Superhydrophobic laser-ablated stainless steel substrates exhibiting Cassie-Baxter stable state

In order to produce long-lasting, non-wettable surfaces, femtosecond laser ablation process was used to produce micro/nano structures on metallic surfaces. The effects of femtosecond laser irradiation process parameters (fluence, scanning speed and laser beam overlap) on the hydrophobicity of the resulting micro/nano-patterned morphologies on stainless steel were studied in detail. Seven distinctly different micro/nano patterns, namely nano-rippled, paraboloidal, sinusoidal, triple roughness, cauliflowered, tulip and scaly patterns, were fabricated. The latter pattern (scaly) was found to be identical to that of the butterfly wing structure. All patterns were classified according to laser intensity and scanning speed. Consequently, the fabricated substrates were chemically treated by silanization (Young contact angle (CA) of about 105 degrees) in order to examine the additional effects of nanopatterning on wettability. The analysis of wettability revealed enhanced superhydrophobicity for most of these structures, particularly for the cauliflowered and scaly patterns with CAs in excess of 160 degrees and contact angle hysteresis of less than 10 degrees. Finally, the cauliflowered pattern was found to possess a stable composite state, which is important for certain applications such as friction (Cassie-Baxter).