Water solves Egyptian puzzle

Physicists at the University of Amsterdam investigated the forces needed to pull weighty objects on a giant wooden sled over desert sand and discovered that dampening the sand in front of the primitive device reduces friction on the sled, making it easier to operate. 

To make their discovery, the researchers picked up on clues from the ancient Egyptians themselves. A wall painting discovered in the ancient tomb of Djehutihotep, which dates back to about 1900 BC, depicts 172 men hauling an immense statue with ropes attached to a sledge. 

In the drawing, a person can be seen standing on the front of the sledge, pouring water over the sand.

“Egyptologists thought it was a purely ceremonial act,” study lead author Daniel Bonn, a physics professor at the University of Amsterdam, explained. “The question was: Why did they do it?” 

Bonn and his colleagues constructed miniature sleds and experimented with pulling heavy objects through trays of sand. When the researchers dragged the sleds over dry sand, they noticed clumps would build up in front of the contraptions, requiring more force to pull them.

Adding water to the sand increased its stiffness and the sleds were able to glide more easily across the surface. This is because droplets of water create bridges between the grains of sand, which helps them stick together, the scientists said. 

It is also the same reason why using wet sand to build a sandcastle is easier than using dry sand, Bonn added. 

But there is a delicate balance, the researchers found.

“If you use dry sand, it won’t work as well, but if the sand is too wet, it won’t work either,” Bonn said. “There’s an optimum stiffness.” 

The amount of water necessary depends on the type of sand but typically the optimal amount falls between two per cent and five per cent of the volume of sand. 

“It turns out that wetting sand can reduce the friction by quite a bit, which implies you need only half of the people to pull a sledge on wet sand, compared to dry sand,” Bonn said. 

The study, published in the journal Physical Review Letters, may explain how the ancient Egyptians constructed the pyramids but the scientists said the research also has modern-day applications.

The findings could help researchers understand the behaviour of other granular materials, such as asphalt, concrete or coal, which could lead to more efficient ways to transport these resources.