A FEW GOOD MEMS Harnessing the Casimir effect (which takes place between the two metal plates in the above diagram) could help researchers build tiny machines, such as microelectromechanical systems (MEMS), that today are hindered by surface interactions that can make nanomaterials sticky to the point of permanent adhesion. Image: © Jeremy Munday, California Institute of Technology
Named for a Dutch physicist, the Casimir effect governs interactions of matter with the energy that is present in a vacuum. Success in harnessing this force could someday help researchers develop low-friction ballistics and even levitating objects that defy gravity. For now, the U.S. Defense Department's Defense Advanced Research Projects Agency (DARPA) has launched a two-year, $10-million project encouraging scientists to work on ways to manipulate this quirk of quantum electrodynamics.
Vacuums generally are thought to be voids, but Hendrik Casimir believed these pockets of nothing do indeed contain fluctuations of electromagnetic waves. He suggested, in work done in the 1940s with fellow Dutch physicist Dirk Polder, that two metal plates held apart in a vacuum could trap the waves, creating vacuum energy that, depending on the situation, could attract or repel the plates. As the boundaries of a region of vacuum move, the variation in vacuum energy (also called zero-point energy) leads to the Casimir effect. Recent research done at Harvard University, Vrije University Amsterdam and elsewhere has proved Casimir correct—and given some experimental underpinning to DARPA's request for research proposals.
Investigators from five institutions—Harvard, Yale University, the University of California, Riverside, and two national labs, Argonne and Los Alamos—received funding. DARPA will assess the groups' progress in early 2011 to see if any practical applications might emerge from the research. "If the program delivers, there's a good chance for a follow-on program to apply" the research, says Thomas Kenny, the DARPA physicist in charge of the initiative.
Program documents on the DARPA Web site state the goal of the Casimir Effect Enhancement program "is to develop new methods to control and manipulate attractive and repulsive forces at surfaces based on engineering of the Casimir force. One could leverage this ability to control phenomena such as adhesion in nanodevices, drag on vehicles, and many other interactions of interest to the [Defense Department]."
Nanoscale design is the most likely place to start and is also the arena where levitation could emerge. Materials scientists working to build tiny machines called microelectromechanical systems (MEMS) struggle with surface interactions, called van der Waals forces, that can make nanomaterials sticky to the point of permanent adhesion, a phenomenon known as "stiction". To defeat stiction, many MEMS devices are coated with Teflon or similar low-friction substances or are studded with tiny springs that keep the surfaces apart. Materials that did not require such fixes could make nanotechnology more reliable. Such materials could skirt another problem posed by adhesion: Because surface stickiness at the nanoscale is much greater than it is for larger objects, MEMS designers resort to making their devices relatively stiff. That reduces adhesion (stiff structures do not readily bend against each other), but it reduces flexibility and increases power demands.
Under certain conditions, manipulating the Casimir effect could create repellant forces between nanoscale surfaces. Hong Tang and his colleagues at Yale School of Engineering & Applied Science sold DARPA on their proposal to assess Casimir forces between miniscule silicon crystals, like those that make up computer chips. "Then we're going to engineer the structure of the surface of the silicon device to get some unusual Casimir forces to produce repulsion," he says. In theory, he adds, that could mean building a device capable of levitation.