Imagine I have a cylindrical pipe closed on both ends with lids. I fill it with sand and compress the sand tightly. Now I hold the cylinder vertically and remove the bottom lid. The sand will counter intuitively not fall off apart from a few loose grains. I understand the reason for this. Essentially the downward gravity force acting on each sand grain is counter balanced by net upward friction force. And the friction force arises due to the contact of each sand particle with it's neighbors.

Now, is there a name for this phenomenon? Or, are there related experiments or applications of this phenomenon?

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    $\begingroup$ This is called jamming, and it's related to some interesting critical phenomena. By the way, it can't be explained by a simple argument involving balance of surface forces. The jamming occurs along spontaneously assembled internal "support structures" that hold the whole sand up. $\endgroup$ – Ryan Thorngren Aug 16 '18 at 6:36
  • $\begingroup$ @RyanThorngren Can you kindly elaborate in an answer what these support structures are? Jamming seems like something that non-Newtonian fluids would do. Does sand always express jamming or only when it's subjected to compressive forces? $\endgroup$ – yathish Aug 17 '18 at 5:32

The phenomenon is called dilatancy.

In my former life as a colloid science I frequently encountered this in concentrated dispersions (the archetypal example of this is oobleck) though the mechanism is subtly different in dispersions since it arises from viscous drag in the medium rather than friction between the dispersed particles. However the phenomenon is basically the same.

In this particular case the volume is constrained by the tube hence the grains cannot flow.

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  • $\begingroup$ Dilatancy pertains to shear deformations. I was imagining the soil to be radially compressed inwards (so no shear force other than the gravity of grains). Still the soil will more or less stick together right? And in this case, there's no dilatancy. The total volume of sand in this case should, in fact, decrease as opposed to the case of dilatancy. Is this correct? $\endgroup$ – yathish Aug 17 '18 at 5:23
  • $\begingroup$ The friction causes shear because it acts at the boundary of the sand. $\endgroup$ – Ryan Thorngren Aug 17 '18 at 5:36
  • $\begingroup$ @yathish the sand is under shear because the centre wants to fall down while the edges are held by the walls of the cylinder. It is in effect like pumping fluid through a pipe. There is a shear stress along the radial direction. This shear stress causes the dilatancy and locks the sand in place. $\endgroup$ – John Rennie Aug 17 '18 at 6:05
  • $\begingroup$ @JohnRennie ok. It's clearer now. If we had some other dispersion which doesn't show dilatancy, it could still be locked in place by just the friction forces, right? Also, constraint of expanding volume locks it in place - isn't this ultimately due to the net upward friction force? $\endgroup$ – yathish Aug 17 '18 at 6:15

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