Sound waves have been drawing special attention from physicists lately because of their peculiar behavior traveling through media such as colloidal suspensions, biological tissue, and powders. Yet another instance of some surprising acoustic behavior is revealed in the 25 January issue of PRL: Researchers in Mexico have for the first time looked at sound waves pulsing through a magnetorheological slurry–a fluid whose acoustic properties can be controlled by a magnetic field. They discovered an unexpected longitudinal mode propagating at remarkably low speeds through the slurry and have come up with a simple model that might explain its origins.
The slurry consists of tiny magnetic particles suspended in a liquid. Without a magnetic field, it behaves like a fluid, but as soon as the field is turned on, the magnetic particles weave themselves into a connected mesh of filaments that form a metallic skeleton in the fluid, completely changing the viscosity and elasticity of the material. The change can happen within milliseconds, so it may be useful in the future for making acoustic devices.
Carlos Ruiz-Suárez and his colleagues at the Center for Research and Advanced Studies in Mérida, Mexico, wanted to study sound propagation in this unusual material. The researchers filled a small cylindrical cell about 3 cm in diameter with tiny particles of iron suspended in glycerine and wound a field-controlling magnetic coil around the cell. They then created a sound wave at one end of the cylinder and measured its travel time for distances up to 60 mm. Since previous theoretical and experimental work indicated that only one low-frequency mode could travel through a colloidal suspension, Ruiz-Suárez and his colleagues were extremely surprised to see a second mode the instant they turned on the magnetic field. Even more startling was the speed of this new mode–at about 50 m/s , it was 40 times slower than the first mode.
The researchers believe that the two modes are propagating through different channels in the slurry. The first mode travels mainly through the glycerine fluid, while the second travels through the metallic skeleton that is formed in the presence of the external field. The experiment supports this explanation because the first mode travels close to the speed of sound in pure glycerine and comes even closer to that speed with increasing field, as the iron particles clump into larger structures. The speed of the second mode also increases with field for the same reason, but is still about 100 times less than the speed of sound in solid iron. The snail’s pace of the mode is partly explained by the fact that the iron skeleton in the slurry is much less stiff than solid iron, but the researchers don’t yet have a complete explanation.
David Weitz of the University of Pennsylvania is intrigued by the discovery of the unexpected second mode. “They have found new behavior in traditional sound waves,” he says. Now, he says, physicists will have to explain why.