The Science of Sandcastles
Jennifer Vail | Sarah Cotts
August 14, 2023
Summer’s blue skies and warm temperatures beckon us to the beach. After all, summer is synonymous with fun in the sun, surf, and sand. And what is a day in the sand without building a sandcastle? While rising tides can be every sandcastle builder’s enemy, the ocean is also their greatest ally. That’s because damp sand sticks together, allowing sandcastle magic. Dry, powdery sand may feel nice but it’s no friend of castle building and a large part of that is down to cohesion. Quick tests of moist and dry sand showed a 10x difference in cohesion!
As it turns out, sandcastles are an excellent example of powder rheology measurements. Other measurements that help tell the story of optimum sandcastle material include unconfined yield strength and flow function, all of which are easily measured with an accessory kit on a rheometer. Yes, the same rheometers used to measure viscosity and shear behavior of materials like oils, slurries, and gels.
The other big indicator of excellent sandcastle material is the Flow Function, which is found by dividing the Major Principal Stress by the Unconfined Yield Strength. When the Flow Function is over 10, the material is free flowing and not ideal for construction of sandcastles. Dry sand is better suited to flow through our fingers than it is to hold up a castle! Sand with 10% water, however, has a Flow Function of around two, so it’s better suited for staying in place. But don’t add too much water: when the sand was mixed with 20% water, the Flow Function jumped back to 10 and the resulting castle was a bit messy!
Of course, we’re not about to start packing up rheometers with our chairs and coolers to go to the beach. But sandcastle construction is surprisingly illustrative of formulation challenges that scientists face in a variety of industries, from making fertilizer to pharmaceutical tablets. Thanks to this demonstration, the next time you hit the sand, you know that a little moisture will go a long way to making it easier to build your architectural masterpiece- and why!