Featured Research

from universities, journals, and other organizations

Scientists Identify Molecular Source Of Friction

Date:
June 4, 1999
Source:
Johns Hopkins University
Summary:
Exactly 300 years after Guillaume Amontons produced the classic laws of friction, physicists have explained why Amontons' equations explain static friction so precisely.

It's "the gunk in the middle"

Exactly 300 years after Guillaume Amontons produced the classic laws of friction, physicists at The Johns Hopkins University have accounted for the notable endurance of Amontons' equations by identifying the molecular origins of static friction.

It's that stuff in the middle.

"Typically, when someone measures friction, what they will report is the ratio of force to the load, which is the coefficient of friction," said Mark Robbins, professor of physics at Johns Hopkins. "But it's never been understood at the molecular level how we get this linear relationship, or why it should hold for a wide array of materials and geometries. Our work now shows that the gunk in the middle--typically hydrocarbon molecules that are almost always present between two surfaces--provides an explanation."

The findings are reported this week in the June 4 issue of the journal "Science" by Robbins and his colleagues, Gang He and Martin Mόser.

Amontons' laws, relied upon extensively and routinely by engineers for three centuries, state that the frictional force needed to slide one body over another is proportional to the load that presses them together and is also independent of the areas of the surfaces. Scientists have argued, however, about the relative importance of surface roughness, chemistry and other factors that might contribute to friction.

Simple theories that leave out the "stuff in the middle" predict that static friction, the force needed to initiate sliding, should vanish between almost any pair of solids. In reality, if that were true, every piece of furniture in a room would slide to one corner and buildings would collapse. The data compiled by Robbins and his colleagues show that hydrocarbon molecules that adsorb on any surface exposed to air resolve this problem. The molecules actually rearrange to lock contacting surfaces together and produce the static friction force that satisfies Amontons' laws.

Scientists have known for centuries that hydrocarbons and other so-called "third bodies" play a significant role in friction. Only in the last decade, however, has a flurry of new experimental probes allowed scientists to study the origins of friction at the atomic scale. At the same time, more powerful computers have given researchers like Robbins and his colleagues an opportunity to accurately simulate the motion of thousands of atoms and determine more about the fundamental nature of friction.

On the whole, Robbins said, "the natural inclination for people approaching the problem has been to try to simplify the analysis as far as possible by disregarding the so-called ‘dirt' between surfaces. Normally, you would think that, by introducing these other molecules, your analysis would just be made more complex. But the fact is, this ‘dirt' makes something that would be very complicated quite simple."

Using molecular dynamics models, Robbins and his colleagues simulated the presence of ‘third body' hydrocarbons along the interface of a variety of contact surfaces. Interestingly, they discovered that varying the length of the molecular chain produced little change in the coefficient of friction. Increasing the number of hydrocarbon layers had no real effect either.

"Friction is a very, very complicated thing," Robbins observed, "and I want to be careful not to suggest that we've now explained it all. But I think most people who have heard about this work seem interested that such a simple picture can give a nice general result that's consistent with Amontons' laws.

"Clearly our model doesn't have all the complexity of the real world--the roughness of real surfaces, the chemical properties of different materials and actual molecules--but it does show how important these third bodies are. From now on, people who do simulations will have to look very carefully at the stuff in the middle."

The research was funded by the National Science Foundation and Intel Corp.

###

RELATED LINKS:
Friction studies
Classical friction explained

###


Story Source:

The above story is based on materials provided by Johns Hopkins University. Note: Materials may be edited for content and length.


Cite This Page:

Johns Hopkins University. "Scientists Identify Molecular Source Of Friction." ScienceDaily. ScienceDaily, 4 June 1999. <www.sciencedaily.com/releases/1999/06/990603163257.htm>.
Johns Hopkins University. (1999, June 4). Scientists Identify Molecular Source Of Friction. ScienceDaily. Retrieved August 28, 2014 from www.sciencedaily.com/releases/1999/06/990603163257.htm
Johns Hopkins University. "Scientists Identify Molecular Source Of Friction." ScienceDaily. www.sciencedaily.com/releases/1999/06/990603163257.htm (accessed August 28, 2014).

Share This




More Matter & Energy News

Thursday, August 28, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Australian Airlines Relax Phone Ban Too

Australian Airlines Relax Phone Ban Too

Reuters - Business Video Online (Aug. 26, 2014) — Qantas and Virgin say passengers can use their smartphones and tablets throughout flights after a regulator relaxed a ban on electronic devices during take-off and landing. As Hayley Platt reports the move comes as the two domestic rivals are expected to post annual net losses later this week. Video provided by Reuters
Powered by NewsLook.com
Hurricane Marie Brings Big Waves to California Coast

Hurricane Marie Brings Big Waves to California Coast

Reuters - US Online Video (Aug. 26, 2014) — Huge waves generated by Hurricane Marie hit the Southern California coast. Rough Cut (no reporter narration). Video provided by Reuters
Powered by NewsLook.com
Chinese Researchers Might Be Creating Supersonic Submarine

Chinese Researchers Might Be Creating Supersonic Submarine

Newsy (Aug. 26, 2014) — Chinese researchers have expanded on Cold War-era tech and are closer to building a submarine that could reach the speed of sound. Video provided by Newsy
Powered by NewsLook.com
Breakingviews: India Coal Strained by Supreme Court Ruling

Breakingviews: India Coal Strained by Supreme Court Ruling

Reuters - Business Video Online (Aug. 26, 2014) — An acute coal shortage is likely to be aggravated as India's supreme court declared government coal allocations illegal, says Breakingviews' Peter Thal Larsen. Video provided by Reuters
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins