Synopsis: Sticky situations

The critical Casimir effect—the thermodynamic analog to the quantum case—appears as a measurable force in colloids.
Synopsis figure
Illustration: A. Gambassi et al., Phys. Rev. E (2009)

When two conducting plates are brought in close proximity to one another, vacuum fluctuations in the electromagnetic field between them create a pressure. This effective force, known as the Casimir effect, has a thermodynamic analog: the “critical Casimir effect.” In this case, thermal fluctuations of a local order parameter (such as density) near a continuous phase transition can attract or repel nearby objects when they are in confinement.

In 2008, a team of scientists in Germany presented direct experimental evidence for the critical Casimir effect by measuring the femtonewton forces that develop between a colloidal sphere and a flat silica surface when both are immersed in a liquid near a critical point [1]. Now, writing in Physical Review E, Andrea Gambassi, now at SISSA in Trieste, Italy, and collaborators at the Max Planck Institute for Metals Research, the University of Stuttgart, and the Polish Academy of Sciences, follow up on this seminal experiment and present a comprehensive examination of their experimental results and theory for the critical Casimir effect.

Success in fabricating MEMS and NEMS (micro- and nanoelectromechanical systems) made it possible to explore facets of the quantum Casimir effect that had for many years only been theoretical curiosities. With the availability of tools to track and measure the minute forces between particles in suspension, scientists are able to do the same with the critical Casimir effect. In fact, it may be possible to tune this thermodynamically driven force in small-scale devices so it offsets the attractive (and potentially damaging) force associated with the quantum Casimir effect. Given its detail, Gambassi et al.’s paper may well become standard reading in this emerging field. – Jessica Thomas

[1] C. Hertlein et al., Nature 451, 172 (2008).


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