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Posted: Jul 12, 2013
Superlubricity can enable microscopic devices to move at speeds of up to 90km/h
(Nanowerk News) A collaboration between researchers at Tsinghua University and Tel Aviv University shows how a phenomenon called superlubricity, long thought to be of purely academic interest, can enable microscopic devices to move at speeds of up to 90km/h, as fast as cars on a highway.
Imagine there’s no friction. It’s not easy, even if you try. Friction gives shoes and tires the grip to move people and cars forward. Without it, roads would be more slippery than ice. But friction is also a great waster of energy. To reduce it, lubricants are used in everything from door hinges to car engines, at considerable expense. Despite lubrication, estimates are that over one third of the fuel energy used in passenger cars is burnt to overcome friction, providing no useful power at all ("Global energy consumption due to friction in passenger cars").
Things only get worse as moving parts shrink from the size of cars to the size of microchips. On the microscopic scale, the ratio of the surface area of objects to their total volume increases dramatically. This means that friction, which is a surface phenomenon, overwhelms the tendency of moving objects to keep moving, which depends on their mass and hence their volume. And introducing lubricants in microscopic machines is tricky, precisely due to their small size.
So an article published in Physical Review Letters last week, by J. Yang et al. ("Observation of High-Speed Microscale Superlubricity in Graphite"), represents a breakthrough in demonstrating the practical utility of a phenomenon called superlubricity – long considered a theoretical concept that could only be demonstrated under extreme conditions – in overcoming friction on the microscopic scale.
The article builds on work by the group of Quanshui Zheng, director of the Centre for Nano and Micro Mechanics (CNMM) at Tsinghua University. The group previously demonstrated that microscopic structures made of crystalline graphite, when sheared, restore themselves to their original shape to minimize the total exposed surface ("Observation of Microscale Superlubricity in Graphite").
This self-retraction phenomenon, as Prof. Zheng and his co-workers have shown, is due to a special structure occurring at the sheared interface, called a twist boundary. This structure means that the crystalline lattice of carbon atoms either side of the sheared interface is misaligned. So rather than neatly locking into place, as layers of carbon atoms do in a normal graphite crystal, the layers at this interface slide easily past each other with almost no friction at all: this is what superlubricity means.
In the latest work, which is a collaboration with an expert in the theory of friction, Michael Urbakh at Tel Aviv University, lead author Jiarui Yang built a device to detect the speed at which the self-retraction occurred, using a laser technique precise and fast enough to catch the fleeting motion in such small objects. Prof. Urbakh remarks “I have been studying superlubricity from a theoretical perspective for years, so suddenly seeing the effect in action like this under an ordinary microscope is a major step forward”.
The results show maximum speeds of 25m/s (90km/h) for a thin, square graphite wafer just three micrometers on a side, as it self-retracts after being sheared by a microprobe. Intriguingly, the highest speeds are reached by heating the graphite to over 100 °C. The reason put forward for this by the researchers, is that the increased jiggling of atoms due to temperature enables the sliding surfaces to overcome inevitable atomic-scaled defects that occur at the interface.
Prior to this work, the velocity of objects exhibiting superlubricity was typically measured in micrometers per second – a snail’s pace. And this was under conditions of ultra-high vacuum, for a specially controlled interface on the scale of nanometers. Commenting on the breakthrough, Prof. Zheng notes that “observing high-speed superlubricity on the much larger scale of micrometers, and even under normal atmospheric conditions, immediately raises the possibility of practical applications”.
Possible uses include future miniaturized hard disk drives and microscopic high-frequency oscillators for telecommunications, in other words microscopic devices whose performance depends on high-speed motion.