Why does sprinkling a little water enable sand to stick together sufficiently to enable the building of sandcastles? The cohesive forces produced are strong enough that one can build a tower that is 2.5 meters high on a mere 20 cm base. A group of scientists have looked into this and come up with an explanation.
The formation of capillary bridges between sand grains are the cause of the stiffness of sculptured wet sand in a sandcastle, as opposed to dry sand which can hardly or not support its own weight. Qualitatively, the liquid leads to the formation of capillary bridges between the sand grains, and the curvature of the liquid interface leads to a capillary pressure causing a force of attraction between the grains. This then creates a network of grains connected by pendular bridges, and allows, for example, creating complex structures such as sandcastles.
In science, whenever one proposes an explanation for something, that explanation becomes more plausible if one can use that same mechanism to predict a new result that may not have struck anybody before the explanation was proposed. (This was, and still remains, the big failure of so-called intelligent design creationism. Its advocates do not even suggest a mechanism for how it works, let along a prediction for a new effect using that mechanism.)
In this case, what these scientists realized was that you could build even bigger castles underwater, to take advantage of the higher pressure there, provided you appropriately reversed some properties by using water-repelling sand.
For normal sand however, this will destroy the liquid bridges between grains and thus the strength of the material. However, if hydrophobic sand is used the roles of water and air interchange completely. In this case the air and not the water “wets” the grains and we can simply interchange water and air, which does not change the bridge force. Since the force between beads remain constant, but the effective density of compacted sand changes from 1.6 g/cm3 to 0.6 g/cm3 when immersed in water. This makes it possible to build underwater sandcastles, which are even more spectacular than normal ones.
And here is one.
Pretty neat, huh? Now fish can take their kids to the beach and build sandcastles too.