This week the featured article is “Morphological clues to wet granular pile stability” by M. Scheel (advanced Nature Materials publication).
As every child playing on the beach quickly learns, there’s a magic degree of “wetness” of the sand to create most robust sand-castles – too dry and the sand structure doesn’t hold with individual sand grains falling apart (water acts like a cohesive element), and if sand gets too wet, the structure easily collapses under its own weight.
Using high energy x-ray microtomography Scheel of Max Planck institute and coworkers obtained detailed 3D images of grain configuration, allowing them to map out a phase diagram of wet granular matter (like sand) as a function of liquid content. The microscopic local particle-particle configuration changes dramatically as the wetness is varied, and macroscopic mechanical properties, such as yield stress and tensile strength, can be derived from the local packing of grains.
Update (2/25/08): this article is now featured on cover of Nature Materials
It is also accompanied by the News and View item by Arshad Kudrolli “Granular Matter: Sticky sand”.