Self-lubricating Material Inspired by Earthworm Skin
A new friction-reducing material that releases a lubricant whenever it is under pressure – like the skin of an earthworm – will open up a wide range of applications in industry and biomedicine.
Earthworms are always clean, even if they come from moist, sticky soil. This is due to a dirt-repellent, lubricating layer that regenerates on their skin. Researchers at the Leibnitz Institute for New Materials (INM) have now artificially recreated this natural process by developing a material with a surface structure that releases a lubricant whenever pressure is applied.
The new material consists of a soft plastic containing droplets of the lubricant silicone oil on its inside. When pressure is applied to the material, the droplets change shape and migrate to the surface. The silicone oil then spreads evenly over the surface to form a hydrophobic and dirt-repellent sliding layer. The droplets reform when the pressure decreases. The sliding layer can be removed and reformed again whenever pressure is applied to the material again. "So it reacts dynamically to pressure – like a 'breathing' system," summarises Jiaxi Cui, head of the Switchable Microfluidics research group.
Coarse surface prolongs lubrication effect
The surface structure of the new material also plays an important role: "Again, we were inspired by the earthworm. Its skin surface is not smooth, but rough. That's what we took into account in our material and roughened the surface," Cui explains. Due to this coarseness, a uniform lubricating film can form and adhere well. The coarseness dictates how much friction can be reduced on the material.
The surface structure is also vital for the longevity of the lubricating effect: "We compared the sliding film on our 'earthworm structures' with a sliding film on a smooth surface: our structures survive 10,000 cycles of friction, whereas sliding films on smooth structures last for only 300 friction cycles," Cui explains. This new material is particularly special because of the combination of a rough surface and lubricating droplets. There have been some structures that reduce friction, including those that are modelled on the functionality of animal skins. Researchers have even investigated systems that release lubricants themselves, but so far the lubricants have only been successful in a fluid environment.
Industrial and biomedical applications
Due to the lubricated material's low friction and anti-microbial properties, researchers predict it will have many applications in the biomedical and industrial fields, particularly in scenarios where a device is required to pass smoothly through a solid surface. The researchers' findings were recently published in the journal "Advanced Materials".