A method to reversibly control Casimir forces using external magnetic fields

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

Researchers from the University of Science and Technology of China recently explored the possibility of selectively tuning the Casimir force, in other words, changing it from attractive to repulsive and vice versa, using external magnetic fields. Their study, included in Natural physicsdemonstrates the successful magnetic field tuning of the Casimir force arising from a gold sphere and a 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 resulting effects,” Changgan Zeng, the paper’s corresponding author, told Phys.org.

“Over the past twenty years I have closely followed developments in the Casimir forces, and I was particularly impressed by an article by Munday et al. Nature. Casimir forces are generally 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 materials involved.

Inspired by this earlier 2009 paper, Zeng began further research aimed at reversibly controlling the Casimir forces through the application of magnetic fields. His hope was to devise a reliable approach to modulate the Casimir effect, which could open new avenues for both research and technological development.

“Initially, we considered controlling the Casimir force by applying an electric field, inspired by the concept of FET devices,” Zeng explains. “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, especially ferrofluids, can be modulated by applying external magnetic fields. Zeng and his students therefore decided to use water-based ferrofluids to tune the Casimir force between a gold sphere and a silica plate.

“I proposed this project to my graduate students, but no one was willing to take it on,” Zeng said. “I ultimately convinced a number of talented students to undertake the project, and they succeeded.”

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

The researchers then conducted an experiment to test their predictions. Using a cantilever that could collect measurements in 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. Collectively, these works could enable the development of new switchable micromechanical devices that harness Casimir’s powers.

“We achieved a 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 change the Casimir force.”

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