Ancient Egyptians had to pull massive statues and pyramid stones weighing 2.5 tons on large sleds across the desert -- without any modern mechanical device. Now, new research shows how adding a small amount of water to sand significantly reduces the sliding friction -- a clever trick that allowed the Egyptians to cut the number of workers needed by half.
To make a good sandcastle, you don’t use dry sand. By adding water, the grains stick to each other, and your castle holds its shape. Same thing with sand transportation: Adding water reduces the sliding friction of any object moving over the sand. With the right amount of dampness, water droplets bind the sand grains together.
An international team led by Daniel Bonn from the University of Amsterdam tested the sliding friction of dry and wet sand by pulling a weighted sled across the surface in a tray. With dry sand, a heap would form in front of the sled, hindering its movement. And as they added water, both the force needed to pull the sled and the amount of friction decreased. As the water made the sand more rigid, the heaps got smaller and smaller until there was no obstacle forming in front of the moving sled.
Their experiments revealed that the required pulling force decreased proportional to the stiffness of the sand. When water was added, capillary bridges arose; these small water droplets act like glue to bind the sand grains together. With the right amount of water, wet desert sand is about twice as stiff as dry sand, allowing the sled to glide far more easily.
“I was very surprised by the amount the pulling force could be reduced -- by as much as 50 percent -- meaning that the Egyptians needed only half the men to pull over wet sand as compared to dry,” Bonn tells the Washington Post. Pictured here is the lab setup: A pile of sand accumulates in front of the sled when it's pulled over dry sand (left), but not with wet sand (right).
But just like with sandcastles, too much water isn’t good either. Water saturation is accompanied by a decrease in stiffness. With very high water contents, the capillary bridges (which used to act like a glue) start to merge and disappear, and the sliding friction increases again. It’s a delicate balance. "If you use dry sand, it won't work as well, but if the sand is too wet, it won't work either," Bonn tells LiveScience. "There's an optimum stiffness." The ideal amount of water falls between 2 and 5 percent of the volume of sand.
The answer had been staring us in the face for a long time. In a wall painting from the tomb of Djehutihotep (schematic above), you can see a worker pouring water on the sand in front of a sled that’s carrying a colossal statue. The sleds were little more than large wooden planks with upturned edges. “Egyptologists had been interpreting the water as part of a purification ritual," Bonn says, "and had never sought a scientific explanation.”
The work was published in Physical Review Letters last week.
Image: Wikimedia (top) & UvA (middle)