Casimir forces in two dimensional materials – CAT

The CAT project aims to explore the quantum forces that appear between neutral objects, known as Casimir-Lifshitz forces (CLF), in two-dimensional materials like graphene. These forces, arising from quantum fluctuations of the electromagnetic field, are extremely weak at large distances but become very strong when objects are separated by just a few hundred nanometers—less than the thickness of a human hair. Understanding and controlling these forces is essential for miniaturized technologies, where tiny mechanical parts can stick together and cause malfunctions.

To achieve this, the project combines advanced theory and precision experiments. On the theoretical side, the French team models graphene at the atomic scale to predict the strength and direction of these quantum forces. This includes accounting for energy dissipation in the material and studying thermal effects, which can alter the force in surprising ways at very small distances. The models also simulate complex structures, such as nanometric grids or patterns on which graphene is placed, to understand how geometry influences these interactions.

Experimentally, the Hong Kong team fabricates and measures these forces with extreme precision. Graphene is prepared either by chemical growth on large substrates or by mechanical exfoliation to obtain thin layers. Samples are placed opposite a small metallic sphere attached to a cantilever—a tiny lever that vibrates slightly. Forces between the graphene and the sphere change the cantilever’s vibration frequency, allowing measurements with piconewton sensitivity.

To improve accuracy, the setup uses two fiber-optic interferometers: one monitors the cantilever’s movement, and the other controls the distance between graphene and the sphere with nanometer precision over hours. Some experiments suspend graphene to remove the influence of the substrate, or place it on nanogrids to create spatial variations, exploring how geometry modulates the force.

A particularly exciting aspect of the project is the ability to control the force in real time. By applying an electric voltage, the Fermi level of graphene can be adjusted, altering its electronic and optical properties—and thus the Casimir force. This opens the door to innovative applications, such as micromotors or frictionless devices where components never touch.

In summary, the CAT project combines advanced theoretical modeling, nanoscale material fabrication, and ultra-sensitive measurements to explore a fundamental quantum phenomenon and make it controllable. These methods not only deepen our understanding of quantum forces but also pave the way for future technologies where such forces become useful tools rather than limitations.

The CAT project has advanced our understanding of Casimir forces, quantum interactions that appear between neutral objects at extremely small scales. Invisible in everyday life, these forces become dominant at the nanometer scale and are particularly important in miniaturized devices, such as microelectromechanical systems (MEMS), where they can cause components to stick together—a phenomenon known as “stiction.”

By using two-dimensional materials, mainly graphene, the French and Hong Kong teams explored previously uncharted regimes. Graphene, a single layer of atoms, has electrical and optical properties very different from conventional materials, allowing the Casimir force to be modified simply by applying a voltage or through chemical adjustments. The project demonstrated that Casimir forces can be modulated in a controlled way, and that significant thermal effects appear at much shorter distances than with traditional 3D materials.

On the experimental side, the HKUST team fabricated partially suspended graphene membranes that are mechanically stable and developed highly precise measurement protocols. Early experiments confirmed theoretical predictions: the force can be influenced by doping, suspension, or the presence of diffraction gratings beneath the 2D material. These results pave the way for more refined measurements of Casimir forces and for designing nanoscale systems where unwanted adhesion can be reduced or controlled.

The theoretical component of the project produced precise models describing interactions in varied configurations: suspended graphene, graphene patterned with periodic structures, or in contact with other 2D materials. These models take into account temperature, electronic properties, and geometry, and are now available to the scientific community to predict and control these forces.

Overall, the project led to 14 international publications, showcasing the productive collaboration between France and Hong Kong and opening new perspectives for exploiting Casimir forces in nanotechnological devices. Applications range from MEMS/NEMS to systems that could harness force modulation or even achieve repulsion. In this way, the project fully met its scientific and technological objectives while establishing a durable and fruitful international partnership.

The results of the CAT project open many exciting avenues for the future. Researchers have shown that it is possible to control Casimir forces using two-dimensional materials such as graphene, by adjusting the chemistry, geometry, or structure of the material. This control could soon make it possible to design nanoscale systems where unwanted adhesion, or “stiction,” is reduced—or even completely avoided.

