Attention, beach-going children: science has something to say to you. You know that towering castle of bucket-ramparts and seashell turrets you built last week with your dad? Can't touch this. In a very poorly copy-edited but technically interesting paper
, materials scientists from Iran, France, and the Netherlands delve into the physics behind why a little bit of water transforms sand into good castle-building material. They calculate the relationship between the width of a sand-tower's base and the height it can reach and verify it by building sand skyscrapers, which you can see to the right. They estimate that a tower with a base radius of 20 centimeters can get to 2.5 meters high before buckling. The sand sticks together because water builds tiny bridges between the grains through capillary action, but the pressure exerted by the surrounding medium, in this case the air, matters too, the team notes. That means, they realize, that you could build castles underwater, to take advantage of the higher pressure there, as long as you use special water-repelling sand. In that situation, air bubbles play the role water plays on dry land, and the pressure exerted by the surrounding water helps hold things up. They built a little row of underwater Grecian pillars to prove it---take a look below. You can't try it out on the beach, unfortunately. But once you get home, you might be able to use these scientists' physics insights to impress your goldfish.
Images courtesy of Scientific Reports / NPG