News Release

APS physics tip sheet #49

News from the American Physical Society

Peer-Reviewed Publication

American Physical Society

Highlights in this issue:

A better time machine model; shaking to reduce nanoscale friction; crack propagation in thin sheets; and the horizontal Brazil nut effect

A New Class of Time Machine
Amos Ori
Phys. Rev. Lett. 95, 021101 (2005)

A physicist at the Israel Institute of Technology in Haifa has proposed a new class of time machines that seems to avoid some of the difficulties inherent in other theoretical time machines. Like many time machine models, the new proposal requires gravitational fields that curve spacetime in ways that allow observers to travel to their own past. However, unlike previous proposals that have typically required exotic and improbable forms of matter, the new time machine core would consist of a toroidal vacuum embedded in sphere of normal matter. Important questions remain, but at the very least the material required to make the machine exists in our universe.

Shaking Reduces Friction
Z. Tshiprut, A. E. Filippov, and M. Urbakh
Phys. Rev. Lett. 95, 016101 (2005)

Lateral vibrations can control friction at the nanoscale, researchers reported in the 1 July 2005 issue of Physical Review Letters. The researchers modeled a tip interacting with a substrate that vibrates in the lateral direction, and showed that vibrations at the correct frequency and amplitude can dramatically reduce friction, and can even make it possible to transform stick-slip motion to smooth sliding. Previous studies have suggested controlling friction with normal vibrations; this paper adds another new method scientists can potentially use to reduce friction. The authors also suggest experiments to test the effects they predict. Being able to control friction in this way may be useful for micromechanical devices and computer disk drives, where friction may cause unwanted stick-slip motion or damage to the device.

Cracks in Thin Sheets
B. Audoly, P. M. Reis, and B. Roman
Phys. Rev. Lett. 95, 025502 (2005)

When a piece of thin material cracks, what determines the shape of the crack? The authors of this paper analyze cracks propagating in thin films, and find simple geometrical rules that explain crack behavior. When a tool tears an elastic thin sheet, oscillatory fracture patterns emerge (think of the jagged tear that forms when you rip open a sealed envelope with a finger or letter opener.) Starting from principles of elasticity, the researchers show that the crack dynamics is determined by a simple geometrical model based on the interplay between crack propagation and out-of-plane deformations of the thin film. Their model accurately reproduces experimental observations of the complex wavy crack pattern that emerges when a tool cuts through a thin sheet.

Pictures and movies of the experiment and simulations can be found at:

The Horizontal Brazil Nut Effect
T. Schnautz et al.
Phys. Rev. Lett. 95, 028001 (2005)

Swirling a circular tray filled with granular materials causes certain grains to migrate to the center or to the edge of the tray, depending on the grain size and density. A collaboration of researchers from Bayreuth University in Germany and Universidad Complutense in Spain has experimentally studied the phenomenon and compared it to theoretical models. They conclude that the effect is a horizontal analogue to the vertical Brazil Nut effect and Reverse Brazil Nut effect, which can cause large particles to migrate to the top (the normal Brazil Nut Effect) or the bottom (the Reverse Brazil Nut effect) of a shaken container filled with a grain mixture.


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