Nav: Home

Calculating the mechanics of a rough sphere

May 31, 2016



WASHINGTON, D.C., May 31, 2016 -- The scientific community uses spheres for all sorts of things -- artificial limbs, cars, molecular chemistry -- but there's always a little uncertainty when this geometric shape is introduced into an experiment. While spheres might look smooth and round to the naked eye, their surfaces are far from it. Even a ball bearing, when held up to intense scrutiny, is riddled with peaks and valleys. These inconsistencies change how the spheres interact with the world and can cause problems for everything from surface measurements to the strength of electrical currents.

A transatlantic team of researchers explain the creation of a simulation model that can help scientists mathematically correct for any errors related to a sphere's roughness this week in Applied Physics Letters, from AIP Publishing.

"Devices that measure surfaces typically use spheres -- the stylus," said Lars Pastewka, a material scientist at Karlsruhe institute of Technology in Germany and the leader of the transatlantic team. "Most researchers assume that the [stylus is] smooth and surface roughness is ignored."

The team's calculations are designed to tell a scientist when they should worry about surface roughness, which should make measurements more accurate, Pastewka explained.

Pastewka and Robbins looked at the surface of spheres on the atomic level. They studied how the roughness -- those irregular peaks and valleys -- interacted mechanically with the surfaces they were pushed against to form areas of intimate atomic contact. By running simulations, Pastewka was able to formulate a mathematical expression that demonstrates how spheres with different types of peaks and valleys will deform when they're met with various amounts of pressure.

While it is possible to create a near perfectly spherical object, most scientists can't afford to do so. Knowing how to correct the imperfections mathematically is the cheapest and most plausible way to tackle this problem.

Pastewka and Robbins' equation shows that the steeper the surface peaks are, the smaller the contact area. This could be exploited to minimize friction that sphere will create when sliding on a surface. But, if the slopes are too steep, they're likely to get damaged.

For spheres meant to conduct electricity, scientists would most likely want lower peaks so more of the sphere is in contact with the medium. But, if a surface becomes too flat, it will stick to the medium it's pushed up against -- and could get stuck permanently. In both cases, there's a sweet spot that Pastewka's equation should help scientists achieve.

"Surfaces that are really flat will stick together, and you don't really want them to stick," Pastewka said. "Surface roughness has the important function of separating interfaces while still allowing interaction."

So far, the team has only looked at how the peaks and valleys react to elastic and adhesive surfaces. Elastic surfaces are like balloons, you can poke them and they bounce back to their original shape. The next step is to focus on plastic surfaces, which will change shape permanently when under pressure.

"The future direction is plasticity, going beyond these elastic calculations [and] allowing the solid to permanently change," Pastewka said.
-end-
The article, "Contact area of rough spheres: Large scale simulations and simple scaling laws," is authored by Lars Pastewka and Mark O. Robbins. The article will appear in the journal Applied Physics Letters on May 31, 2016 (DOI: 10.1063/1.4950802). After that date, it can be accessed at: http://scitation.aip.org/content/aip/journal/apl/108/22/10.1063/1.4950802

ABOUT THE JOURNAL


Applied Physics Letters features concise, rapid reports on significant new findings in applied physics. The journal covers new experimental and theoretical research on applications of physics phenomena related to all branches of science, engineering, and modern technology. See http://apl.aip.org.

American Institute of Physics

Related Scientists Articles:

Scientists have found longevity biomarkers
An international group of scientists studied the effects of 17 different lifespan-extending interventions on gene activity in mice and discovered genetic biomarkers of longevity.
Coaching scientists to play well together
When scientists from different disciplines collaborate -- as is increasingly necessary to confront the complexity of challenging research problems -- interpersonal tussles often arise.
Scientists proposed a novel configuration of nanoscopes
TPU scientists proposed using special diffraction gratings with gold plates instead of microlenses to accelerate the generation of images from nanoscopes without losing any magnification power.
Children grow in a different way, scientists demonstrate
An international group of scientists under the supervision of a staff member of Sechenov University (Russia) and Karolinska Institute (Sweden) found out that earlier views on the mechanisms that provide and regulate skeletal growth were wrong.
'Doing science,' rather than 'being scientists,' more encouraging to girls
Asking young girls to 'do science' leads them to show greater persistence in science activities than does asking them to 'be scientists,' finds a new psychology study by researchers at New York University and Princeton University.
More Scientists News and Scientists Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...