Spontaneous Separation of Charged Grains

An experiment at Rutgers shows how two populations of sand grains mixed together and held in a hopper will, when shaken out into a beaker, spontaneously segregate themselves, all because of static electrical interactions. This phenomenon, the opposite of mixing, might have practical uses in the powder industry.

In the recent report, the two types of sand grains (“art sand”), one colored blue and the other red, are mechanically alike but acquire slightly different charge (see the series of figures at http://www.aip.org/png/2007/282.htm). Through a process not well understood, the grains lose some electrons owing to their jostling motion (“tribocharging”) in the hopper, and become positively charged.

(The degree to which the like-charged grains repel each other as they sit in close proximity in the hopper can be seen in the middle figure in the sequence, which shows how some grains come flying off the surface to heights as great as 2 meters.) At one point in the experiment, the red and blue grains separate themselves in falling off the hopper platform.

Below they land in the vicinity of a van de Graaf generator, which is also positively charged. Troy Shinbrot and his colleagues at Rutgers were somewhat puzzled when they noticed that the more positively charged blue grains landed closer to the generator; shouldn’t the repulsion have kept them further away? Indeed Shinbrot (732-445-4500, x6310, shinbrot@soemail.rutgers.edu) asked his graduate student to repeat the test numerous times in order to correct this seeming error before discovering the explanation: a beard of positively charged grains had formed immediately under the lip of the platform.

The beard’s static repulsion, although weak, predominated over the repulsion posed by the generator because the beard was much closer to the grains as they fell off the lip onto the pile below. This effect is not unlike the static attraction between milk droplets pouring from a glass cup; the droplets, although electrically neutral, consist of polar molecules (mostly water) which are attracted to the lip of the glass, causing the milk to dribble down the edge of the glass rather than pouring smoothly.

(Mehrotra et al., Physical Review Letters, upcoming article; lab website, http://coewww.rutgers.edu/~shinbrot/temp/TriboBalloons.html)