Opposites Attract, Likes Repel? Scientists Overturn Fundamental Principle of Physics

Hexagonal Cluster Formation

A new study has overturned a fundamental principle of physics by demonstrating that similarly charged particles can attract each other in a solution, with the effect varying between positive and negative charges depending on the solvent. This discovery has significant implications for various scientific processes, including self-assembly and crystallization. The research reveals the importance of solvent structure at the interface in determining interparticle interactions, challenging long-held beliefs and indicating a need for a re-evaluation of our understanding of electromagnetic forces. Credit: Zhang Kang

“Opposites charges attract; like charges repel” is a fundamental principle of basic physics. However, a new study from Oxford University, recently published in the journal Nature Nanotechnology, has demonstrated that similarly charged particles in solution can, in fact, attract each other over long distances.

Just as surprisingly, the team found that the effect is different for positively and negatively charged particles, depending on the solvent.

Besides overturning long-held beliefs, these results have immediate implications for a range of processes that involve interparticle and intermolecular interactions across various length-scales, including self-assembly, crystallization, and phase separation.

The team of researchers, based at Oxford’s Department of Chemistry, found that negatively charged particles attract each other at large separations whereas positively charged particles repel, while the reverse was the case for solvents such as alcohols.

These findings are surprising because they seem to contradict the central electromagnetic principle that the force between charges of the same sign is repulsive at all separations.

Experimental Observations

Now, using bright-field microscopy, the team tracked negatively charged silica microparticles suspended in water and found that the particles attracted each other to form hexagonally arranged clusters. Positively charged aminated silica particles, however, did not form clusters in water.

Using a theory of interparticle interactions that considers the structure of the solvent at the interface, the team established that for negatively charged particles in water, there is an attractive force that outweighs electrostatic repulsion at large separations, leading to cluster formation. For positively charged particles in water, this solvent-driven interaction is always repulsive, and no clusters form.

This effect was found to be pH dependent: the team was able to control the formation (or not) of clusters for negatively charged particles by varying the pH. No matter the pH, the positively charged particles did not form clusters.

Solvent-Specific Effects and Further Discoveries

Naturally, the team wondered whether the effect on charged particles could be switched, such that the positively charged particles form clusters and the negatives do not. By changing the solvent to alcohols, such as ethanol, which has different interface behavior to water, this is exactly what they observed: positively charged aminated silica particles formed hexagonal clusters, whereas negatively charged silica did not.

According to the researchers, this study implies a fundamental re-calibration in understanding that will influence the way we think about processes as different as the stability of pharmaceutical and fine chemical products or the pathological malfunction associated with molecular aggregation in human disease. The new findings also provide evidence for the ability to probe properties of the interfacial electrical potential due to the solvent, such as its sign and magnitude, which were previously thought immeasurable.

Professor Madhavi Krishnan (Department of Chemistry, Oxford University), who led the study, says: “I am really very proud of my two graduate students, as well as the undergraduates, who have all worked together to move the needle on this fundamental discovery.”

Sida Wang (Department of Chemistry, Oxford University), a first-author on the study, says: “I still find it fascinating to see these particles attract, even having seen this a thousand times.”

Reference: “A charge-dependent long-ranged force drives tailored assembly of matter in solution” by Sida Wang, Rowan Walker-Gibbons, Bethany Watkins, Melissa Flynn and Madhavi Krishnan, 30 February 2024, Nature Nanotechnology.
DOI: 10.1038/s41565-024-01621-5

7 Comments on "Opposites Attract, Likes Repel? Scientists Overturn Fundamental Principle of Physics"

  1. Please corect February 30 at the end of this article!

  2. Well, that explains my marriage, that’s for sure.

  3. When I first read this news article I was perplexed to why it was making news, since all it felt like to me was a team of researchers basically confirming the existence of DLVO theory.

  4. Fixed gravity for you. | March 10, 2024 at 9:57 am | Reply

    British physicists discover that reality can be usefully quantized into basic half-truths.

  5. No they didn’t

  6. ” …SciTechDaily: Home of the best science and technology news since 1998…”
    I too have pointed out (via emails direct to the editors) how this doesn’t accord with numerous articles presented by this publication, quoting obvious blatant bias and pseudo-science.
    They promised to look into it: nothing has changed…?

  7. Galaxy and information technology.

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