Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Smooth surfaces are tacky at small scales

15.03.2002


Spot-welds stick sliding metals


Metals stick as they slip if pressed together and pushed.
© GettyImages



Two smooth, cold, metal surfaces are like pieces of tacky Sellotape. They form tiny spot welds that have to be broken apart before they can slide over each other. This, claim two physicists in California1, is another reason why metals stick as they slip if they are pressed together and pushed.

Such microscopic causes of friction and wear are increasingly important as the scale of mechanical engineering shrinks to below what’s visible. Here, conventional methods of lubrication start to fail.


Stick–slip motion is common between surfaces that are not lubricated. It causes chalk to screech on a blackboard, makes a violin string vibrate when a bow passes across it, and can wear out watch mechanisms and cutting tools.

Roughness is thought to be behind most stick–slip. Even an apparently smooth sheet of metal or glass is usually covered with tiny ridges, pits and scratches. These can interlock like teeth until the driving force becomes great enough to break the irregularities or push them over one another. Then the surface lurches forward until the protrusions enmesh further along.

This is not the whole story, say Raffi Budakian and Seth Putterman of the University of California at Los Angeles. They found that tiny strings formed and broke between two tiny gold balls as they rubbed together.

Ball game

Budakian and Putterman glued one ball, a fifth of a millimetre across, to the tip of an optical fibre. The other, two-millimetre ball they attached to a platform that they could move precisely.

When the small ball moved, the optical-fibre beam moved with it. Thus, the researchers could accurately measure the ball’s displacement, and the forces acting on it. From changes in an electrical current flowing between the balls, they also deduced the size of the area of contact between them.

First, the duo measured how hard it was to pull the balls apart vertically. They found that this rupture stress increased as the area of contact got bigger.

They concluded that the metal balls are tacky at nanometre scales: held together for several seconds, narrow bridges of gold form between the two surfaces, which stretch and break as the balls are separated. The force needed to break these necks of gold depends on how thick they are.

Then the researchers looked at horizontal sliding motions. They found that the force needed to initiate a slip was the same as the force needed to rupture a gold neck as the balls were pulled apart vertically. In other words, it seems that tiny strings of gold are created, stretched and snapped as the metal surfaces move over one another.

References

  1. Budakian, R. & Putterman, S.J. Time scales for cold-welding and the origins of stick–slip behaviour. Preprint, (2001).


PHILIP BALL | Nature Science News

More articles from Physics and Astronomy:

nachricht Cherned up to the maximum
10.07.2020 | Max-Planck-Institut für Chemische Physik fester Stoffe

nachricht Porous graphene ribbons doped with nitrogen for electronics and quantum computing
09.07.2020 | University of Basel

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The spin state story: Observation of the quantum spin liquid state in novel material

New insight into the spin behavior in an exotic state of matter puts us closer to next-generation spintronic devices

Aside from the deep understanding of the natural world that quantum physics theory offers, scientists worldwide are working tirelessly to bring forth a...

Im Focus: Excitation of robust materials

Kiel physics team observed extremely fast electronic changes in real time in a special material class

In physics, they are currently the subject of intensive research; in electronics, they could enable completely new functions. So-called topological materials...

Im Focus: Electrons in the fast lane

Solar cells based on perovskite compounds could soon make electricity generation from sunlight even more efficient and cheaper. The laboratory efficiency of these perovskite solar cells already exceeds that of the well-known silicon solar cells. An international team led by Stefan Weber from the Max Planck Institute for Polymer Research (MPI-P) in Mainz has found microscopic structures in perovskite crystals that can guide the charge transport in the solar cell. Clever alignment of these "electron highways" could make perovskite solar cells even more powerful.

Solar cells convert sunlight into electricity. During this process, the electrons of the material inside the cell absorb the energy of the light....

Im Focus: The lightest electromagnetic shielding material in the world

Empa researchers have succeeded in applying aerogels to microelectronics: Aerogels based on cellulose nanofibers can effectively shield electromagnetic radiation over a wide frequency range – and they are unrivalled in terms of weight.

Electric motors and electronic devices generate electromagnetic fields that sometimes have to be shielded in order not to affect neighboring electronic...

Im Focus: Gentle wall contact – the right scenario for a fusion power plant

Quasi-continuous power exhaust developed as a wall-friendly method on ASDEX Upgrade

A promising operating mode for the plasma of a future power plant has been developed at the ASDEX Upgrade fusion device at Max Planck Institute for Plasma...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Contact Tracing Apps against COVID-19: German National Academy Leopoldina hosts international virtual panel discussion

07.07.2020 | Event News

International conference QuApps shows status quo of quantum technology

02.07.2020 | Event News

Dresden Nexus Conference 2020: Same Time, Virtual Format, Registration Opened

19.05.2020 | Event News

 
Latest News

X-ray scattering shines light on protein folding

10.07.2020 | Life Sciences

Looking at linkers helps to join the dots

10.07.2020 | Materials Sciences

Surprisingly many peculiar long introns found in brain genes

10.07.2020 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>