Physics Tip Sheet #61 - May 6, 2006

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

Highlights in this issue: A possible solution to the mystery of Martian razorbacks; and DNA and carbon nanotube micromachines that convert rotation to translation and back again.

The Mystery of Martian Razorbacks Solved in the Lab . . . Maybe
Troy Shinbrot et al.
Phys. Rev. Lett. 96, 178002 (2006)
http://link.aps.org/abstract/PRL/v96/e178002  

Experiments with fined-grained powders have led to shapes that may solve the mystery of freestanding, plate-like “razorback” structures recorded during the 2004 Mars Rover missions (see a photo and a brief article about the razorbacks at http://antwrp.gsfc.nasa.gov/apod/ap040727.html). Researchers at Rutgers University in New Jersey found they could duplicate the shapes by pouring a stream of sand onto a sheet of acrylic. Under conditions of low relative humidity, the grains became statically charged, which caused some of the grains to launch into the air with audible pops and others stick together to form razorback-like conglomerations. The researchers propose that Martian winds or motion of the grains against each other due to gravity may lead to electrical charging in the planet’s arid atmosphere, which in turn causes the self-forming razorbacks. Previous speculation had focused on the possibility that the Martian razorbacks were due to geological processes, despite the fact that the thin, fragile structures would have been unlikely to survive erosion for the periods required for them to form geologically.

DNA Machines Turn Rotation into Translation
Alexandre Dawid et al.
Physical Review Letters
Physical Review Letters (upcoming article, available to journalists on request)

Carbon Nanotubes Turn Translation into Rotation
Haiyi Liang and Moneesh Upmanyu
Phys. Rev. Let. 96, 165501 (2006)
http://link.aps.org/abstract/PRL/v96/e165501  

Some DNA molecules may serve as handy micromachines to convert rotational motion into linear translation, an important function for positioning minuscule parts in microscopic devices. Researchers at the École Normale Supérieure in France made the micro-translators by securing one end of a palindromic DNA molecule and untwisting the other end of the helical molecule. A palindromic DNA molecule consists of an amino acid chain that reads the same forwards and backwards, just as is true of the letters in palindromic words such as “rotator” and “deified.” The researchers found that by grasping the end of a DNA molecule with a pair of magnetic tweezers and twisting in the opposite direction of the molecule’s helix, the DNA would buckle in the middle and form a cross-shaped molecule (see http://www.nd.edu/~aseriann/palindna.html for an example of palindromic and cruciform DNA). Cruciform DNA is shorter than the linear configuration, and grows ever shorter as the molecule is untwisted.

Meanwhile, researchers at the Colorado School of Mines have shown theoretically that carbon nanotubes can perform the reverse task, i.e. converting linear motion to rotation. Simulations show that stretching or compressing a carbon nanotube along its length makes it twist, provided the nanotube has a helical, spring-like structure. Future experiments will be needed to confirm the phenomenon.

Converting rotational motion to translation and converting translational motion to rotation are vital tasks in macroscopic machines that are usually performed by worm screws and gears. Unfortunately, they are are difficult to duplicate on small scales. Together, these two studies suggest that DNA molecules and carbon nanotubes could solve the problem, and may soon be important components in micromachine parts bins that researchers will turn to when assembling tiny, complex machines.

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