Today I made a phone call with Martin van Hecke of the university of Leiden (group: Granular Matter) for I wanted to know a bit more about the physics of sand running trough a small hole in a sand glass.
Normally I use silver sand that I buy in the next building materials shop. I sieve it, and that’s it! During this I experienced that I could never make the hole smaller that 5 mm without the risk sand stream being stopped. But it would be nice to be able to use a smaller hole, so the amount of sand would last longer.
Martin explained the basic rule: the hole must be about 5 times as big as the particles.
He suggested I could use materials that are used for blast cleaning from for example the firm pneumix, they sell little balls of glass for blast cleaning. But you need to be really careful with this material: when you spill it on a flat surface, it gets slippery as hell. So I decided on the spot that this would not be ideal in the streets of Madrid. We do not want to break the hips of old grandmothers.
For test materials he and his students even go to the next garden shop to buy several kinds of sand: just to have some samples to test Granular behavior in different circumstances!
Martin also explained that what determines the capacities of running trough a small hole: The smaller the size of the sand the better it runs in totally dry circumstances. But normally there is a certain amount of moist in the air… to particles smaller that 0,8 mm tent to get sticky. The sand needs to be a round as possible. The particles should be consistent of size: not bigger that 1 mm, not smaller than 0,8 mm. So it might be helpful to use drainage sand. This is sieved so that also the more small particles are removed.