Colored sand unmixes when jostled

Mark Frauenfelder 12:30 pm Thu Jul 12, 2007

"Are these sand grains violating the second law of thermodynamics? Certainly not, but the explanation is nonetheless interesting, including reference to "a 'beard' of positively charged grains [that] had formed immediately under the lip of the platform…" Explained in Physics News Update #832.

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 charg. Through a process not well understood, the grains lose some electrons owing to their jostling motion (“tribocharging”) in the hopper, and become positively charged.

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