Two different polymers at the contact point (right) prevent the polymers becoming entangled (left) [Credit: Nolux Media]

Lubricant system mimics that used in joints

A method of lubrication that mimics the joints of animals could drastically reduce friction in axles, hinges and pistons.

Researchers trying to create new lubricants are faced with the opposing challenge of keeping them as thin as possible to reduce friction, while making them viscous enough that they stay in the contact gap and do not leak away.

Nature has solved this problem in the joints of animals using a layer of polymers – densely packed, long-chain molecules – that protrude from the cartilage at the end of bones to form "polymer brushes", which attract the extremely thin and watery fluid lubricant and keep it in place at the contact point.

Over the last 20 years, numerous attempts have been made to imitate the natural, but with little success as the tentacle-like polymers on surfaces opposite each other tend to get tangled up in each other, slowing each other down and detaching from the surfaces.

But in a paper published in journal Nature Communications yesterday Professor Martin Müser, a physicist at the Forschungszentrum Jülich research centre in Germany, explains who he came up with the idea of using two different polymers at the contact point to prevent the polymers becoming entangled.

"Using supercomputers, we simulated what would happen if we applied water-soluble polymers to one side and water-repellent polymers to the other side," says head of the NIC (John von Neumann Institute for Computing) group Computational Materials Physics at the Jülich Supercomputing Centre (JSC).

" This combination of water-based and oil-based liquids as a lubricant reduced the friction by two orders of magnitude – around a factor of 90 – compared to a system comprising just one type of polymer."

Measurements with an atomic force microscope at the University of Twente in the Netherlands verified the results.

"The two different phases of the liquid separate because they repel each other. This simultaneously holds the polymers back and prevents them from protruding beyond the borders," says Dr Sissi de Beer, who recently moved from Müser's group to the University of Twente.

The low-friction two-component lubricant could be particularly useful in simple piston systems, like syringes used to precisely administer tiny amounts of a drug, as well as areas where high pressures and forces occur locally – for example, axle bearings and hinges.

Unfortunately, the system is unlikely to work as a replacement for the most common lubricant – engine oil – as conventional polymer brushes are unable to withstand the high temperatures involved.

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