Chart. KMODDL - Kinematic Models for Design Digital Library. Why sandcastles are so easy to build - fundamentals - 11 February 2008. Every child knows that you don't have to follow an exact recipe to build a sandcastle. All you need is sand plus a splash of water - and now physicists understand why. Mario Scheel at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, and colleagues studied how a liquid squeezes between grains. Using a series of X-ray images to build up a three-dimensional picture of the sand pile, they found that with the driest sand-and-water recipe, the grains were linked by liquid bridges shaped like a double-ended trumpet.
When enough of these bridges form, the mixture is able to hold its shape. From then on adding more liquid doesn't make much difference, unless so much is added that the mixture becomes saturated. "The liquid goes into the crevices and fuses the bridges together," explains Martin Brinkmann, one of Scheel's collaborators. The result could be applied to many different liquids and granular materials, and help us understand other mixing processes. Sixty Symbols - Physics and Astronomy videos. Middle World.