  Contact electrification of dielectric grains forms the basis for a myriad of
physical phenomena. However, even the basic aspects of collisional charging
between grains are still unclear. Here we develop a new experimental method,
based on acoustic levitation, which allows us to controllably and repeatedly
collide two sub-millimeter grains and measure the evolution of their electric
charges. This is therefore the first tribocharging experiment to provide
complete electric isolation for the grain-grain system from its surroundings.
We use this method to measure collisional charging rates between pairs of
grains for three different material combinations: polyethylene-polyethylene,
polystyrene-polystyrene, and polystyrene-sulfonated polystyrene. The ability to
directly and noninvasively collide particles of different constituent
materials, chemical functionality, size, and shape opens the door to detailed
studies of collisional charging in granular materials.
