Opportunities and challenges involving repulsive Casimir forces in nanotechnology.

This article reviews four primary approaches to engineering repulsive Casimir forces—quantum electrodynamic interactions typically attractive between closely spaced metallic surfaces—that are relevant for mitigating stiction and enabling novel functionalities in nano- and microelectromechanical systems. The approaches include (1) introducing liquid media between objects to achieve repulsion via intermediate permittivity, (2) employing magnetic materials or metamaterials to generate repulsive interactions through contrasting electromagnetic responses, (3) creating nonequilibrium conditions such as temperature differences or photon chemical potential imbalances to induce repulsive forces, and (4) designing special object geometries, notably a metallic needle above a perforated metallic plate, that yield repulsion purely from shape effects. While repulsive Casimir forces have been experimentally demonstrated in liquid environments, challenges remain for realizing repulsion in vacuum or air, particularly with magnetic materials and nonequilibrium systems; the geometry-based approach offers a promising theoretical pathway but requires precise fabrication. The review emphasizes the need for further experimental validation to harness repulsive Casimir forces for applications like quantum levitation and ultralow friction devices.