Physics behind static electricity still unknown

Science : It has long been known that rubbing two different materials together creates static electricity because of the materials’ differing abilities to trap and hold electrons. However, real-world results, such as explosions in grain elevators, indicate that rubbing bits of the same material together can also generate static electricity. To explain why, researchers in 1986 proposed that the phenomenon is due to the size of the particles: Large grains of a material can end up with a positive charge and small grains with a negative charge. Now that theory has been disproved. Using a high-speed camera, Heinrich Jaeger of the University of Chicago and colleagues tracked tens of thousands of particles of zirconium dioxide–silicate falling through a horizontal electric field. Although the smaller ones did tend to be negatively charged and the larger ones positively charged, the difference wasn’t enough to explain the static buildup. The researchers suggest that another factor, such as hydroxide ions, is responsible.