January 2003

From American Physical Society

Physics tip sheet #31 - January 15, 2003

1) Buckyball bearings make perfect lubricant for nanomachines
K. Miura, S. Kamiya, N. Sasaki
Physical Review Letters (to appear)

Early work on nanomachines kept crashing to a halt. The machines would get stuck due to intermolecular forces. Now, a technique for eliminating friction has been tested by physicists. Their solution is to use C60 molecules (also called buckyballs and looking like soccer balls of carbon atoms) as bearings between flat plates of graphite (another form of carbon). Although this system has a small amount of friction when starting from rest, friction is completely eliminated once it is moving.

Journal article: Available on request

2) Mystery of rain drop formation clears up
R. McGraw, Y. Liu
Physical Review Letters (Print issue: January 10, 2003)

A longstanding mystery in cloud physics has been how small rain droplets can fall. Modeling small drop formation is critical for understanding the indirect effect of aerosols on climate change. Raindrops grow through three processes: nucleation (which seeds the raindrop), condensation (where water vapor collects on the nucleated drop) and collection (where a falling drop bumps into other drops to grow). However, condensation is very slow for newly-nucleated drops and the time it would take for small drops to form by this process is typically longer than the lifetime of the cloud. Similarly, collection only happens for drops large enough to be falling through the cloud. A new model of raindrop growth shows how small drops can form and explains some of the differences between continental (polluted) clouds and maritime (clean) clouds.

Physical Review Focus: http://focus.aps.org/story/v11/st2
Journal article: http://link.aps.org/abstract/PRL/v90/e018501

3) Solving the timetabling problem with a quantum computer
J. Zhang, H. Wang, Z. Deng, Z. Lu
arXiv preprint server

The timetabling problem (finding non-conflicting meeting times for a set of people with different requirements) has been mathematically proven to be very difficult. However, it is this sort of problem that for which quantum computers can be ideal. Some new experiments have solved a small timetabling problem using a nuclear magnetic resonance (NMR) quantum computer in chloroform by adapting the existing quantum algorithm used to do fast searches of databases.

Preprint: http://www.arxiv.org/abs/quant-ph/0301033

4) How your breakfast cereal separates
Physical Review Letters
Print issue: January 10, 2003) A. P. J. Breu, H.-M. Ensner, C. A. Kruelle, I. Rehberg

Why are all the nuts in your breakfast muesli at the top of the packet when you open it? This Brazil nut effect, as it is known, has been the subject of debate for many years with a wide variety of explanations posed. Then a physicist noticed that the reverse problem can occur the nuts can sink to the bottom. This paper describes experiments to study the Brazil nut and reverse Brazil nut problems. It finds that both effects do occur with the large particles moving upwards for gentle shaking and the large particles moving downward for strong shaking. However, the large particles will never reach the bottom if the cereal box is too deep.

Journal article: http://link.aps.org/abstract/PRL/v90/e014302

5) How the cochlear hears
Physical Review Letters (to appear)
M. O. Magnasco

The cochlear plays a vital role in converting the sounds we hear into signals the brain can interpret. How the cochlear performs this task is now sounding clearer to one physicist. He has shown that a relatively simple model can explain how the cochlear dissects complex sounds for the brain to understand. Sounds coming into the ear are forced by the geometry of the cochlear to travel in one direction. Different pitches travel at different speeds and thereby split apart. These different pitches cause vibrations in different parts of the cochlear and the spatial separation of sound frequencies helps the brain to very sensitively determine the components of a complex noise.

Journal article: Available on request

6) Patterns of vegetation in dry regions
Physical Review Letters (to appear)
N. M. Shnerb, P. Sara, H. Lavee, S. Solomon

Vegetation grows in characteristic patterns in arid and semi-arid regions. Plants compete for limited amounts of moisture and so tend to grow far apart but the region around a plant has the most water and so is the most likely place for vegetation to spread. These competing effects contribute to creating the patchy patterns observed in deserts. If there is more than some threshold amount of water, the vegetation manages to trap enough of it to spread widely. A new simple model is able to reproduce these effects and displays behavior similar to that observed in real life field measurements.

Journal article: Available on request

7) Saving energy in pumping fluids
J.-P. Matas, J. F. Morris, E. Guazzelli
Physical Review Letters (Print issue: January 10, 2003)

When water is pumped through pipes, energy is lost if the flow becomes turbulent. If turbulence can be prevented, savings will be made. Although the transition to turbulence has been studied for over a century, there is still much to learn because it is such a difficult problem. This study looks at the transition to turbulence for fluids carrying particles, such as pipeline flows of slurries. A set of experiments characterizes how the particles influence the onset of turbulence and determines the relevant properties of the pipes carrying the flow.

Journal article: http://link.aps.org/abstract/PRL/v90/e014501











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