Historical research guide · 0502 · 2025
Casimir Force Control Enabled by 3D Nanostructures
Calum Shelden · Benjamin Spreng · Joseph L. Garrett · Tahmid S. Rahman · Jongbum Kim · Jeremy N. Munday
In one minute
The Casimir force is what empty space does to two objects held very close together: shut some of the vacuum’s electromagnetic modes out of the gap and the pressure that remains pushes the objects toward each other. Below a micrometre it dominates everything else, and it is a main reason tiny machined parts stick together and stop working. Jeremy Munday’s group, with Calum Shelden and Benjamin Spreng, show that you can engineer it away — or amplify it — using shape alone. Holding the material fixed, they measured the force between a gold-coated sphere and three sculpted gold surfaces: single pillars, pillars with a hole down the middle, and periodic pillar arrays. A single 300-nanometre pillar cuts the force by roughly a factor of ten. A hole in the pillar bends the force-versus-distance curve the opposite way. A dense array does the reverse again and nearly triples it. All of it in ordinary air at room temperature, with plain theory close enough to design from.
Connection
Chapter 2 turns on the vacuum being a real medium you can measure pushing back, and chapter 6 needs a handle on that push that an engineer can actually turn. This paper is that handle: same metal, same ambient air, force strength and force-distance law both set by geometry, tenfold down with one pillar and threefold up with an array — the design vocabulary any Casimir-driven actuator or vacuum-energy cell will be built from.
From the source
The Casimir force results from the alteration of the zero-point energy of electromagnetic fields in the presence of boundaries; it exists independent of any electric charge on the objects and is strongly dependent on the geometry of the surfaces involved.Introduction, opening paragraph
Engineered 3D nanostructures dramatically modify the force behavior between objects of identical material: a single pillar suppresses the Casimir force by a factor of ten, and the separation dependence changes shape as well — shrinking the pillar makes the force gradient fall off faster with distance than the sphere-and-plate case, while adding a hole to the pillar makes it fall off more slowly.Abstract; Figure 2c and 2d and the surrounding text
A dense periodic pillar array enhances rather than suppresses the interaction: at a pillar-radius-to-gap ratio of one half the force gradient is nearly three times that of a single isolated pillar at a surface separation of about 85 nm, before converging to a sphere-and-rough-plate result at the largest separations.Figure 3a and inset; results text on pillar arrays
Which surface dominates the force is itself a design parameter. As the separation grows from 10 to 300 nm above a 300 nm pillar, the supporting plate’s contribution to the force gradient rises from 0.5 percent to 94 percent, whereas above a 3 micrometre pillar the plate’s contribution rises by only about 11 percent over the same range.Figure 2e and 2f; pressure-gradient map discussion
The proximity force approximation agrees strikingly with measurement even for sharp-edged nanostructures whose dimensions are comparable to the separation — within a factor of about three — which supports the view that it is the confinement of modes between nanostructures, not the nanostructure itself, that drives the strong deviations seen in high-aspect-ratio gratings.Abstract; concluding paragraph
No single version of the proximity force approximation is best across all separations: the standard form, the Derjaguin approximation and the piece-wise Derjaguin approximation each win over a different separation range, and numerically exact scattering calculations for a sphere above a pillar array converge only for separations above about 3 micrometres — well outside the experimental regime.Figure 4 and its insets; final results section
The way in
https://doi.org/10.1021/acs.nanolett.5c01101How to cite it
Calum Shelden, Benjamin Spreng, Joseph L. Garrett, Tahmid S. Rahman, Jongbum Kim, Jeremy N. Munday (2025) Casimir Force Control Enabled by 3D Nanostructures doi:10.1021/acs.nanolett.5c01101