Going Beyond Traditional Roughness Metrics for Floor Tiles: Measuring Topography Down to the Nanoscale

Slipping is a major cause of injury and hospitalization in the USA and globally. Slipping occurs when the instantaneous friction between the shoe and floor is less than the required friction. While floor roughness is a key factor contributing to friction, prior investigations have primarily used stylus profilometry, which is incapable of measuring roughness at small scales, below approximately 1 micron in lateral size. In the present research, the small-scale roughness was quantified using cross-section scanning electron microscopy (SEM). Three different flooring materials were investigated, including tiles of ceramic and two different types of quarry stones, whose friction coefficients had been previously characterized. The surfaces were cross-sectioned, imaged at magnifications from 250 to 100,000 times, and then the surface profiles were extracted using image analysis. The SEM topography was combined with stylus profilometry measurements, using the power spectral density (PSD), to achieve multi-scale characterization of features ranging from a scan size of 4 mm down to a resolution of 10 nm. The results demonstrate meaningful differences in topography at different length scales, where surfaces with widely varying roughness at one scale were indistinguishable at another. The measurements further showed that floor-tile roughness has self-affine fractal-like character, with hierarchical roughness extending from the micron-scale down to the nanoscale, much of which is undetectable using conventional techniques. Overall, this research supports the investigation of small-scale roughness as a potential missing factor in the understanding of floor topography and its causal effect on slip-and-fall accidents.