Physics Tip Sheet #56 - November 2, 2005

Contact: James Riordon
riordon@aps.org
301-209-3238
American Physical Society

Highlights in this issue: insights into bacterial water ballet, faster medical microscopy, and modeling a new material for armor and blast shields.

Bacterial Water Ballet
J. Hernandez-Ortiz, C. Stoltz, and M. Graham
Physical Review Letters (forthcoming article)

Bacteria swimming in water can produce striking patterns when their concentration is high enough, which may lead to novel ways to mix fluids in microscopic experiments. Scientists at the University of Wisconsin-Madison have developed a model that shows that each microscopic swimmer pushes the fluid around as it swims, and in turn is pushed around by moving fluid stirred up by other swimmers. At low concentrations, the swimmers move around in an uncoordinated manner, but at higher concentrations they swarm together into whirling vortex patterns that closely resemble those observed in experiments. Because microorganisms stir up the fluid as they swim, this could suggest an approach to mixing fluids in microscale devices where conventional mixing techniques don’t work effectively.

Faster Medical Microscopy
E. Baleine and A. Dogariu
Phyiscal Review Letters (forthcoming article)

A new microscopy technique could surpass even cutting-edge techniques when it comes to efficiency and screening capabilities. Common medical techniques for imaging small structures within the body rely on time-consuming scanning probes. However, improved screening may be possible with a technique called variable coherence scattering microscopy (VCSM), according to recent theoretical arguments and a proof-of-concept experiment by University of Florida researchers. This technique requires no moving parts, little processing time, and provides detailed pictures of structures over larger areas than is possible with scanning probes.

In VCSM, the medium is illuminated with light that varies in spatial coherence – spatial coherence refers to how the properties of light at one point in space are correlated to those at another point in space. Some of the light illuminating the medium will scatter, and the brightness of the scattered light is measured as the spatial coherence changes. Scientists can then use the data to create a detailed map of the medium over a large field of view.

Blocking Blasts with Graduated Balls
R. Doney and S. Sen
Physical Review E, October 2005
http://link.aps.org/abstract/PRE/v72/e041304  

Rows of graduated spheres offer a promising new type of blast-proofing and shock absorption, according a numerical simulation performed by researchers at the Army’s Aberdeen Proving Grounds in Maryland and the State University of New York. The study showed that the energy of an impact on a large sphere at the head of a row of progressively smaller spheres resulted in significantly reduced energy transmitted to the last sphere in the row, which would potentially shield anything behind the last sphere from the full brunt of the impact. The researchers studied rows of between three and twenty spheres, with varying degrees of resilience, and with different amounts of graduation in sphere size. They found that the graduation of the sphere size was the most effective parameter in changing the ability of the spheres to dissipate energy. The study could lead to lighter and more effective types of battlefield armor and blast proofing.

Kendra Rand, James Riordon, and Ernie Tretkoff contributed to these news tips.

Journal articles are available to journalists on request.

For media assistance with these or other physics stories, contact:

James Riordon
Head of Media Relations
American Physical Society
Ph: +1 301 209 3238
Fax: +1 301 209 3264
Email: riordon@aps.org