For the sake of the scoop, Ben & Jerry's managed to do what no refrigerator manufacturer has: jolt into development the world's most eco-friendly ice-cream freezer. Unveiled for Earth Day 2004 at a Manhattan scoop shop, the chiller relies not on greenhouse gases but on sound waves to keep the precious stuff cold. Research on thermoacoustic refrigeration limped along for 20 years or so until the ice cream duo forked over $600,000 to a Penn State team, which made the prototype in two years. The freezer is an ordinary deli cabinet piped to a 14-inch-high canister with soundproof stainless steel walls. A loudspeaker pumps sound waves (at 190 decibels, louder than a rocket launch) into the canister, expanding and contracting helium gas inside. The pressure changes chill the icebox as efficiently as a conventional freezer. Cool? Sure. But even with investors now vying to make the technology commercially viable, the long-standing reign of cheap, functional ozone-polluting fridges won't be trumped anytime soon. "Historically, environmental friendliness doesn't sell," says Penn State acoustician Matt Poese. But don't tell that to Ben & Jerry's. The company will test and promote the rig at its Vermont tour facility, summer 2004.

 

A sound wave consists of oscillations in pressure, temperature, and displacement. Although the temperature oscillations are small, research during the past two decades has shown that this "thermoacoustic" effect can be harnessed to produce powerful, reasonably efficient heat engines, including heat pumps, and refrigerators. Thermoacoustic engines typically have no moving parts; at most, there is a single oscillating part (such as a loudspeaker) with no sliding seals. Thus, these engines have the potential to be both simple and reliable. Research by Greg Swift at Los Alamos National Laboratory on the thermodynamics of the thermoacoustic process has led to the development of prototype refrigerators with cooling powers up to tens of watts, and prototype engines with efficiencies approaching those of conventional engines. The research has spawned collaborative efforts that have resulted in advances in the theory, design, and construction of thermoacoustic devices.

Scientific Impact: Los Alamos' leadership in both the scientific and technological aspects of thermoacoustics since the mid-1980s has generated a sizeable academic research community around the world. The first international workshop on thermoacoustics will be held in 2001.

Social Impact: Thermoacoustic energy conversion (including conversion of heat to acoustic power, acoustic power to refrigeration, and acoustic power to mixture separation) is reasonably efficient and should be inexpensive and reliable in mass production. Efforts are under way to develop a natural-gas liquefier for use in remote locations, a residential co-generation system to produce both electricity and gas heat, an electric generator for deep-space probes, and a water chiller for use on submarines.

Reference: S. Backhaus and G.W. Swift. "A thermoacoustic-Stirling heat engine." Nature, 399:335-338, 1999.

G. W. Swift. "Thermoacoustic engines and refrigerators." Physics Today, pages 22-28, July 1995.

G. W. Swift. "Thermoacoustic engines." J. Acoust. Soc. Am., 84:1145-1180, 1988.

Liquids are now at the mercy of a breakthrough material from Bell Labs. Flip an electric switch and the material acts like a sponge. Flip again and it behaves like a rain slicker. Applications could turn up wherever liquids meet solids (read: practically everywhere). Lead researcher Tom Krupenkin envisions near-frictionless torpedoes, self-cleaning windshields and more efficient batteries.

Water clings to most materials, either soaking in or beading up, depending on surface area and composition. The new material, etched from silicon, resembles a microscopic bed of grass. Each "blade" is a few nanometers thick--about 100,000 times smaller in diameter than a single human hair. When liquid drops onto the tiny blades, it suspends itself on their tips without sinking between. The blades "reduce the surface area the droplet feels," says Krupenkin, so the liquid beads up effortlessly. When the researchers charge the silicon with electricity, the energy field pulls the liquid down into the gaps, and the "nanograss" wets instantly.