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How to Build the Perfect Sandcastle, According to a Physicist

Motherboard interviewed Daniel Bonn, a physicist from the University of Amsterdam, about how to build sandcastles that will make you the envy of the beach.
August 13, 2015, 2:27pm

Physicists investigate the nature of the universe. Quarks, string theory, the multiverse, all important lines of inquiry that will advance our understanding of the world around us. But there's been one field that they've ignored, until now. How do you build the perfect sandcastle?

Three years ago, an international team of physicists and material scientists from universities in Belgium and the Netherlands finally tackled this age-old question. They ran experiments to determine the ideal composition of a sandcastle, and published their findings in Nature.

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The team filled cylindrical PVC pipes of different diameters with sand, then sliced the pipes down the sides and removed them, creating columns of sand of varying heights.

The observations from their experiments confirm what any veteran sandcastle builder already knows. Building a castle with a wider base allows you the support to build it higher, just like any other structure. In fact, if you can wet and condense the sand in the appropriate amounts, there's no theoretical limit to how tall and how large you can get your sandcastle. It might seem like just a hobby, but the act of building a good sandcastle shares a lot of similarities with any other civil engineering project.

Motherboard interviewed Daniel Bonn, a physicist from the University of Amsterdam, about how to build sandcastles that will make you the envy of the beach.

MOTHERBOARD: You helped author a paper several years ago titled "How to Build the Perfect Sandcastle." An interesting subject matter for a group of physicists to tackle. What was the inspiration for this research?
Daniel Bonn: We were trying to understand the mechanical properties of wet sand; this is of tremendous importance for all kinds of applications (building houses, roads and bridges on sandy soils, or the very Dutch problem of the stability of dikes [levees], for instance).

You mention that "capillary bridges" between sand grains are responsible for how sand and water interact to make a structure that can support its own weight. Can you briefly explain what capillary bridges are, and what it has to do with the interaction of sand and water?
Water drops or soap bubbles prefer to be spherical because this shape minimizes the total surface area—creating surface area costs energy. Likewise, a small amount of water between two sand grains forms a small liquid bridge that minimizes the surface area between the water and the air. If one then moves one grain with respect to the other, one automatically creates surface area. This costs energy, and therefore, there will be a resistance to deformation.

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One can feel this easily if one puts a drinking glass with a flat bottom in the kitchen sink. If the sink is dry, it is easy to lift the glass. However, if one puts a drop of water between the glass and sink, this becomes very hard.

What factors make for a 'perfect' sandcastle, that is both impressive and structurally sound?
One needs the optimum amount of water, but one also needs to very carefully compact the sand. Professional sandcastle builders use thumpers [machines designed to mechanically compact sand] all the time to make sure the sand becomes very compact. For the characteristics of the sand, see below. For the rest you'd have to ask a professional…

Vibroseis trucks, or "thumper trucks," use heavy metal plates to compact the ground or create a seismic source. Image: Noble Energy/Wikimedia

We know that the radius of a structure, how dense its sand is packed, and the composition of the sand all make a difference. Are there any other influences that builders have to consider, like weather, atmospheric pressure, saltwater vs freshwater, etc.?
A high air humidity and low temperature help to prevent evaporation of the water; again, professional builders have spraying systems to keep the sand moist. Saltwater undoubtedly helps: Should the water dry, salt crystals will form that generally stick well to sand grains, so these too can act as a glue.

I know geology isn't your focus, but are there other types of sand—like hydrophobic sand— that are better than others for this purpose?
I don't think hydrophobic sand exists in nature, although sand contaminated with crude oil from near oil wells might do the trick. Angular grains seem to help, clays that are naturally present in river sand also help, and having small grains with a wide range of grain sizes is also good.

You mention in the discussion section that, for typical beach sand, a castle column with a radius of 20 cm can get as high as 2.5 meters. Based on your findings, is there any upper limit, a point where increasing the radius of the base of your structure does not return you any increase in height?
No, not from the point of physics, in any case. But if you would want to construct the tower of Babel, you'd probably get into other trouble such as the fact that the temperature and humidity change when you go very high….

Based on your observations, what's the best way to get that kind of dense packing necessary under normal conditions? Any tools or tricks that would be helpful?
Thumpers! Thumper trucks, ideally. and leaving the sand lying around for a long time also helps a great deal: this is what one usually does e.g., for building bridges.

This interview has been condensed and edited for clarity. Hell or Salt Water is a series on Motherboard about exploring and preserving our oceans. Follow along here.