Advances in Casimir-Polder Interactions between Atoms and Substrates

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The quantum vacuum is a busy place, with particles popping into and out of existence for short periods of time. This activity gives rise to amazing phenomena, such as the attractive force between closely-spaced uncharged conducting plates, or between neutral atoms, discovered by Casimir more than 70 years ago. A beginning is being made to see how to exploit these Casimir forces in practical devices and nanomachinery. By changing the properties of the objects that interact, and the environment in which they are immersed, the forces can be repulsive rather than attractive. When the objects and atoms are in motion, quantum frictional forces arise between atoms and surfaces even when they are not in contact. This project advances the progress of science and promotes the education of a diverse group of students and researchers. This interdisciplinary work has significant impacts in biology, chemistry, atomic and nuclear physics, and is even finding applications in planetary science.Specific topics to be considered include: (1) Negative Casimir entropies. It is now well known that the interaction entropy between atoms, or between atoms and metallic surfaces, for example, are often negative. Although it was suggested that the positive self-entropies of the atoms themselves would cancel this effect, we have now discovered that this is in general not the case, and that self interactions typically lead to a region of negative entropy. (2) Quantum vacuum forces between atoms and surfaces in inhomogeneous media. Nearly all work on Casimir forces between bodies assumes they are separated by vacuum, or by a homogeneous dielectric medium. But if they they are separated by a spatially varying medium, divergences appear which are not well understood. The group has made some preliminary proposals as to how to extract meaningful interaction energies, but the general situation will require much more work. (3) Repulsive Casimir forces and nonmonotonic torques. Casimir forces can turn repulsive, and not merely with exotic combinations of materials. This is not unrelated to the negative entropies seen ubiquitously, and reflects the nonmonotonicity of the free energy. With anisotropic materials, Casimir torques can undergo sign changes with the distances between atoms and surfaces. Real-world applications to such long-standing problems such as the freezing of ice, and the interaction of greenhouse gases with substrates are being explored. (4) Casimir friction. When an atom or a dielectric plate is moved parallel to nearby plate, a frictional force is experienced due to the interaction with the fluctuations in the electromagnetic vacuum. This requires understanding nonequilibrium dissipative effects. Systematic ways of treating such phenomena are being developed, with the intent of proposing accessible experimental signatures.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.