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Tweaking quantum force lowers barrier to tiny devices

July 14, 2008
Courtesy University of Florida
and World Science staff

Cym­bals don’t clash on their own—in our world, an­y­way.

But the quan­tum world, that of sub­a­tom­ic par­t­i­cles, is bi­zarrely dif­fer­ent. Two met­al plates, placed al­most in­fin­i­tesi­mally close to­geth­er, spon­ta­ne­ously at­tract each oth­er.

What seems like mag­ic is known as the Casimir force, doc­u­mented in many ex­pe­ri­ments. The cause goes to the heart of quan­tum phys­ics: Seem­ingly emp­ty space is not ac­tu­ally emp­ty but con­tains “vir­tu­al” par­t­i­cles as­so­ci­at­ed with fluc­tu­at­ing fields of elec­tro­mag­netic force. 

A scan­ning elec­tron mi­cro­graph, tak­en with an elec­tron mi­cro­scope, shows the comb-like struc­ture of a met­al plate at the cen­ter of new­ly pub­lished re­search on quan­tum phys­ics. Phys­i­cists found that cor­ru­gat­ing the plate re­duced the Ca­si­mir force, a quan­tum force that draws to­geth­er very close ob­jects. The discovery may prove use­ful as ti­ny “mi­cro­elec­trome sys­tems -- so-called MEMS de­vices that are al­ready used in a wide ar­ray of con­sum­er prod­ucts -- be­come so small they are af­fect­ed by quan­tum forc­es. (Cour­tesy Yil­iang Bao and Jie Zoue/U. of Flor­i­da)  

These par­t­i­cles push the plates from both the in­side and the out­side. How­ev­er, the space be­tween the plates is poorer in par­t­i­cles than that out­side. This is be­cause par­t­i­cles ex­ist sim­ul­ta­ne­ously as waves, and only waves of shorter lengths, or wave­lengths, fit be­tween the plates.

The plates move to­geth­er, be­cause there are more par­t­i­cles bounc­ing around out­side of them—push­ing them against each oth­er—than there are par­t­i­cles be­tween them push­ing them apart.

Now, Uni­ver­s­ity of Flor­i­da phys­i­cists have found they can re­duce the Casimir force by al­ter­ing the plates’ sur­face. They say the dis­cov­ery could prove use­ful as ti­ny “mi­cro­elec­tro­me­chani­cal” sys­tems — so-called MEMS de­vices al­ready used in a wide ar­ray of con­sum­er prod­ucts — be­come so small they are af­fect­ed by quan­tum forc­es.

These quan­tum ef­fects could come in­to play “if the de­vices con­tin­ue to be smaller and small­er,” said the uni­ver­s­ity’s Ho Bun Chan, an au­thor of a pa­per on the find­ings that ap­pears to­day in the on­line edi­tion of the jour­nal Phys­i­cal Re­view Let­ters. The find­ing, re­search­ers added, could one day help re­duce what MEMS en­gi­neers call “stic­tion”—when two very small, very close ob­jects tend to stick to­geth­er.

Al­though stic­tion has many causes—in­clud­ing the pres­ence of wa­ter mo­le­cules that tend to clump to­geth­er—the Casimir force can con­trib­ute. Such quan­tum ef­fects could prove im­por­tant as the separa­t­ions be­tween com­po­nents in ti­ny ma­chin­ery shrink from mil­lionths to bil­lionths of a me­ter, Chan said.

“A lot of peo­ple are think­ing of ways to re­duce stic­tion, and this re­search opens up one pos­si­bil­ity,” he said.

Dutch phys­i­cist Hen­drik Casimir first pre­dicted that two closely spaced met­al plates would be mu­tu­ally at­tracted in 1948. It took sev­er­al dec­ades, but in 1996, phys­i­cist Steve Lam­ore­aux, then at the Uni­ver­s­ity of Wash­ing­ton, per­formed the first ac­cu­rate meas­ure­ment of the Casimir force us­ing a tor­sional pen­du­lum, an in­stru­ment for meas­ur­ing very weak forc­es.

In a pa­per pub­lished in the jour­nal Sci­ence in 2001, Chan and oth­er mem­bers of a Bell Labs team re­ported tap­ping the Casimir force to move a ti­ny met­al see-saw. The re­search­ers sus­pended a met­al sphere an ex­tremely ti­ny but well-con­trolled dis­tance above the see-saw to “push” it up and down. It was the first demon­stra­t­ion of the Casimir force af­fect­ing a MEMS de­vice.

