Why anyone can build a sandcastle

Researchers at the Max Planck Institute for Dynamics and Self-Organization in Göttingen unravel the complex structure of wet granulates

February 11, 2008

For the construction of a sandcastle skill and imagination are as necessary ingredients as the water, which transforms the sand into a mouldable material. Surprisingly, however, nobody needs to follow a recipe for the precise amount of water: the mechanical properties of the wet sand are quite independent of the liquid content. Researchers from the Max Planck Institute for Dynamics and Self-Organization in Göttingen, the Australia National University, the University of Erlangen, and the ESRF in Grenoble have now studied this phenomenon on a microscopic level using x-ray micro tomography. They discovered that when the liquid content of the mixture is increased from less than 1% to well above 10%, the distribution of the fluid between the grains of sand changes dramatically. The mechanical stiffness, however, remains practically constant. (Nature Materials, published online on February 10th, 2008).

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In medicine, x-ray micro tomography is also known as computer tomography. Scientists irradiate an object with x-rays from various angles to produce two-dimensional images. These images are subsequently analyzed by means of a computer, which reveals the three-dimensional structure of the object under study. When scientists use a bright x-ray source like the synchrotron source at the ESRF in Grenoble, the computer tomography yields a spatial resolution of about one thousandth of a millimetre. This is sufficient to resolve the tiny and highly complex fluid structures that form inside moist granules.

What the research team found by analyzing these three-dimensional images is quite astonishing: The fluid does not fill the granulate structure completely, thereby forcing all of the air out of the tiny spaces between the grains. Instead, fluid and air coexist in the mixture, forming a delicate geometry.

In hindsight, the reason for this type of distribution is straightforward to understand. As the fluid coats the grains, it tries to surround itself with as much "grain" as possible. This is best achieved where two grains touch. The "empty" space in between is relatively unattractive for the fluid and can therefore be filled with air.

As the scientists carried out more exact studies, they were surprised again: not only did all the structures exhibit the same pressure; the pressure was also independent of the fluid content. This explains the universal stiffness of the material. The constant pressure corresponds to a constant force inside the structure and therefore leads to the same mechanical properties.

"These properties are not only significant for the construction of sandcastles", says Stephan Herminghaus from the Max Planck Institute for Dynamics and Self-Organization. They are also relevant to the pharmaceutical and food-production industries and help to understand certain natural catastrophes such as landslides. Herminghaus adds: "Wet granulates are relevant in very many fields, and now we have a better understanding of their mechanical properties."

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