Our daily lives have been invaded by granular materials for a long time now. Sugar and salt in our kitchens, piles of grain, sacks of cement and sand in business companies: all examples of grains to which we no longer even pay attention. Nonetheless the way they pile up responds to precise physical laws is not known, the study of which is the speciality of the GRASP (Group for Research and Applications in Statistical Physics) created at the University of Liège in 1999 by Professor Nicolas Vandewalle. Researchers who could only but be attracted by very strange towers of sand…

If we allow sand to flow little by little, it forms itself into a heap – a conical pyramid – which will spread to a greater or lesser extent according to an angle called the avalanche angle. The same is true if we repeat the experiment when the sand flows into water. The only difference with the wet sand is that the avalanche angle will not have the same value (in fact, the difference is very small; one might almost say that the angle doesn’t change), and the shape of the pile will thus not be identical even if other parameters have not varied. But these two situations – dry sand and sand entirely under water – are not the only ones possible. We could in effect consider the case of grains of sand which flow onto a pile whose base is under water but whose peak is in the open air. In other words there is an air-water interface which traverses the cone of sand. It is this situation which has been studied by Stéphane Dorbolo, a FNRS Research Associate, and two postdoctoral researchers who are currently carrying out research at the GRASP, Florian Moreau and Felipe Pacheco-Vàzquez. And there was a surprise in store: in certain conditions the grains of falling sand do not form a pretty heap…but climb into a narrow tower! (watch the video)

To understand this spectacular phenomenon, one has to look closely at what is happening between the grains of sand. ‘When liquid is present,’ explains Stéphane Dorbolo, ‘four situations are possible, and they are determined by the degree of wetness (of saturation) of the pile of sand onto which the grains fall.’ The first is called pendular. In this case, there is little liquid, so the degree of wetness is low. That is expressed by the formation of simple liquid bridges between certain grains of sand, which is enough to ensure a certain cohesion between the grains (see illustration).