A particularly promising direction concerns controlled repulsion between surfaces. By tuning the electronic or optical properties of 2D layers, it may become possible to create nanoscale devices in which certain components float or separate on their own, without mechanical contact. This could offer unprecedented opportunities for microsystems and nanomachines, improving both their reliability and lifespan.

The project also lays the groundwork for exploring new emerging 2D materials, such as semimetals or topological materials, which have even more unusual properties than graphene. These materials could allow Casimir forces to be amplified or precisely modulated, opening the way to applications in electronics, photonics, or even nanoscale energy conversion.

Finally, the already productive France–Hong Kong collaboration will continue, combining theory and experiments. Next steps include direct measurements of non-reciprocal forces and the development of devices that actively exploit force modulation. The goal is to move toward technologies where quantum interactions are no longer an obstacle but a tool to design innovative, reliable, and miniaturized systems.

In summary, the CAT project opens a new frontier for nanotechnology: a future where quantum physics, harnessed through 2D materials, could enable microsystems that are more efficient, longer-lasting, and capable of properties previously thought impossible.

CAT is an international collaborative project aiming at investigating Casimir forces in two-dimensional materials, by combining the theoretical (France) and experimental (Hong Kong) expertise of the two teams involved. Casimir-Lifshitz forces refer to the interactions between electrically neutral bodies that arise due to quantum fluctuations. They originate from the fluctuations of the electromagnetic field and the corresponding polarization fluctuations induced in the bodies. This force increases rapidly with decreasing distance between the objects, and becomes the dominant interaction between uncharged bodies at submicron separations. While the Casimir force is of fundamental interests due to its quantum nature, it is also relevant to technological applications. One example is micro- and nano-mechanical devices that are used in everyday life, from inertial sensing in automobiles to radio frequency filters in mobile phones. Under the trend of miniaturization, a thorough understanding of the fundamental interaction between surfaces is essential for future devices to function properly. For instance, attractive Casimir forces could initiate two micromechanical components to jump into contact and stick together, resulting in malfunction of the devices due to undesirable “stiction” (a blend word from sticking and friction). On the other hand, recent theoretical advances have explored constructive uses of the Casimir force, such as the possibility of changing its sign to avoid stiction. In addition, repulsive Casimir forces have also been proposed for levitating objects and for constructing frictionless gears. This project leverages recent advance in the development of two-dimensional materials with novel electronic and optical properties to study Casimir effects in regimes not yet explored. For example, the dispersion of the quasiparticles in graphene leads to thermal effects on the Casimir force occurring at distances significantly smaller compared to traditional 3D materials. At these short distances, the Casimir force is expected to be strong enough to be measured. 2D materials therefore open new opportunities for investigating and exploiting thermal Casimir forces. In addition, by placing the 2D material on top of a nanoscale grating, the interplay between geometry, optical properties of the materials and thermal effects is expected to modify the Casimir force in a non-trivial fashion that has yet to be investigated. Another advantage of 2D materials is that they offer other ways to tune the Casimir force, such as by controlling the chemical potential, which is difficult to achieve with 3D systems. Joint theoretical and experimental efforts of the CAT project will focus on the following objectives: (i) Studying the new features of Casimir Forces stemming from the properties of 2D materials; (ii) Investigating Casimir Forces on suspended layers of 2D materials, free from the influence of substrates; (iii) Exploring the effects of gratings on the Casimir Forces on 2D materials, including the interplay between geometry and thermal effects. The CAT team have strong track records in the experiments (HK) and theory (France) of Casimir forces respectively. While existing theoretical results are largely based on graphene, the team will also consider other 2D systems. Successful completion of the objectives will enable a fundamental understanding of the Casimir forces in 2D materials that is essential for exploiting them in micro-nanomechanical systems. The proposed work is particularly relevant and timely given the recent breakthroughs in Casimir physics and the immense progress in the field of 2D materials. Ongoing collaboration between the two teams, as evidenced by one already published paper and by two working papers, will be further strengthened.