Solid yet liquid: common materials conceal rich physics
Physicists from the FOM Foundation and Leiden University have discovered that granular materials do not always stiffen if they have been packed sufficiently closely. This insight provides a new perspective on the hybrid behaviour of soft materials such as sand, foam and mayonnaise: how these materials switch from being liquid to solid as a result of compression works in a completely different manner than had previously been thought. Surprisingly, even if these materials strongly resist compression, under certain conditions they do not offer any resistance against flow. The common nature of sand, toothpaste, shaving foam and mayonnaise sometimes makes us forget just how rich their physics is. Now the question is how we can use this knowledge to better understand, control and manipulate their complex mechanics. FOM postdoc Dr. Simon Dagois-Bohy and his colleagues together with workgroup leader Prof.Dr. Martin van Hecke published their results last week in the leading journal Physical Review Letters. The article was highlighted as an 'editor's suggestion'. At the same time a Viewpoint commentary about this work was published in the journal Physics: physics.aps.org/articles/v5/97.
Over the past decade a lot of work has been carried out into the 'jamming transition'. This transition occurs if the particles in soft granular materials are pressed close enough together and the material becomes stiff. The most simple version of this jamming occurs if soft elastic particles are compressed without fiction or attraction. If these particles are close enough together, they will come into contact with each other at a certain moment and then both the pressure and the resistance to further compression will start to increase.
Jamming
For so-called 'strict jamming' resistance to shearing is also needed. The researchers have now observed that a denser packing does not offer any resistance to shearing: close to the jamming point the probability that the material is unstable is even 100%. Compression is therefore not enough for the material to become solid and so the jamming transition occurs in a fundamentally different manner from what was previously thought.
The percentage of packings that are unstable depends on both the pressure and the number of particles in the system. Furthermore the percentage eventually decreases to zero if the system is made large enough. Such finite size scaling reveals that the jamming transition shares certain important characteristics with thermodynamic phase transitions.
Reference
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).
Information
For further information please contact:
Simon Dagois-Bohy +31 (0)527 55 17 or Martin van Hecke +31 (0)71 527 54 82