Synergetic effect of laser patterning and micro coining for controlled lubricant propagation

In this study, the anisotropic spreading behavior of Poly-(alpha)-olefin oil (kinematic viscosity of 7.8 cSt at 100 degrees C) on stainless steel samples (AISI 403) having periodic, channel-like structures produced by hot micro-coining (periodicity of 400 mu m and depth of 40 mu m) as well as multi-scale structures (coining and laser patterning) was investigated. These results were compared to the behavior of periodic channels fabricated by direct laser interference patterning (periodicity of 5 mu m and depth of 1 mu m). The spreading behavior of a droplet (3 mu l) was studied for a polished reference as well as for all modified surfaces and recorded by a digital light microscope. From this study, it can be concluded that the polished reference leads to an isotropic spreading behavior resulting from the stochastic surface roughness without any preferential orientation whereas all structured samples induce an anisotropic spreading behavior but with different degrees of anisotropy. The observed behavior can be well correlated with pinning induced by the grooves thus hindering the droplet propagation perpendicular to the grooves and the generation of capillary forces which favor the droplet movement along the grooves. It could be proved that the structural depth is a very desicive parameter with regard to the resulting spreading behavior. The multi-scale surface combining large structural depths and the steeper pattern geometry of the micro-coined surface with much smaller grooves of the laser-structure shows the largest anisotropic spreading behavior due to a stronger pinning and increased capillary forces.