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It is easier to push a light moving box across the floor than a heavy one. The heavier, the more friction. However, this does not apply if there is hardly any friction between the moving box and the floor. Then a heavy moving box slides just as freely as a light one.
The phenomenon in which friction between two surfaces almost completely disappears is called superlubrication. An international research group has now discovered how and why superlubrication has counterintuitive properties, such as weight not mattering. Their results appeared last week in Physical Examination Letters.
Friction is estimated to be responsible for about a quarter of energy losses worldwide as it requires additional energy and causes wear and tear. Super lubrication could reduce the load.
Superlubrication occurs on surfaces that are superflat, down to the size of atoms. Friction occurs when the atoms on two surfaces come into contact. Even the flattest materials look like miniature mountains or hills on the scale of atoms. When two surfaces touch, the peaks of one can fall into the valleys of the other. If they fit together exactly, it takes a lot of effort to slide all the tips over the tips of the other surface.
However, when you rotate the same surfaces relative to each other, the peaks and valleys no longer collapse and they slide over each other more easily. “You can compare it to two old-fashioned washboards,” says physicist Joost Frenken, who is not involved in the research. “If one person’s ridges fit exactly into the other’s, then they are safe. When you turn them, they simply slide over each other.” The super lubrication means it glides so well that there is hardly any friction.
Graphite flakes
About twenty years ago, Frenken, now dean of the University of Groningen, first demonstrated this phenomenon of superlubrication on graphite surfaces, which consist of flat layers of carbon atoms. Frenken: “That explained why graphite flakes, which have been used as a lubricant for locks and hinges for decades, work so well.”
Frenken and his team had also previously found that temperature further reduces friction: “Atoms vibrate due to the heat, making it easier for them to push over hills.”
The new research shows that the reverse can also be true. When you move surfaces quickly over one another, the friction actually increases as the temperature increases. The researchers show that this is because heat creates small curvatures in the flat surfaces. At high speeds, these curves cause more friction.
They also describe why friction does not increase with increasing weight. In “normal” friction, atomic surfaces are messy. If you press them together harder and with more weight, the contact between them increases – and thus the friction. With super lubrication, surface contact is already optimal and does not increase with weight.
“Thanks to this and other studies, we are increasingly understanding how superlubrication works,” says Frenken. “This means we can use it for more applications.”
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