Factors affecting the measurement of roughness factor of surfaces and its implications for wetting studies

Roughness factor is widely used for topography characterization of surfaces. The measurement of meaningful values of roughness factor depends on the instrument settings, e.g. spatial resolution and scan-size, the instrument characteristics (voxel dimensions), the post-treatment of discrete data array (tessellation algorithm), and finally the nature of the surface texture (e.g. fractal). To analyze the influence of all these parameters on the value of roughness factor and evaluate the influence of each parameter, different synthetic (mathematically defined) surfaces and acid-etched/passivated titanium surfaces were used. The titanium surface topographies were studied using two different microscopes: white light confocal microscopy (WLCM) and atomic force microscopy (AFM). In decreasing order of influence, roughness factor values are sensitive to the specific surface nature (fractal or non-fractal), the spatial resolution, the scan-size and the tessellation algorithm, whereas the instrumentation does not seem to be an important parameter in this study. The effect of the variability in the roughness factor depending on the above parameters in interpretation of the Young's contact angle and solid–vapor interfacial energy was studied based on the apparent contact angle observed and the Wenzel's equation. It was found that depending on how roughness factor is measured variations up to 20% in the Young's contact angle or solid–vapor interfacial energy may be found.