Presumed mute, liquid crystals sound off.

The springs, the jewels, and the tiny wheels are gone, but a ghost of the ticks and tocks of traditional clocks may linger inside today's digital watches, a new and surprising discovery suggests.

In fact, an imperceptible murmur may emanate from cell phones, calculators, and other devices with liquid-crystal displays.

"It's an amusing thing," comments Robert B. Meyer of Brandeis University in Waltham, Mass. "People will think, `My liquid crystal is actually chirping at me.'"

A liquid crystal is a fluid of rodlike molecules that line up in an orderly way. Manipulating the alignment can affect the fluid's transparency (SN: 8/7/99, p. 87; 6/1/96, p. 348). In most liquid-crystal displays, layers of glass coated with thin, ( transparent electrodes sandwich a film of oil-like liquid crystal about 10 micrometers thick. Rods of the liquid crystal normally lie stacked in planes parallel to the glass surface, a horizontal orientation that allows light through the cell. Application of a voltage flips the molecules to a vertical position, turning the cell opaque enough to appear dark.

Ordinarily, no one hears the newly reported chirps because everyday liquid-crystal displays use low voltages and their components are probably not the best size to make loud sounds, says Jay S. Patel of Pennsylvania State University in State College.

Young Jin Kim, a researcher in Patel's lab, chanced upon the sounds while testing an experimental optical switch, or cell, that operates at more than 30 volts, much higher than the few volts of most displays. With each voltage pulse, the switch gave off a click "like the sound of a ratchet," Kim says. A voltage varying as a sine wave transformed the click into a steady hum.

In search of the sounds' cause, the incredulous scientists made and tested hundreds of cells, using a sensitive microphone and amplifiers. They concentrated on nematic liquid crystals, the most widely used type of liquid-crystal material.

To confirm that the liquid-crystal rods themselves make the sound, the researchers applied voltages to cells containing disordered liquids. Then, they didn't hear a peep. They also heated the liquid crystal above a temperature at which orderly liquid-crystal behavior breaks down. Again, no sounds.

They performed other tests as well. "From so many studies, we can confirm that the sound is from the motion of liquid-crystal molecules," Kim says.

The experiments demonstrate electrical control over both optical and acoustic properties of a cell, remarks Peter Coilings of Swarthmore (Pa.) College. "That's why I see some cause for excitement," he says.

Patel says that it's too early to tell, however, whether the unexpected effect might lead to novel practical applications for liquid crystals.

He and Kim speculate that the sloshing of molecules as they flip from horizontal to vertical and back somehow vibrates the glass. The container, in turn, emits sounds at frequencies from 1 to 14 kilohertz--within human hearing range. Sound volume peaks when molecules flip at natural vibration frequencies of the containers, the researchers found.

How such weakly powered motion could agitate the glass is "a mystery," Meyer says. "It begs some kind of analysis."

The researchers report their discovery in the Sept. 27 APPLIED PHYSICS LETTERS. They claim there that until now, "there have been no reports of sound generation from liquid crystals" due to a changing electric field.

However, Antal Jakli of Kent (Ohio) State University disputes their claim. He cites more than 20 publications since 1985, many of which he coauthored, that describe similar effects in the types of liquid crystals known as smectic and ferroelectric.

Patel admits that his group was unaware of the work of Jakli and his colleagues. Nonetheless, for the widely used nematic liquid crystals that his group has investigated, "nothing like this has ever been done before," Patel asserts.