Historical research guide · 0807 · 2009

Measured long-range repulsive Casimir–Lifshitz forces

J. N. Munday · Federico Capasso · V. Adrian Parsegian

In one minute

Jeremy Munday, Federico Capasso and Adrian Parsegian did something the vacuum was widely assumed not to allow: they measured a Casimir force that pushes instead of pulls. The Casimir–Lifshitz force is the pressure of the quantum vacuum on nearby bodies, and every measurement before this one found attraction. Lifshitz theory says the sign follows the materials: if the fluid between two solids responds to light more strongly than one of them and less strongly than the other, the force reverses. Gold, bromobenzene and silica satisfy that ordering across a wide span of frequencies. So the team hung a gold-coated sphere on an atomic force microscope cantilever, submerged the whole cell in bromobenzene, and watched the cantilever bend toward a gold plate and away from a silica one, on approach and on retraction alike. The repulsion is weaker than the attraction, and both grow as the surfaces close. Quantum levitation in a fluid, they note, follows from it.

Connection

Chapter 2’s case that the vacuum is a real medium rests on forces you can measure, and this is the measurement that shows the sign of those forces is an engineering choice: pick the materials and the fluid and the vacuum pushes. Chapter 6’s vacuum-force devices need exactly that handle, since a repulsive branch means levitation, no stiction and near-frictionless moving parts.

From the source

Lifshitz theory sets the sign of the long-range quantum-fluctuation force by the ordering of the three materials’ dielectric responses at imaginary frequency: between two like materials the terms are negative and the force attracts, but when the fluid’s response sits between those of the two solids the terms turn positive and the force repels — and gold, bromobenzene and silica satisfy that ordering across a large frequency range.Equations 1 and 2 with Figure 1a and 1b
The experiment measured it directly: a 39.8 micrometre polystyrene sphere coated with a 100 nanometre gold film, mounted on an atomic force microscope cantilever inside a cell filled with filtered bromobenzene, deflects toward a gold plate — attraction — and away from a silica plate — repulsion — with the same sphere and the same fluid.Figure 2a to 2c and the paragraph beginning ‘Raw deflection versus piezo displacement data’
The repulsion is not a fluid or a charge artefact. It persists on both approach and retraction, whereas hydrodynamic drag opposes the motion and would change sign with direction, and any charge trapped on the silica would induce an opposite image charge in the metal sphere and pull the two together; electrostatic force microscopy on both plates found no excess charge accumulation.Paragraph following Figure 2; Supplementary Information on electrostatic effects
Calibrated against the hydrodynamic force — linear in velocity, so running the piezo at two speeds and subtracting isolates it — and averaged over fifty runs per configuration, the repulsive interaction measures weaker than the attractive one, both grow as the separation shrinks, and the repulsion follows the temperature-dependent Lifshitz calculation once the measured surface roughness is included; force magnitudes below about 10 piconewtons are left undetermined because of the spread in the data.Paragraphs on detector calibration; Abstract; Figure 3a to 3c
The measurement that would tighten the comparison is optical: the authors attribute the remaining gap between theory and experiment mainly to the dielectric data for bromobenzene, whose two-oscillator model they judge insufficient and whose properties have not been measured over a large spectral range, with roughness corrections also expected to fail once the roughness approaches the separation.Paragraph beginning ‘Uncertainties in the optical properties’
The payoff the authors name is mechanical: with materials chosen so the long-range force repels, one surface can be made to levitate above another in a fluid, which would suppress stiction and open a class of switchable nanoscale devices and sensors with ultra-low static friction.Abstract, final sentence; closing paragraph

The way in

https://doi.org/10.1038/nature07610

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

Published as Nature 457, 170 to 173, on 8 January 2009, by Jeremy N. Munday and Federico Capasso of the School of Engineering and Applied Sciences at Harvard University with V. Adrian Parsegian of the National Institutes of Health. Licence checked directly: the article carries the Springer text-and-data-mining licence rather than a Creative Commons grant, and the accepted author manuscript deposited at PubMed Central as PMC4169270, manuscript NIHMS620257, carries the publisher’s copyright and no Creative Commons statement. So this sheet carries the summary, the claims and the authors’ own abstract and sends the reader to the source; the author manuscript is free to read at PubMed Central. The summary and claims below were written from that complete author manuscript, and the locators cite its figures and paragraphs. The work was supported in part by the Center for Nanoscale Systems at Harvard and by the intramural research programme of the National Institutes of Health. Three companion sheets in this library carry the rest of this conversation: Bressi and colleagues on the parallel-plate Casimir measurement at /library/stm-208d347532, Sushkov and colleagues on the thermal Casimir force at /library/stm-4fb469cffa, and Munday’s later work on Casimir force control with three-dimensional nanostructures at /library/stm-b3d5907768.

How to cite it

J. N. Munday, Federico Capasso, V. Adrian Parsegian (2009) Measured long-range repulsive Casimir–Lifshitz forces doi:10.1038/nature07610

Where it sits in the curriculum

Casimir physics and vacuum-force engineering