Scientists have finally explained how rubbing a surface creates electrical current, drawing a parallel with stroking a cat although the findings have far wider ramifications.
Static electricity was first observed in 600 BCE. Yet for many years modern researchers have struggled to explain how rubbing causes it. For thousands of years humans have understood that rubbing induces static charging in all insulators. What they did not understand is the ‘why’.
It would now appear, according to Northwestern researchers, that nanosized surface deformations are a factor together with different electrical charges building up on the front and back parts of a sliding object, creating a current.
With the former – the deformations – rubbing two materials together bends tiny protrusions on the surfaces of those materials. Those bent, deformed protrusions give rise to voltages.
When an object slides, the front and back parts of that object experience different forces. This difference in forces causes different electrical charges to build up on the front and back parts of the object. And the difference in electrical charges creates a current, leading to a light ‘zap’.
In other words, having different deformations — and therefore different charges — at the front and back of something sliding leads to a current.
The researchers have also been able to develop a new model that calculates electrical current. The values for the current for a range of different cases are in good agreement with experimental results.
A concept called “elastic shear” is part of the new model. Elastic shear can occur when a material resists a sliding force. If a person pushes a plate across a table, the plate will resist sliding. As soon as the person stops pushing it, the plate stops moving. This added friction — caused by the resistance to sliding — causes electrical charges to move.
To put it simply, sliding and shear are intimately connected.
Static electricity can lead to serious problems and hence gaining an understanding of its properties is important. For example, sparks from static electricity cause industrial fires and even explosions. It also can hinder consistent dosing for powdered pharmaceuticals. The new findings should help with the quest towards improving safety and industrial processes.
The research has been published in the journal Nano Letters. The research is titled “What puts the ‘tribo’ in triboelectricity”.
