The science of sandcastles

04 July 2017

Interview with 

Matthew Arran, Cambridge University

SAND-CASTLE

Sandcastle on the beach

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To kick of Marine Month the Naked Scientists are taking a virtual trip to the beach, by brining the sand to the studio. Granular materials scientist Matthew Arran from the University of Cambridge is an expert on the stuff. He explained how to make a supreme sandcastle and explained to Chris Smith exactly what it's made from... 

Matthew - Most sand is lots of rocks that have been ground down over millions of year in the oceans or in the rivers. It can be corals that are ground down on some tropical beaches, or in the case of Iceland or some bits of Scotland it can be volcanoes that have been ground down to make black sand.

Chris - And that’s why the sand differs in colour, and texture, and composition?

Matthew - Yes, exactly. And that turns out to be important for sandcastles too.

Chris - Speaking of which… what have you got in front of you?

Matthew - In front of me I have a big metal tray like you’d use for doing a roast. In the tray I have three plastic cylinders that are about 20cms high, and in each cylinder I have some sand as you’d expect. In one tube it’s dry, in another tube I have it completely saturated with water just as you’d find at the beach. In the third tube I have it half saturated with water. So the key is to show what amount of water do you want to add to build the best sandcastle?

Chris - We’ve got this tube. It’s an open top and bottom cylinder three quarters full of sand and you’re going to lift it up. Well the sand in there’s dry, and I would speculate that in the same way I’d turn my salt cellar over and salt comes out, I suspect the sand is just going to go everywhere.

Matthew - Exactly. So dry sand doesn’t work for sandcastles.

Chris - Okay. I didn’t need to have a PhD to know that one. I think that was relatively easy. So which one are we going to do next?

Matthew - Now wet sand should work so I’m just going to top up this cylinder so it has lots of water in It has as much water as the sand can hold.

Matthew - And I just pull up the tube… and we see it slumps out at the bottom.

Chris - It did come out in a sort of sausage shape, but then it quite quickly just, as you say, slumped and it has flowed almost like it was a liquid over the bottom of the tray. We know have a very poor representation of a sandcastle from that one.

Matthew - It’s not what you’d do.

Chris - So is the goldilocks - the just right one?

Matthew - Yeah. If you have a mix of about a quarter as much water as you have sand, and you fill the tube with it. I’m just going to tap this down to stop it coming up with the tube.

Chris - So the magic ratio is a quarter as much water as sand?

Matthew - It depends on what you are trying to achieve with your sandcastle, but a good bet is about a quarter. So if I pull up…

Chris - You’re just drawing the tube upwards… it’s holding. It’s looking pretty good. So what we’ve basically got is a sandcastle about the shape and size of a big beer can.

Matthew - Yeah.

Chris - And it’s holding. It’s standing there. So granular material scientist that you are, why does that work and yet the one that had lots more water - clearly water is important because it’s helping this one to stand up so beautifully. Why was too much water bad and no water terrible?

Matthew - The key to this is surface tension. This is the same force that keeps the droplets on your tap from falling down immediately and sand is interesting because about half of it is just empty space, so when you add water to sand that water can fill in the empty space.

With the dry sand you have no water, so you have no forces bringing these grains of sand together and so it just flows out. When the sand is full of water, all of the water’s there so you have no surface tension because there’s no air. There’s no gaps and the water can just flow along with sand and it flows out.

But when you have a mix of about a quarter as much water as you have sand, then the water forms bridges between the grains. The surface tension brings the sand grains together making it cohesive.

Chris - It’s fabulous to now know how I can make such an amazing sandcastle Matt. But the thing is I don’t want people to go away thinking that you do nothing but gratuitous sandcastles for research because this is really important. There is an important science side to this as well, isn’t there?

Matthew - This sort of cohesion and the strength constrained by water is very relevant to coastal erosion or landslips. We need to know how wet a soil needs to be before it collapses and flows, which can damage homes and people’s livelihoods.

Chris - Obviously you don’t go building sandcastles in the laboratory very often, or do you? Is there not say a computer model that you could build for these sorts of interactions in order to work out how much sand to add to your cement, and how runny it’s going to be, or how likely the hillside is likely to collapse if we have a heavy rainfall for example?

Matthew - Famously, there are an awful lot of grains of sand on a beach, so it’s very hard to be able to computer model that includes all of them, or even any significant number. In this tube there’s going to be about a million, at least, grains of sand and you can’t build a computer model that takes all of the physics into account for every one of them. So sometimes we have to do actual experiments just to work out the basic physics that are going on.

Chris - It’s good old fashioned leg work, and spade work then.

Chris: Thank you Matthew Arran, from Cambridge University.

Chris - That’s the science of actual granular materials Danni, but what about the biology of the sands and sediments, that’s pretty important too surely?

Danni - It’s really important actually. It’s been estimated that in just less than one gram of sand there’s over 2,000 different species of microbes. Which isn’t really surprising if you think about it. There’s probably even more in mud. But these microbes actually excrete extracellular polymeric substances and these help to cement and glue together the sand grains, which is really important for stabilising sediments. This is why they’re trying to stop people from driving four wheel drives over sandy beaches because it breaks this bond, and means it can lead to more erosion as well.

Chris - The sea doesn’t do that itself when the tide comes in?

Danni - It does as well but further up on the shore where you only get the tide reaching at really high tides, it’s quite important to have stabilised sediments.

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