The rich physics of toothpaste and mayonnaise

Researchers led by Leiden physicist Prof. Martin van Hecke have discoverd that granular materials such as toothpaste, sand, shaving foam and mayonnaise do not always stiffen if they are tightly packed. Even if these materials resist compression, they often do not offer any resistance against flow.

Complex mechanics

This discovery provides a new perspective on the hybrid behaviour of a number of everyday soft materials: how these materials switch from liquid to solid as a result of compression works in a completely different manner from had previously been thought. Surprisingly, they do not offer any resistance to flow under certain conditions, whereas they do exert great resistance to compression. This is how toothpaste, shaving foam, sand and mayonnaise exhibit their rich physics. The question now is how this newly acquired knowledge can be used to understand, control and manipulate the complex mechanics of these materials.

Publication in Physical Review Letters

FOM postdoc Dr Simon Dagois-Bohy of Leiden University and his colleagues published together with workgroup leader Prof. Martin van Hecke, who is a professor at Leiden University in the field of Condensed Matter Physics, their results last week in the renowned journal Physical Review Letters. The article was selected as the editor’s suggestion. A Viewpoint was also published about this work in Physics.

Jamming transition

In the last decade much work has been carried out on the ‘jamming transition’. This transition occurs if the particles in soft granular materials are pressed close enough together and the material becomes stiff. The simplest version of jamming occurs if soft elastic particles are compressed without friction or attraction. If these particles are close enough together they come into contact with each other at a certain point, and then both the pressure and the resistance to further compression starts to increase.

Strict jamming

For what is known as ‘strict jamming’ resistance to occur shearing is also needed. The researchers have now seen that a denser packing does not offer this: close to the jamming point the probability that the material is instable is as great as 100%. Compression is therefore not sufficient for the material to become solid, and the jamming transition is therefore fundamentally different from was previously thought.

As long as the system is big enough...

The percentage of packings that are instable depends on both the pressure and the number of particles in the system. Furthermore, the percentage ultimately decreases to zero if the system is made large enough. Such finite size scaling demonstrates that the jamming transition shares important characteristics with thermodynamic phase transitions.

Soft-Sphere Packings at Finite Pressure but Unstable to Shear, S. Dagois-Bohy, B. P. Tighe, J. Simon, S. Henkes, and M. van Hecke, Phys. Rev. Lett. 109, 095703 (2012).