samedi 3 mai 2014

Transport des blocs utilisés dans la construction des pyramides : la théorie d’un groupe de physiciens hollandais

Ils s’appellent A. Fall, B. Weber, M. Pakpour, N. Lenoir, N. Shahidzadeh, J. Fiscina, C. Wagner et D. Bonn. Ces chercheurs hollandais de l'Université d'Amsterdam et de la Fondation pour la recherche fondamentale sur la matière, inspirés sans doute par la célèbre fresque de la sépulture de Djéhoutyhotep où l’on voit un homme en train de verser un liquide (sans doute de l'eau) devant le gigantesque traîneau tiré par deux groupes de haleurs, ont effectué des tests en laboratoire sur les réactions du sable mouillé servant de surface au transport de blocs de pierre au moyen de traîneaux.

Le résultat de leur “découverte” a été annoncé dans la revuePhysical Review Letters- 112, 175502 (29 April 2014) et peut être consulté ici :


“We show experimentally that the sliding friction on sand is greatly reduced by the addition of some - but not too much - water. The formation of capillary water bridges increases the shear modulus of the sand, which facilitates the sliding. Too much water, on the other hand, makes the capillary bridges coalesce, resulting in a decrease of the modulus ; in this case, we observe that the friction coefficient increases again. Our results, therefore, show that the friction coefficient is directly related to the shear modulus ; this has important repercussions for the transport of granular materials. In addition, the polydispersity of the sand is shown to also have a large effect on the friction coefficient.”

Wall painting from 1880 B.C. on the tomb of Djehutihotep. 
The figure standing at the front of the sled is pouring water onto the sand.

Force-displacement curves for wet and dry Iranian sand. Inset: Picture of the setup. The picture on the left was taken while sliding over dry normalized sand. The picture on the right was taken while sliding over normalized sand wetted with 5% water. In the dry sand, a heap clearly builds up in front of the sled. The 11×7.5  cm sled is made out of PVC with rounded edges (as the Egyptian sled) and a roughness of 35  μm with sandpaper on its bottom; the results were qualitatively similar but less reproducible with a smooth bottom.