Surface geometrical model modification and contact angle prediction for the laser patterned steel surface

Our previous article [1] described an attempt to achieve a hydrophobic surface by means of laser ablation process to implement micro-patterns on polished hydrophilic steel surface. By changing the geometric parameters of pattern, the water contact angle of the patterned surface was successfully increased to about 130°, compared to the original 68.5° found from a plain smooth hydrophilic steel surface (with a roughness of Ra ≤ 0.01 μm). The present article focuses on the relationship between the controlled patterns and the related contact angles. An assumption of overhangs which can suspend the free liquid surface in indentations as described by Herminghaus has been used in conjunction with the Cassie–Baxter model to analyze the hydrophilic substrate [2]. When the experimental results were discussed, the deformation in local surface induced by laser heat was also considered in details. On the patterned surface, the molten substrate was observed to have formed slopes and piled to build up ridges along the sides of grooves. Different geometrical models of surface profile were employed to approximate the actual laser patterned surface so that the contact angle could be predicted with accuracy.