X-rays reveal mystery of sand castles
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Building sandcastles is child's play, as any beach-goer knows. Now scientists have explained why: you just add water.
Using x-ray imaging, the scientists show why you can sculpt structures from almost any sand, so long as it is not bone dry.
The work appears online in the journal Nature Materials.
"You don't need a recipe to build a sandcastle," researcher Dr Adrian Sheppard from the Australian National University in Canberra said.
"Properly dry sand doesn't stick together at all. But any kind of wet sand, from fairly wet to very wet or in fact relatively dry, behaves almost the same way."
To understand why this should be, Dr Sheppard and colleagues in Germany and France used a technique known as x-ray microtomography to get a closer look.
The researchers, led by Professor Stephan Herminghaus from the Max Planck Institute for Dynamics and Self-Organisation, took thousands of x-ray images of wet sand in a small tube.
Using computer software, they combined those images to create a detailed 3D view of how the water and sand were interacting.
They then repeated that process with smaller and smaller amounts of water.
"We looked at exactly where the water was and what kind of shapes the water formed," Dr Sheppard said.
"Essentially, what we learned was that the water was forming lots of small bridges that connect right across the structure."
Although those water bridges became thinner as the water concentration dropped, "their strength was almost independent from how wet the sand was", Dr Sheppard said.
He says the work has implications beyond the beach.
For example, the results might be valuable for the minerals processing and pharmaceutical industries, where powders and granular materials are big business.
"Dealing with things in powdered form is of enormous importance in all sorts of industrial applications," Dr Sheppard said.
"You need to know how they're going to behave, and very small amounts of moisture can affect that in a big way."
