Sonochemistry: the ultrasound and the fury.

Sonochemistry: The ultrasound and the fury

The scene is downright hellish: Millions of tiny gas bubbles continuously form, grow and implode with such rapidity and force that their contents reach temperatures common at the sun's surface. Shock waves radiating from the miniature depth-charges create localized pressures approaching those of the deepest ocean trench. Hieronymus Bosch would have loved it.

Chemists Kenneth S. Suslick and Stephen J. Doktycz do. Working at the University of Illinois in Urbana-Champaign, they use ultrasound, or sound pitched beyond human hearing, to create these turbulent micro-environments safely within glass vessels filled with solvents and solid particles, in an attempt to solve a puzzle. For years, scientists have wondered why ultrasound accelerates -- sometimes up to a millionfold -- chemical reactions involving metal catalysts.

In the March 2 SCIENCE, Suslick and Doktycz report some details underlying these swift "sonochemical" reactions. "This set of experiments allowed us for the first time to get at least a rough feeling for the conditions that occur when ultrasound irradiates a liquid containing solid powder," Suslick told SCIENCE NEWS.

The ultrasonic shock waves force particles to collide, much as a huge ocean wave might smash two surfers together. By examining fine dissolved particles of transition metals--zinc, tin, nickel, chromium, tungsten and others, which collide and can melt together during ultrasonic irradiation -- the researchers have inferred just how fast and hot particle collisions can get.

Tiny metallic "necks" that bridge the fused particles provide the empirical clue for reconstructing the unseen collisions. A neck's volume, ranging from about one-half of six-trillionths of a cubic centimeter, indicates how much metal melts during the collisions and therefore how much energy is needed to drive the melting process. Presumably the particles carried this energy into the collisions in the form of kinetic energy. The researchers calculate that particles with diameters in the 5-to-10-micron range attain velocities ranging from 100 to 500 meters per second.

They also found that metals with melting temperatures below molybdenum's 2,617 [degrees]C form well-defined necks, while tungsten particles, which melt at 3,410 [degrees]C show no signs of fusing. Collisions between molybdenum particles result in less-pronounced links, which the researchers describe as "spot welds." They conclude that peak temperatures during interparticle collisions fall between 2,600 [degrees]C and 3,400 [degrees]C and that some of the molten necks must cool at rates of more than 1 billion [degrees]C per second, since the particles would otherwise separate as they recoil after the collision.

Such findings should help scientists uncover the mechanisms by which ultrasound enhances the catalytic power of metal particles, says sonochemist Philip Boudjouk of North Dakota State University in Fargo.