A layer of carbon compounds determines the charge of identical materials
DENVER — Static electricity is a delicate subject.
Touch or rub two materials together and they can exchange electrical charges. But the details of the static electricity phenomenon are poorly understood. Now, scientists have identified a hidden factor at play. A thin coating of carbon-rich molecules changes the way identical materials exchange charges, scientists report in March 19. Nature. This suggests that surface contamination plays a major role in static electricity.
“Static electricity is not child’s play,” physicist Scott Waitukaitis said in a statement. March 16 conference at the American Physical Society World Physics Summit. “Literally, this could be why we have ground to stand on. » The charge created by the collision of particles in protoplanetary disks is thought to help planets, including Earth, form. It is also the source of volcanic lightninghelps trap sand kicked up during dust storms and can cause industrial accidents such as sawmill fires.
When two identical particles collide, one acquires a positive charge and the other becomes negative. But scientists didn’t know what determines which particle gets which charge. Waitukaitis and his colleagues studied this effect in silicon dioxide, or silica, a material commonly found in sand, rock and glass.
The researchers bounced a tiny silica sphere off a silica plate and measured the charge acquired by the sphere. To do this, the scientists used a technique called acoustic levitationharnessing sound waves to suspend the silica bead half a millimeter in the air before dropping it. This technique avoided any adverse effects linked to physical contact with the object.
Some spheres became positively charged while the plate became negatively charged. But some interactions went the other way. However, if the researchers heated the sphere or plate to 200° Celsius for two hours and then let it cool, they could manipulate the effect. A previously heated sphere almost always negatively charged an untreated plate, while a heated plate made the sphere positively charged. The same thing happened when the spheres were exposed to plasma, a mixture of electrically charged particles. In both cases, the treated object took on a negative charge and the pure object became positively charged.
Close inspection of the materials revealed that the heat and plasma treatments removed a thin layer of carbon-rich molecules from the silica surface. Almost all objects exposed to air are similarly encrusted, thanks to the ubiquitous organic molecules that float around. “This carbon cake grows on everything, in all environments,” says Waitukaitis, of the Austrian Institute of Science and Technology in Klosterneuburg.
After a sphere was heat treated, its carbon layer returned after several hours, due to exposure to carbon-rich molecules in the air. The sphere’s charging behavior moved closer to its original baseline in parallel with the growth of the carbon layer, suggesting that the carbon layer was responsible for the change in how the objects charged.
“It really showed that both things were evolving on the same time scale,” says chemical engineer Daniel Lacks of Case Western Reserve University in Cleveland, who was not involved in the research.
Scientists had long suspected that contamination of surfaces – by carbon or other substances – was important to understanding static electricity. The new study “proves very clearly the general point that uncontrolled surface contamination plays a major role,” says materials scientist Laurence Marks of Northwestern University in Evanston, Illinois. But that’s not the end of the story. Previous experiments, he says, revealed that other factors were relevant, such as the curvature of a surface.
Even a small change in the carbon layer can change the results of an experiment, says physicist Rolf Möller of the University of Duisburg-Essen in Germany. “This work clearly shows that we must be very careful about… the influence of contamination. »
The study’s findings apply to silica and related materials, called insulating oxides. But the importance of understanding surface effects is general. In an earlier study, Waitukaitis and colleagues found that static electricity between sponge polymers depends on how many times they had been hit and how smooth their surfaces were thus.
Researchers still don’t know how changing the carbon layer changes the outcome of an experiment, or even how charges are actually exchanged between objects. But this work could provide a better understanding of an electrifying phenomenon.