It’s a fundamental principle of physics that particles with opposite charges attract each other, while those with the same charge repel. But now, scientists at the University of Oxford have found that under certain circumstances, particles can attract those of the same charge.
Particles can gain either a positive or negative charge, and this dictates how they behave around other particles. Put two with opposite charges close together and they’ll be attracted to each other, while two of the same charge will repel each other. This electrostatic force gets stronger as the total charge increases and the particles get closer together, which is known as Coulomb’s law.
But in a new study, scientists have discovered exceptions to the rule. When suspended in certain solutions, some charged particles can attract particles of the same charge, even over relatively long distances. Weirder still, particles with positive and negative charges behave differently in different solutions.
In tests, the team suspended negatively charged silica microparticles in water, and found that under certain pH levels they could be made to attract each other to form hexagonal shaped clusters. That seems to violate a basic electromagnetic principle that says particles of the same charge should be repulsive at any distance. But when the researchers looked at the effect using a theory of interparticle interactions that takes into account the solvent’s structure, a new attractive force was found that can overcome electrostatic repulsion.
This wasn’t the case for positively charged aminated silica particles, though. In water at any pH level, this interaction stayed repulsive. So, the team wondered if they could flip it around, and found that by switching to a different solvent – in this case, alcohols – positively charged particles clustered together, while negative ones stayed repulsive.
The team says this discovery could force a major rethink of our assumptions, and could be put to use in practical chemistry for processes like self-assembly, crystallization, and phase separation.
The research was published in the journal Nature Nanotechnology.
Source: University of Oxford
