Walking along the beach presents us with a curious paradox: when we lift our feet from the wet sand we see how the sand appears briefly dry. If we stop to think about it a bit, it is intriguing. Pressing with our foot the sand should react like sponges when we squeeze them: oozing water. However, we do not see that thin layer of water on top of our footprint but on the contrary, it appears dry. Anyone can do the following experiment. Sand and some water are introduced in a flexible plastic boat. By squeezing the walls, the water disappears into the arena. This happens because by pressing the sand, the grains are redistributed, which leads to an increase in volume between them. There is more space for the water and, therefore, it fits more.
The interesting thing is that this silly observation is very important when it comes to making the most of the oil or wine extraction process : higher pressure does not necessarily produce an extraction of the liquid.
And since we are talking about the relationship between water and sand, there is something that kids discover right away: that if they want to build a castle in the sand, they must use wet sand… in the right measure. If they use dry sand, the most they get is a scattered mound; if they use too much water, as happens when a wave comes, the construction collapses. Behind it all is a relatively old friend of ours: electricity .
By wetting the sand, the water covers the surface of the grains with a thin layer. Water is an extraordinary substance and among its properties is that the molecule has poorly distributed electrical charges. In short, one part of the molecule has a positive charge and the opposite, negative. The grains of sand also have curious distributions of electrical charges , which means that when a molecule of water approaches the grain of sand it sticks, in the same way that when we played at attracting pieces of paper with a pen rubbed on the sleeve of the sweater. In this way, the water trapped between the surface of two grains acts as a weak but effective glue. If there is too much water, the grains will separate and that will be the end of the sandcastle.
Why is wet sand darker than dry sand?
It all has to do with light scattering , the collision between light and grains of sand. Light tends to bounce in all directions if it hits small particles, and forwards if they hit slightly larger ones. An astute reader might wonder what this has to do with what we are discussing, because dry grains of sand are the same size as wet ones. You are partly right, but only partly. The point is that light, when it enters the spaces between the grains of wet sand, is not moving through the air but through the water , where it moves more slowly. And this has the same effect as hitting large grains of sand. In fact, light needs a minimum of six collisions to get out again when it hits wet sand, while two is enough for dry sand. When suffering a greater number of collisions, the light loses intensity and that is why we see the darker wet sand.
A sea of umbrellas
If there is something that also characterizes a beach in summer, it is another sea, that of umbrellas. Who invented them? It all started in Babylon 3,400 years ago . Symbol of rank and distinction, we all remember those movies where a slave or a serf walks behind the king, pharaoh or chief of the tribe with an umbrella. However, for the Egyptians the parasol had a clear religious meaning . For them, the celestial vault was formed by the body of the goddess Nut that covered the earth completely and only supported herself on the ground using her fingers and toes. In this way, the parasols, made of stretched palm fronds, feathers and papyrus, became earthly incarnations of the goddess and could therefore only protect the heads of nobles. Being invited to stand under the pharaoh’s parasol was a very high honor, since it implied being under the protection of the monarch.
In Greece and Rome the parasol was part of the female attire. Men could only use it without fear of ridicule if they accompanied a woman . The noble women, who carried white umbrellas, celebrated the Festival of the Parasols once a year. Party that of course, celebrated fertility.