In the lat­est re­search, the phys­i­cists radic­ally al­tered the shape of the met­al plates, cor­ru­gat­ing them in­to evenly spaced trench­es so that they re­sem­bled a kind of three-di­men­sion­ comb. They then com­pared the Casimir forc­es gen­er­at­ed by these cor­ru­gat­ed ob­jects with those gen­er­at­ed by stand­ard plates, all al­so against a met­al sphere.

The re­sult? “The force is smaller for the cor­ru­gat­ed ob­ject but not as small as we an­ti­cipat­ed,” Chan said, adding that if cor­ru­gat­ing the met­al re­duced its to­tal ar­ea by half, the Casimir force dropped by only 30 to 40 per­cent.

Chan said the ex­pe­ri­ment shows that it is not pos­si­ble to simply add the force on the con­stit­u­ent sol­id parts of the plate — in this case, the tines — to ar­rive at the to­tal force. Rath­er, he said, “the force ac­tu­ally de­pends on the ge­om­e­try of the ob­ject.” 

“Un­til now, no sig­nif­i­cant or non­triv­i­al cor­rec­tions to the Casimir force due to bound­a­ry con­di­tions have been ob­served ex­pe­ri­men­tal­ly,” wrote Lam­ore­aux, now at Yale Uni­ver­s­ity, in a com­men­tary ac­com­pa­nying pub­lica­t­ion of the pa­per.

* * *

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Cymbals don’t clash on their own — in our world, anyway. But the quantum world, that of subatomic particles, is bizarrely different. Two metal plates, placed almost infinitesimally close together, spontaneously attract each other. What seems like magic is known as the Casimir force, documented in many experiments. The cause goes to the heart of quantum physics: Seemingly empty space is not actually empty but contains “virtual” particles associated with fluctuating fields of electromagnetic force. These particles push the plates from both the inside and the outside. However, the space between the plates is poorer in particles than that outside. This is because particles exist simultaneously as waves, and only waves of shorter lengths, or wavelengths, fit between the plates. The plates move together, because there are more particles bouncing around outside of them, pushing them against each other, than there are particles between them pushing them apart. Now, University of Florida physicists have found they can reduce the Casimir force by altering the plates’ surface. They say the discovery could prove useful as tiny “microelectromechanical” systems — so-called MEMS devices that are already used in a wide array of consumer products — become so small they are affected by quantum forces. These quantum effects could come into play “if the devices continue to be smaller and smaller,” said the university’s Ho Bun Chan, an author of a paper on the findings that appears today in the online edition of the journal Physical Review Letters. The finding, researchers added, could one day help reduce what MEMS engineers call “stiction” — when two very small, very close objects tend to stick together. Although stiction has many causes — including, for example, the presence of water molecules that tend to clump together — the Casimir force can contribute. Such quantum effects could prove important as the separations between components in tiny machinery shrink from millionths to billionths of a meter in size, Chan said. “A lot of people are thinking of ways to reduce stiction, and this research opens up one possibility,” he said. Dutch physicist Hendrik Casimir first predicted that two closely spaced metal plates would be mutually attracted in 1948. It took several decades, but in 1996, physicist Steve Lamoreaux, then at the University of Washington, performed the first accurate measurement of the Casimir force using a torsional pendulum, an instrument for measuring very weak forces. In a paper published in the journal Science in 2001, Chan and other members of a Bell Labs team reported tapping the Casimir force to move a tiny metal see-saw. The researchers suspended a metal sphere an extremely tiny but well-controlled distance above the see-saw to “push” it up and down. It was the first demonstration of the Casimir force affecting a micromechanical device. In the latest research, the physicists radically altered the shape of the metal plates, corrugating them into evenly spaced trenches so that they resembled a kind of three-dimensional comb. They then compared the Casimir forces generated by these corrugated objects with those generated by standard plates, all also against a metal sphere. The result? “The force is smaller for the corrugated object but not as small as we anticipated,” Chan said, adding that if corrugating the metal reduced its total area by half, the Casimir force dropped by only 30 to 40 percent. Chan said the experiment shows that it is not possible to simply add the force on the constituent solid parts of the plate — in this case, the tines — to arrive at the total force. Rather, he said, “the force actually depends on the geometry of the object.” “Until now, no significant or nontrivial corrections to the Casimir force due to boundary conditions have been observed experimentally,” wrote Lamoreaux, now at Yale University, in a commentary accompanying publication of the paper.