A method to reversibly control Casimir forces using external magnetic fields

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

Researchers at University of Science and Technology of China have recently been exploring the possibility of selectively tuning the Casimir force, in other words switching it from attractive to repulsive and vice-versa, using external magnetic fields. Their study, featured in Nature Physics, demonstrates the successful magnetic field-tuning of the Casimir force arising from a gold sphere and silica plate immersed in water-based ferrofluids.

"My research area is condensed matter physics, but I also have a strong interest in fundamental physics, such as quantum fluctuations and their induced effects," Changgan Zeng, the 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 a paper by Munday et al. in Nature. Casimir forces are typically attractive, which poses challenges for applications, such as in 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 involved materials."

Inspired by this previous paper published in 2009, Zeng set out to pursue further research aimed at reversibly controlling Casimir forces by applying magnetic fields. His hope was to devise a reliable approach to modulate the Casimir effect, which 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 involved, these permittivities are generally not sensitive to external fields. On the other hand, according to Lifshitz theory, the Casimir force also depends on the magnetic permeabilities of the materials."

The magnetic permeability of many magnetic materials, particularly ferrofluids, can be modulated by applying external magnetic fields. Zeng and his students thus decided to use water-based ferrofluids to enable the tuning of the Casimir force between a gold sphere and a silica plate.

"I proposed this project to my graduate students, but none were willing to take it on," Zeng said. "Ultimately, I managed to persuade some talented undergraduates to undertake 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 switched from attractive to repulsive simply by modulating an external magnetic field, the distance between their two material samples and the volume of ferrofluids they employed.

The researchers then conducted an experiment designed to test their predictions. Using a cantilever that could collect measurements inside 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 at effectively tuning the Casimir effect using external fields. Collectively, these works could enable the development of new switchable micromechanical devices that leverage Casimir forces.

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

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