Effects of Varying Shoe Surface Roughness on COF Between Shoe and Floor Material in the Presence of a Liquid Contaminant

Slip and fall accidents represent a serious occupational and public health concern. Yet, the tribological mechanisms that cause shoe and floor surfaces to be slippery are still not well understood. Previous attempts to model shoe-floor-contaminant friction under mixed-lubrication have ignored the effects of boundary lubrication. The purpose of this study was to examine the affect of roughness and viscosity on shoe-floor-contaminant lubrication by examining its effect on boundary and hydrodynamic lubrication. COF-velocity curves generated by a pin-on-disk tribometer were analyzed to determine the effects of roughness and viscosity on boundary and mixed lubrication. COF was collected under varying shoe roughness (7.3μm, 8.2μm, and 9.34μm), contaminant concentrations (water, 0.89cP; 1.5% diluted detergent, 1.28cP; 25% diluted glycerol, 1.9cP; 50% diluted glycerol, 5.54cP; and 75% diluted glycerol, 41cP), and speed (0.05–1.0m·sec⁻¹). A single shoe material (polyurethane) and a single floor material (vinyl tile) were tested. Reduction in COF with increasing sliding speeds, consistent with regions of boundary and mixed lubrication were observed for all viscosities except the highest (41cP) and the lowest viscosity (water, 0.89cP). An exponential regression model was fit to the data to determine the effect of roughness and viscosity on the rate of COF decay, τ_hydro, and COF when velocity is 0, COF_BL, indicative of boundary lubrication. τ_hydro was inferred to be a measure of the hydrodynamic lubricating effect. Fluid contaminant significantly affected both COF_BL and τ_hydro. Post-hoc analyses revealed that COF_BL decreased with higher concentrations of glycerol. τ_hydro was significantly higher under 1.28cP (diluted detergent) viscosity lubrication when compared to 1.9cP and 5.54cP (diluted glycerol) viscosity lubrication, indicating a slower rate of decrease. No significant effect of shoe material roughness on COF_BL or τ_hydro was identified. Fluid contaminant had a significant effect on both boundary and hydrodynamic lubrication. The change in boundary lubrication coefficient of friction for varying lubricants was primarily attributed to a higher proportion of glycerol molecules, which is a much longer molecule than water, coating the shoe and floor surfaces. The hydrodynamic effect was significant between the glycerol-water lubrication compared to the lower viscosity, 1.5% detergent lubrication, which indicates that the higher viscosity fluids caused a greater rate of friction decrease. This effect is likely due to the wedge term effect of the Reynolds equation. Absence of roughness effects on both boundary and hydrodynamic variables could be due to the soft, shoe material deforming and therefore shoe roughness having a decreased affect on asperity interaction.