Physics News Update No. 687

The American Institute of Physics Bulletin of Physics News Number 687
June 4, 2004 by Phillip F. Schewe, Ben Stein

REVERSING TIME TO CATCH SNIPERS. At last week's 75th anniversary meeting of the Acoustical Society of America in New York City, researchers presented a system that uses "time-reversed" acoustics to pinpoint the exact locations of gunfire and explosions in an urban environment.

Coming from the U.S. Army's Cold Regions Research and Engineering Laboratory and the University of Connecticut, the researchers (Donald.G.Albert@erdc.usace.army.mil and Lanbo.Liu@erdc.usace.army.mil)?tested the system in a small "training" village consisting mainly of two-story concrete-block buildings. In their tests, they fired a gun at an arbitrary location within the village. The gunshot echoed from building walls and other surfaces. A network of simple audio sensors recorded the reverberations at unique vantage points.

The researchers then turned to a computer, which contained a 2-D computer model of the village. Inside this "virtual village," the computer generated a backwards version of each recorded sound wave. Displaying a map of the village, the computer broadcasted the time-reversed waves from the locations corresponding to the sensors that recorded the original waves. In the computer map of the village, the time-reversed waves eventually returned and converged at the spot corresponding to the source of the gunshot.

The researchers are hoping to develop the system for real-world use, for example by reducing the amount of computer processing time associated with the procedure so that it can potentially pinpoint snipers and explosions in real-time. (Paper 5aPAb5; pictures, movies and lay-language text at http://www.acoustics.org/press/147th/liu-albert.html)

OBSERVING SUPERFLUIDITY IN HYDROGEN MOLECULES is difficult since the predicted temperature at which liquid H2 would become superfluid (losing all viscosity), about 2 K, is well below the triple point of hydrogen (14 K), the temperature below which H2 exists only as a solid. To make H2 into a superfluid, H2 molecules would have to be supercooled, cooled rapidly below their freezing point.

A new experiment at the Instituto de Estructura de la Materia-CSIC in Madrid has not yet observed superfluid H2, but physicists there have, for the first time, proved that tiny H2 droplets---tiny clusters, with up to 8 molecules, in a gas jet---are liquid in form. The scientists (from Madrid, a Max Planck Institute in Goettingen, and Washington State University) determined the liquid status of the individual cluster sizes through Raman scattering, the process in which the energy of a laser beam is depleted ever so slightly when it passes through a molecular medium (in this case the H2 droplets) by the excitation of the molecules. This proved for the first time that a Raman spectrum can be obtained for H2 clusters.

Why so much fuss over whether hydrogen can be made superfluid? If successful it would be the first direct evidence for the existence of another superfluid besides helium, at present the only known liquid superfluid. H2 is the simplest and most abundant molecule in the universe, and scientists rely on it to point to properties in other atoms and molecules. Furthermore, hydrogen is the primary fuel in stars, while on Earth hydrogen might also play an important role as fuel since it has the highest chemical energy density by mass. (Tejeda et al., Physical Review Letters, 4 June 2004).

MICROFLUIDIC TANGO: SORTING WITHOUT DIFFUSION. Separations of complex biological mixtures such as the contents of a cell require biomolecules to be sorted by their size or density. To accomplish this, molecular biologists usually employ methods that rely on diffusion, the often gradual migrations of particles from one zone to another.

However, diffusion-based sorting requires patience, since the particles must randomly wander over a large number of possible paths. Now, a multidisciplinary Princeton team (Robert Austin, Austin@princeton.edu)?has produced a potentially faster, non-diffusion-based sorting method. The researchers tap into the power of microfluidics, the control of liquids using microscopic structures. Their microfluidic method allows them to sort objects in a nonrandom (deterministic) fashion.

In their technique, a smooth fluid carries the biomolecules of interest in a downward stream. Encountering arrays of obstacles staggered in a certain way, smaller molecules zig-zag back and forth through the obstacles but must proceed on the average straight down. However, if a biomolecule is big enough, it moves steadily at an angle to the zig-zag motion, taking tango-like dance steps as it veers to the left or right, thereby separating itself from the smaller molecules.

In their initial demonstrations, the researchers have sorted fragments of artificial bacteria chromosomes to within 12% of their molecular weight in 10 minutes, already an order of magnitude faster than conventional methods. In tests with sub-micron polymer bead particles, the device can rapidly and continuously sort them into an array of output channels with a resolution of 1% of the particles' radius or less. Thus the device may find applications in the area of sorting inorganic nanoparticles as well. (Huang et al., Science, 14 May 2004.)

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