Hair and Balloon Magic Trick Explained After 2,500 Years

A common childhood magic trick was to get small pieces of papers glued to a plastic ruler or a comb or a piece of silk cloth. The trick lies in rubbing two surfaces, which develop charges on the surfaces and as a result, attraction takes place. This has been a millennia-old phenomenon, the first case being reported in 600 BC by the Greek philosopher Thales of Miletus. Everyone has experienced it since then and for the last 2,500 years, people have engaged in focused research, but ironically, what thermodynamics forces the charge transfer had remained elusive till date.

A paper published yesterday in the journal Physical Review Letter has claimed to have an answer about the thermodynamic driver of this charge separation on two insulating surfaces when they are rubbed. The study finds that the answer lies in the nanoscale level.

The study carried out by a team from the Northwestern University shows that at the nanoscale, surfaces of all materials are rough with tiny protrusions in countless numbers. When two materials come into contact and rub against one another, these protrusions bend and deform.

The team found that it’s this deformation of the protrusions that gives rise to voltages between the surfaces that ultimately causes static charging. Known as flexoelectric effect, this phenomenon occurs when separation of charge in an insulator arises from deformations such as bending.

The Northwestern University team, taking resort to a simple model, showed that the voltages that arise from bending of the protrusions during the rubbing are indeed large enough to cause static electricity. As a result, we see the hair-raising effect when a balloon is rubbed over one’s hair. This experiment also explains a number of observations found in experiments such as why charges are produced when two pieces of the same material are rubbed together and also could predict experimentally measured charges with remarkable accuracy.

Unlike the electric current flowing through a power line, static electricity remains at a place without moving. Known as the triboelectricity, this form of electric effect typically builds up in materials that don’t conduct charge very well, like that of rubber or plastic, which causes it to get stuck. These insulators accumulate a static charge when rubbed against one another.

L D Marks, the corresponding author of the paper, said, “Our finding suggests that triboelectricity, flexoelectricity and friction are inextricably linked. This provides much insight into tailoring triboelectric performance for current applications and expanding functionality to new technologies."

"This is a great example of how fundamental research can explain everyday phenomena which hadn't been understood previously, and of how research in one area -- in this case friction and wear -- can lead to unexpected advances in another area," said Andrew Wells, a program director at the National Science Foundation (NSF), which funded the research.

This new understanding could help engineers to generate more static electricity by optimising materials, which could be used optimally in doing things such as charge wearable technology. The findings could also prove important in improving safety in places like oil refineries where even a spark can cause an explosion no smaller than a catastrophe.