X-raying Sandcastles

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”.

Granular “Splash”, new results

What happens when you drop a steel bearing ball into a fine sand? Turns out you get a granular post-impact “splash”, similar to what you would expect in a liquid, followed by a narrow upward-directed granular jet emanating from the sand. What is a bit counter-intuitive is that the air plays an important coupling mechanism in formation of such granular jet. For example, when you pump out air and repeat experiment in vacuum, the jet ceases to exist (see picture on the left).

Move to wordpress and this week’s items (4)

This is my first post since recent move to wordpress.

This week we have four items:

1. An excellent review in this week’s Science by Simon Billinge and Igor Levin titled “The Problem with Determining Atomic Structure at the Nanoscale”. The fundamental problem of determining atomic structure of non-periodic materials is an old one, but is becoming especially critical in context of nano-structures, where finite correlation lengths on the order of nanometers, even in case of crystalline structures, makes it impossible to apply standard crystallography methods applicable for materials with long-range periodicity. Authors focus on PDF, TEM, EXAFS and other techniques, but also briefly mention newly available coherent x-ray scattering techniques based on phase-retrieval algorithms.