Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Smooth surfaces are tacky at small scales


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.


  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 Move over, lasers: Scientists can now create holograms from neutrons, too
21.10.2016 | National Institute of Standards and Technology (NIST)

nachricht Finding the lightest superdeformed triaxial atomic nucleus
20.10.2016 | The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences

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: New 3-D wiring technique brings scalable quantum computers closer to reality

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...

Im Focus: Scientists develop a semiconductor nanocomposite material that moves in response to light

In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...

Im Focus: Diamonds aren't forever: Sandia, Harvard team create first quantum computer bridge

By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.

"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...

Im Focus: New Products - Highlights of COMPAMED 2016

COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.

In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...

Im Focus: Ultra-thin ferroelectric material for next-generation electronics

'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.

Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...

All Focus news of the innovation-report >>>



Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

Agricultural Trade Developments and Potentials in Central Asia and the South Caucasus

14.10.2016 | Event News

World Health Summit – Day Three: A Call to Action

12.10.2016 | Event News

Latest News

Resolving the mystery of preeclampsia

21.10.2016 | Health and Medicine

Stanford researchers create new special-purpose computer that may someday save us billions

21.10.2016 | Information Technology

From ancient fossils to future cars

21.10.2016 | Materials Sciences

More VideoLinks >>>