Wet sand flows better than dry sand

As any child knows, the best way to build a sandcastle is to use wet sand. Wet grains stick together due to capillary forces [1]. A castle made from dry sand will collapse under its own weight. However, we have demonstrated that, under some circumstances, wet sand can flow better than the dry sand !

Wet granular materials became recently the central point of intensive researches [1-4]. Indeed, those systems show granular cohesion [4] which the clue for understanding powders and other industrial products. Wet sand is a complex fluid because it is a mixture of three different phases : air (about 35%), water (about 2 %) and solid particles (about 63%).

In our last paper [3], we studied that the stress-strain behavior of sand with and without small amounts of liquid under steady and oscillatory shear. Since dry sand has a lower shear modulus, one would expect it deforms more easily. Using a new technique based on a Poiseuille-like flow in a tube, we observe that the resistance against deformation of the wet sand is much smaller than that of the dry sand, and that the latter dissipates more energy under flow. This is also observed in large amplitude oscillatory shear measurements using a rotational rheometer, showing that the effect is robust and holds for different types of flows.

The explanation why wet sand flows more easily than dry, is that the capillary bridges in the wet sand leads to agglomeration of the sand grains.  This is useful for building sandcastles, but also provides an effectively negative confining pressure for the agglomerated sand grains as a whole. Since for sand it is known that shear stress and confining pressure are proportional, this then leads to a smaller resistance to flow for the wet sand.

[1] P.Schiffer, Granular physics: A bridge to sandpile stability, Nature Phys. 1, 21 (2005); M.Parkpour et al, Scientific Reports 2, 549 (2012)
[2] J.E.Fiscina et al, Phys. Rev. Lett. 105, 048001 (2010) – PDF
[3] J.E.Fiscina, M.Pakpour, A.Fall, N.Vandewalle, C.Wagner and D.Bonn, Phys. Rev. E 86, 020103(R) (2012) – PDF
[4] G.Lumay and N.Vandewalle, New J. Phys. 9, 406 (2007) – PDF

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About nicovdw

Professor in Physics
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