Another law of physics broken: Not only opposites attract

An experiment at Oxford University has turned upside down one of the fundamental principles of physics: that opposites attract and like charges repel.

We’ve all experienced this principle when trying to push like-charged magnet ends together.

The new study which calls this into question is published in Nature Nanotechnology. The researchers found similarly charged particles suspended in a solution can actually attract each other over long distances. And this effect is different for positive and negative particles in some solvents.

It may be slightly unsettling, but the surprising result may have implications for applications that involve interactions at different length scales like self assembly, crystallisation and phase separation.

The experimentalists suspended silica particles about 4–5 micrometres across in different liquid solutions.

They used bright-field microscopy to track the microparticles.

Negatively charged particles in water were found to attract when separated by several hundred nanometres. They ended up forming hexagonal patterns. Meanwhile, positively charged particles repelled. A different solution, alcohol, reverses this.

The level of attraction is also closely linked to the solution’s pH. Varying the pH, the researchers were able to turn on or off the negatively charged particles’ attraction to each other in water. No matter the pH, positively charged particles did not attract in water.

The study is based on a theory in which the electrostatic repulsion of like charges is overcome by the interactions of the solvent on the surface of the submerged objects.

“I still find it fascinating to see these particles attract, even having seen this a thousand times,” says first author Sida Wang, a PhD student in Oxford’s Department of Chemistry.

Sign up to our weekly newsletter

Please login to favourite this article.