We all know that walking along the beach on wet sand is easier than dragging our feet through the dry stuff. We all know that wet sand is a very different material from dry – the strange inter-granular capillary physics of wet sand allows us to build sandcastles and, it seems, made it a little easier to drag ridiculously heavy objects around the desert for the ancient Egyptians intent on claiming their places in posterity.
Around 4,000 years ago in ancient Egypt lived a typically self-aggrandising nomarch (no, not typo, but the term for a semi-feudal governor), Djehutihotep. His tomb is famous for its paintings, a drawing of one of which is shown above. Hundreds of slaves are dragging a sledge bearing a gigantic statue of Djehutihotep, undoubtedly looking on his works and despairing. Look closely, and you will see a man pouring water, not on the perspiring workers, but on the sand, and research just published has indicated that the Egyptians knew exactly what they were doing in terms of the behaviours of granular materials. As reported on Sci-News.com:
A multinational group of physicists led by Prof Daniel Bonn from the University of Amsterdam has hypothesized that Egyptians probably made the desert sand in front of the sledge wet … In the presence of the correct quantity of water, wet desert sand is about twice as stiff as dry sand. A sledge glides far more easily over firm desert sand simply because the sand does not pile up in front of the sledge as it does in the case of dry sand.
The university news release describes the experiments and the results as follows:
Ancient Egyptians transported pyramid stones over wet sand
29 April 2014
Physicists from the University of Amsterdam have discovered that the ancient Egyptians used a clever trick to make it easier to transport heavy pyramid stones by sledge, allowing them to halve the number of workers needed. The researchers published this discovery in the prestigious journal Physical Review Letters.
For the construction of the pyramids, the ancient Egyptians had to transport heavy blocks of stone and large statues across the desert. The Egyptians therefore placed the heavy objects on a sledge that workers pulled over the sand. Research from the University Amsterdam has now revealed that the Egyptians probably made the desert sand in front of the sledge wet. Experiments have demonstrated that the correct amount of dampness in the sand halves the pulling force required.
The physicists placed a laboratory version of the Egyptian sledge in a tray of sand. They determined both the required pulling force and the stiffness of the sand as a function of the quantity of water in the sand. To determine the stiffness they used a rheometer, which shows how much force is needed to deform a certain volume of sand.
Experiments revealed that the required pulling force decreased proportional to the stiffness of the sand. Capillary bridges arise when water is added to the sand. These are small water droplets that bind the sand grains together. In the presence of the correct quantity of water, wet desert sand is about twice as stiff as dry sand. A sledge glides far more easily over firm desert sand simply because the sand does not pile up in front of the sledge as it does in the case of dry sand.
The Egyptians were probably aware of this handy trick. A wall painting in the tomb of Djehutihotep clearly shows a person standing on the front of the pulled sledge and pouring water over the sand just in front of it.
Besides revealing something about the Egyptians, the results are also interesting for modern-day applications. We still do not fully understand the behaviour of granular material like sand. Granular materials are, however, very common. Other examples are asphalt, concrete and coal. The research results could therefore be useful for examining how to optimise the transport and processing of granular material, which at present accounts for about ten percent of the worldwide energy consumption.
The research was supervised by group leader professor Daniel Bonn and is part of the FOM programme 'Fundamental aspects of friction
It’s interesting to note the comment “in the presence of the correct quantity of water” – this graphic from the paper in Physical Review Letters demonstrates the subtlety and fickleness of granular behaviour:
The images show the experimental sledge being pulled through the experimental sand, dry on the left, correctly wetted on the right. The graph shows the force required to move the sledge with across the dry sand (the red line) and with different proportions of water. The green line (1.5% water) reduces the force required a little, but the purple line demonstrates how adding 5% water eases task very significantly – fewer slaves required, although undoubtedly they were given other things to do. However, add a little more water – the black line represents 7.4% – and pulling the sledge becomes harder than in dry sand, the water between the grains turning the material into something more like mud.
So it would seem that the ancient Egyptians’ understanding of the physics of granular materials helped achieve their megalomaniacal ambitions. And the research has provided Egyptologists with an explanation of what that person was doing on the front of Djehutihotep’s sledge - as reported in an article in The Washington Post:
“This was the question,” Bonn wrote in an e-mail to The Post. “In fact, Egyptologists had been interpreting the water as part of a purification ritual, and had never sought a scientific explanation. And friction is a terribly complicated problem; even if you realize that wet sand is harder – as in a sandcastle, you cannot build on dry sand — the consequences of that for friction are hard to predict.”