Spot-welds stick sliding metals
Metals stick as they slip if pressed together and pushed.
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 whats visible. Here, conventional methods of lubrication start to fail.
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 balls 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.
PHILIP BALL | Nature Science News
Airborne thermometer to measure Arctic temperatures
11.01.2017 | Moscow Institute of Physics and Technology
Next-generation optics offer the widest real-time views of vast regions of the sun
11.01.2017 | New Jersey Institute of Technology
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).
Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...
Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.
The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...
UMD, NOAA collaboration demonstrates suitability of in-orbit datasets for weather satellite calibration
"Traffic and weather, together on the hour!" blasts your local radio station, while your smartphone knows the weather halfway across the world. A network of...
Fiber-reinforced plastics (FRP) are frequently used in the aeronautic and automobile industry. However, the repair of workpieces made of these composite materials is often less profitable than exchanging the part. In order to increase the lifetime of FRP parts and to make them more eco-efficient, the Laser Zentrum Hannover e.V. (LZH) and the Apodius GmbH want to combine a new measuring device for fiber layer orientation with an innovative laser-based repair process.
Defects in FRP pieces may be production or operation-related. Whether or not repair is cost-effective depends on the geometry of the defective area, the tools...
10.01.2017 | Event News
09.01.2017 | Event News
05.01.2017 | Event News
16.01.2017 | Trade Fair News
16.01.2017 | Automotive Engineering
16.01.2017 | Life Sciences