Two repulsive interactions lead to a strong attraction – who knew? The researchers at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw, that's who.
According to a recent press release from the institute, "When two objects repel each other under the action of one force, we usually expect that addition of another force, also repulsive one, will accelerate separation. This intuitive view is, however, not always true."
To be fair, researchers at the University of Stuttgart, Germany, were the first to witness the unexpected effect. But the Polish researchers took the experiment to the next level.
"Starting from the basics, we have developed a theoretical model of the system studied in Germany and successfully verified its predictions with experimental evidence. That's why we are able to explain how superposition of two repulsive interactions transforms into attraction," says Prof. Alina Ciach from the IPC PAS.
The system modelled at the IPC PAS was a mixture of water and an oily organic liquid -- lutidine. The mixture also included salt ions. The fluid was placed between two electrically charged walls -- one hydrophilic and another one hydrophobic.
Water and lutidine mix only in a certain temperature range. An interesting situation arises close to the critical temperature, where the system cannot "make a decision" if the components should mix or separate. Ultimately, a force emerges to push the walls apart.
The unusual behaviour of the modelled system was revealed after electric charge of the same sign was applied to both walls. A second, electrostatic, repulsion was acting then between the walls, and the walls were becoming attractive.
I guess this is where the phrase "If you can't beat 'em, join 'em," would apply.
For more about this experiment, read the release.
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