To Levitate Nano-Objects, Researchers Exploit a Force of Quantum Repulsion

By Eliza Strickland | January 8, 2009 10:37 am

Casimir forceBy harnessing a quantum mechanic force of repulsion researchers have caused nanoparticles to repel each other, and in their next experiment they plan to levitate a tiny gold nanosphere. The quantum force is part of the Casimir effect, first predicted in 1948 by the Dutch physicist Hendrik Casimir, which describes both the attraction and repulsion that occur between two tiny objects held close together in a vacuum. While the attractive force has previously been demonstrated, the new experiment marks the first time the repulsive force has been seen in a lab.

But the experiment wasn’t just a neat physics trick; the researchers say the repulsive force may one day be used in nanoscale devices. Lead author Jeremy Munday says the research may lend itself to producing ultrasensitive detectors and almost friction-free devices by separating their components via Casimir repulsion. “Where you would normally have friction,” he says, “you can start to greatly reduce that by having a repulsive interaction that doesn’t let the surfaces come into contact” [Scientific American].

The Casimir force’s attractive properties go on display when two parallel metal plates are placed near each other in a vacuum, and begin to attract each other. This happens because even a vacuum is actually fizzing with a quantum field of particles, constantly popping in and out of existence. They can even fleetingly interact with and push on the plates. However, the small space between the two plates restricts the kind of particles that can appear, so the pressure from behind the plates overwhelms that from between them. The result is an attractive force that gums up nanoscale machines [New Scientist].

But in the 1960s a Russian physicist theorized that the Casimir force could also produce the opposite effect, forcing objects away from each other. In the new experiment, reported in Nature [subscription required], researchers achieved this effect with a silicon plate and a tiny gold-plated bead placed in a liquid, bromobenzene. The Casimir attraction between the liquid and the silica plate is stronger than that between the gold bead and the silica, so the fluid forces its way around the bead, pushing it away from the plate [New Scientist].

While commercially available devices aren’t yet small enough to run into problems due to the Casimir effect, study coauthor Federico Capasso says it’s just a matter of time before engineers are routinely working in the nanoscale. The research team’s next step is to use the phenomenon to levitate a tiny piece of gold, or other material, in the liquid. “The levitation experiment should be straightforward,” says Capasso, and, if so, could be exploited in tiny machines within 10 years. “I have a hunch that something useful will come out of this,” he adds [Nature News].

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Image: Jay Penni / Federico Capasso

CATEGORIZED UNDER: Physics & Math, Technology
  • Thomas Prevenslik

    Casimir extended the microscopic van der Waals force between atoms in a gas to the attraction between macroscopic bodies in a vacuum. However, recent experiments at Harvard have suggested that the Casimir force can be changed from attraction to repulsion by immersing micron sized gold and silicon structures in liquid bromobenzene.

    But this experiment not only falsely presupposes the Casimir attractive force exists, but then extends that falsity to conclude the attractive Casimir force can be changed to repulsion.

    Indeed, the Casimir force is shown to not exist because Casimir did not conserve the EM radiation in the gap between parallel plates, for if he would have, Casimir would have found the frequency of the EM radiation increases by QED as the gap decreases because the EM energy in the gap must be constant. The gradient of the constant EM energy with respect to the gap therefore vanishes and there is no Casimir force. See

    But then what is the source of the Casimir force being measured?

    The answer is electrostatics. At gaps less than 100 nm, the frequency of the EM radiation reaches VUV levels and the plates charge oppositely by the photoelectric effect. Hence, the attractive force measured in Casimir experiments is electrostatic caused by the charging of the plates by VUV radiation.

    The usual attractive QED induced electrostatic force of oppositely charged gold and silicon structures is changed to repulsion upon immersion in bromobenzene because the latter is an electron scavenger that alters charge distribution.

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