Like charges repel, unlike charges attract. The simplest way to show this is to charge up different pieces of insulating plastic by rubbing them on your shirt and watching what they do when brought up close to one another. Amit Mehrotra and colleagues use a similar idea to separate a mixture of red and blue sand grains falling into a hollow acrylic cylinder, purely through the different amount of charge each is carrying (A. Mehrotra et al. Phys. Rev. Lett. 99, 058001; 2007).

The red and blue grains were all of the same size and positively charged, with the charge density of the blue grains being about six times that of the red. The authors also made the cylinder positively charged by rubbing it lightly with nitrile gloves. The grains were mixed up on a vibratory feeder, and then discharged into the cylinder from a metal chute.

On entering the cylinder, the charged grains separated spontaneously into red and blue components (pictured). Oddly, however, it was the more positively charged blue grains that moved towards the positively charged cylinder walls — rather than being more strongly repelled, as basic electrostatics would seem to demand.


The authors show through simulations that the sand particles are not, in fact, going against the grain. The effect is caused by negative charges induced on the underside of the metal chute, whose concentrated attraction causes a 'beard' of falling sand grains to grow on the lip of the chute. This beard is sufficiently repulsive that the more highly charged blue grains levitate more strongly off the end of the chute, resulting in two falling streams separated according to colour.

Pretty as it is, the experiment also has a practical aspect. The ability to separate grains by how much charge they carry, rather than by charge sign, could have applications in technologies that exploit electrostatic charging — aerosol drug delivery, xerography and filtration, for example.