Predominant and Generic Parameters Governing the Wettability Characteristics of Selected Laser-Modified Engineering Materials

Laser processing has extremely high levels of accuracy, resolution and controllability which facilitate tailored wettability characteristics modifi cation. This work has contributed to this by establishing generic relationships between the laser-modifi ed wettability characteristics through measured contact angle, θ, in terms of surface oxygen content, surface roughness and polar component of the surface energy, γ^P. Although surface oxygen content, surface roughness and γ^P are interrelated, this work isolated, to different extents, each of these three and determined the qualitative effect of each on selected laser surface treated ceramics, metals and polymers. Depending on the material studied, varying degrees of success in isolating each of the three factors were achieved. For the ceramics studied, the predominant factor influencing the wettability characteristics was increased γ^P. Surface oxygen content was found to effect changes to a much lesser extent and surface roughness was shown to play a very minor role. For the metals studied, the increase in surface roughness governed the modifi cation of the wettability characteristics, whilst increases in the surface oxygen content and γ^P were shown to play lesser roles. It was more diffi cult to isolate the effects of γ^P and surface oxygen content for the metals. Even so, it was possible to infer from the fi ndings that the factor playing the predominant role was generic. For the polymer studied, surface energy was found to exercise a more dominant role than either surface oxygen content or surface roughness; however, this was only the case to a certain extent as laser-induced patterning of the polymer brought about a marked increase in θ that was contrary to Wenzel's equation. The periodic nature of the laser-induced patterned surface gave rise to a liquid droplet in an equilibrium state that cannot be explained through the Wenzel and Cassie- Baxter wetting regimes. A mixed-state wetting regime is proposed to be occurring along the liquid-surface interface.