Lubricant migration on stainless steel induced by bio-inspired multi-scale surface patterns

This manuscript aims at investigating the spreading dynamics of an additive-free Poly-(alpha)-olefin (PAO) oil on stainless steel surfaces (AISI 304) having multi-scale surface patterns with and without an applied temperature gradient. For this purpose, single-scale patterns were fabricated by micro-coining, direct laser interference patterning and ultrashort-pulse laser patterning. In a second step, multi-scale patterns were realized by superimposing micro-coined surfaces with the respective laser patterns. In order to precisely adjust the temperature gradient and to study the lubricant's migration on the patterned surfaces, a specially designed test rig was used. Experiments without temperature gradient demonstrated a preferential lubricant spreading parallel to the pattern for all samples. In this context, the multi-scale samples showed a faster lubricant spreading compared to the single-scale patterns, which can be attributed to additional capillary forces and an increased roughness. In case of an applied temperature gradient, purely micro-coined samples showed an increased rate of lubricant spreading compared to the other single-scale samples due to the larger volume to surface area ratio. The multi-scale surfaces demonstrated again the fastest lubricant spreading, which clearly underlines the ability to actively guide lubricant by multi-scale patterning. Keywords: Lubricant spreading, Micro-coining, Laser patterning, Multi-scale, Wetting