structural dynamics, or changes in the loads these materials undergo with time. But for years, scientists have been scratching their heads trying to understand what else goes on in these materials. 

The international team found out that the other process that happens in these pastes is what is called contact-controlled aging, which helps to explain age-related changes in them. 

Here Is Why Toothpaste and Cement Harden over Time
Professor Eric Furst from the University of Delaware, Source: Joy Smoker/University of Delaware

The team discovered that some contacts form between particles in the pastes that ultimately stabilize their microstructure. Then, these contacts stiffen, making the entire paste become solid. 

Professor Eric Furst of the University of Delaware explained "When people think about aging in materials and the mechanical properties of materials as they age, especially in rheology or the study of how things flow, this mechanism has been overshadowed by changes in the organization, or microstructure, of the material."

This is a novel discovery that will most likely be extremely useful. Most importantly, it'll help people who use these pastes to design better ways to predict and minimize unwanted changes over time. This includes cement, clays, soils, inks, paints, and more — materials used over a wide range of industries.

"This paper has some broad-ranging implications because there are a lot of types of problems out there where this type of contact aging may be really important," said Furst.

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