A method of reversible control of Casimir forces using external magnetic fields

The so-called Casimir force or Casimir effect is a quantum mechanical phenomenon resulting from the fluctuation of the electromagnetic field between two conductive or dielectric surfaces that are a short distance apart. Studies have shown that this force can be either attractive or repulsive, depending on the dielectric and magnetic properties of the materials used in the experiments.

Researchers at the University of Science and Technology of China recently explored the possibility of selectively tuning the Casimir force, in other words, switching from attractive to repulsive and vice versa using external magnetic fields. Their study, reported in Natural physicsdemonstrates the successful tuning of the magnetic field of the Casimir force arising from a gold sphere and a silicon slab immersed in water-based ferrofluids.

“My research area is condensed matter physics, but I am also very interested in fundamental physics such as quantum fluctuations and their induced effects,” Changgan Zeng, corresponding author of the paper, told Phys.org.

“Over the past two decades, I have closely followed developments in the field of Casimir forces, and I was particularly impressed by the article by Munday et al. Nature. Casimir forces are typically attractive, which poses challenges for applications such as microelectromechanical systems (MEMS). In their paper, the authors devised an elegant experiment to achieve repulsive Casimir forces by carefully selecting the dielectric permittivities of the materials used.”

Inspired by this previous paper published in 2009, Zeng decided to pursue further research into the reversible control of Casimir forces by applying magnetic fields. His hope was to devise a reliable approach to modulating the Casimir effect that could open new avenues for both research and technology development.

“Initially, we considered controlling the Casimir force by applying an electric field, inspired by the concept of FET devices,” Zeng explained. “Although it is well known that the Casimir force depends on the dielectric permittivities of the materials used, these permittivities are generally not sensitive to external fields. On the other hand, according to Lifshitz’s theory, the Casimir force also depends on the magnetic permeabilities of the materials.”

The magnetic permeability of many magnetic materials, especially ferrofluids, can be modulated by applying external magnetic fields. Zeng and his students therefore decided to use water-based ferrofluids to enable tuning of the Casimir force between the gold sphere and the silicon slab.

“I proposed this project to my graduate students, but no one was willing to accept it,” Zeng said. “Eventually I was able to convince some talented college students to take on the project and we succeeded.”

Zeng and his students first performed a series of theoretical calculations. These calculations suggested that the Casimir force could be changed from attractive to repulsive simply by modulating the external magnetic field, the distance between their two material samples, and the volume of ferrofluid they used.

The researchers then conducted an experiment designed to test their predictions. Using a cantilever that could collect measurements inside the ferrofluids, they observed how the changes they implemented affected the Casimir effect.

The findings of this recent study could soon pave the way for further efforts to effectively tune the Casimir effect using external fields. Together, this work could enable the development of new switchable micromechanical devices that exploit Casimir forces.

“We have achieved reversible tuning of the Casimir force from attractive to repulsive using a magnetic field, opening the way for the development of switchable micromechanical devices based on the tunable Casimir effect,” added Zeng. “In our next studies, we plan to control the Casimir force with light. For example, plasmons in metal plates can be excited by light, which should effectively change the Casimir force.”

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