The Tribological Properties Of PDMS Surface Composite Gradient Pillar Patterns

In nature, creatures have evolved special organ skins which can easily adapt to its surroundings, following the survival of the fittest, for millions of years, for instance, the skin of gecko's paw, shark, and desert beetle's shells. With the thorough exploration, it was found that there were lots of arrayed micro or nano-structures, provided the skin many special functions, on the biological surfaces. Inspired by the biological design, researchers have been committed to bionic copy of the microstructures on artificial materials, to get the similar functions of the natural skins. Detailed study of microstructures on the biological skins, including tree frogs' sole, newts' toe pads, and lotus leaf, was carried out by us, by bionic replicating microstructures, in this paper.In this research, the nature-like composite gradient micro-pillars were fabricated, with the process of photolithography, wet etching, lithography and complex mode, on the surface of Polydimethylsiloxane. Then, part of the samples' surfaces was treated by oxygen plasma. In experimental section, the friction force was tested, using Type-32 measurement and pin-on-disk tribo-meter, and the wettability of samples' surface were also achieved by the survey of surfaces' contact angle. Besides, the numerical contact model of a rigid ball and the PDMS was presented in this article, and the effects of contact pressure and area were discussed, in detail, caused by micro-pillars on surface. The main conclusions are as follows:Firstly, micro-pillars on surface could reduce the contact area effectively when a rigid ball was pressed into PDMS, with the contact stress concentrating at the edge of the pillars. With the height of pillars increasing, the contact pressure increased, while the contact area decreased; reversely, the contact pressure and area both decreased because of increased area density.Secondly, composite gradient micro-pillars could change the surface wettability of PDMS easily. When the surface is hydrophobic, it would be more hydrophobic, otherwise more hydrophilic.Lastly, composite gradient micro-pillars contributed to reducing the static and sliding friction coefficient between the steel ball and PDMS under dry friction condition. As the surface of PDMS is hydrophobic, composite gradient micro-pillars could also help reduce the static and sliding friction coefficient; however it performed to increase static and sliding friction coefficient under specific parameters, when the surface is hydrophilic.